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pandas contains extensive capabilities and features for working with time series data for all domains.
Using the NumPy datetime64 and timedelta64 dtypes, pandas has consolidated a large number of
features from other Python libraries like scikits.timeseries as well as created
a tremendous amount of new functionality for manipulating time series data.
For example, pandas supports:
Parsing time series information from various sources and formats
.. ipython:: python
import datetime
dti = pd.to_datetime(['1/1/2018', np.datetime64('2018-01-01'),
datetime.datetime(2018, 1, 1)])
dti
Generate sequences of fixed-frequency dates and time spans
.. ipython:: python
dti = pd.date_range('2018-01-01', periods=3, freq='H')
dti
Manipulating and converting date times with timezone information
.. ipython:: python
dti = dti.tz_localize('UTC')
dti
dti.tz_convert('US/Pacific')
Resampling or converting a time series to a particular frequency
.. ipython:: python
idx = pd.date_range('2018-01-01', periods=5, freq='H')
ts = pd.Series(range(len(idx)), index=idx)
ts
ts.resample('2H').mean()
Performing date and time arithmetic with absolute or relative time increments
.. ipython:: python
friday = pd.Timestamp('2018-01-05')
friday.day_name()
# Add 1 day
saturday = friday + pd.Timedelta('1 day')
saturday.day_name()
# Add 1 business day (Friday --> Monday)
monday = friday + pd.offsets.BDay()
monday.day_name()
pandas provides a relatively compact and self-contained set of tools for performing the above tasks and more.
pandas captures 4 general time related concepts:
- Date times: A specific date and time with timezone support. Similar to
datetime.datetimefrom the standard library. - Time deltas: An absolute time duration. Similar to
datetime.timedeltafrom the standard library. - Time spans: A span of time defined by a point in time and its associated frequency.
- Date offsets: A relative time duration that respects calendar arithmetic. Similar to
dateutil.relativedelta.relativedeltafrom thedateutilpackage.
| Concept | Scalar Class | Array Class | pandas Data Type | Primary Creation Method |
|---|---|---|---|---|
| Date times | Timestamp |
DatetimeIndex |
datetime64[ns] or datetime64[ns, tz] |
to_datetime or date_range |
| Time deltas | Timedelta |
TimedeltaIndex |
timedelta64[ns] |
to_timedelta or timedelta_range |
| Time spans | Period |
PeriodIndex |
period[freq] |
Period or period_range |
| Date offsets | DateOffset |
None |
None |
DateOffset |
For time series data, it's conventional to represent the time component in the index of a :class:`Series` or :class:`DataFrame` so manipulations can be performed with respect to the time element.
.. ipython:: python
pd.Series(range(3), index=pd.date_range('2000', freq='D', periods=3))
However, :class:`Series` and :class:`DataFrame` can directly also support the time component as data itself.
.. ipython:: python
pd.Series(pd.date_range('2000', freq='D', periods=3))
:class:`Series` and :class:`DataFrame` have extended data type support and functionality for datetime, timedelta
and Period data when passed into those constructors. DateOffset
data however will be stored as object data.
.. ipython:: python
pd.Series(pd.period_range('1/1/2011', freq='M', periods=3))
pd.Series([pd.DateOffset(1), pd.DateOffset(2)])
pd.Series(pd.date_range('1/1/2011', freq='M', periods=3))
Lastly, pandas represents null date times, time deltas, and time spans as NaT which
is useful for representing missing or null date like values and behaves similar
as np.nan does for float data.
.. ipython:: python pd.Timestamp(pd.NaT) pd.Timedelta(pd.NaT) pd.Period(pd.NaT) # Equality acts as np.nan would pd.NaT == pd.NaT
Timestamped data is the most basic type of time series data that associates values with points in time. For pandas objects it means using the points in time.
.. ipython:: python
pd.Timestamp(datetime.datetime(2012, 5, 1))
pd.Timestamp('2012-05-01')
pd.Timestamp(2012, 5, 1)
However, in many cases it is more natural to associate things like change
variables with a time span instead. The span represented by Period can be
specified explicitly, or inferred from datetime string format.
For example:
.. ipython:: python
pd.Period('2011-01')
pd.Period('2012-05', freq='D')
:class:`Timestamp` and :class:`Period` can serve as an index. Lists of
Timestamp and Period are automatically coerced to :class:`DatetimeIndex`
and :class:`PeriodIndex` respectively.
.. ipython:: python
dates = [pd.Timestamp('2012-05-01'),
pd.Timestamp('2012-05-02'),
pd.Timestamp('2012-05-03')]
ts = pd.Series(np.random.randn(3), dates)
type(ts.index)
ts.index
ts
periods = [pd.Period('2012-01'), pd.Period('2012-02'), pd.Period('2012-03')]
ts = pd.Series(np.random.randn(3), periods)
type(ts.index)
ts.index
ts
pandas allows you to capture both representations and
convert between them. Under the hood, pandas represents timestamps using
instances of Timestamp and sequences of timestamps using instances of
DatetimeIndex. For regular time spans, pandas uses Period objects for
scalar values and PeriodIndex for sequences of spans. Better support for
irregular intervals with arbitrary start and end points are forth-coming in
future releases.
To convert a :class:`Series` or list-like object of date-like objects e.g. strings,
epochs, or a mixture, you can use the to_datetime function. When passed
a Series, this returns a Series (with the same index), while a list-like
is converted to a DatetimeIndex:
.. ipython:: python
pd.to_datetime(pd.Series(['Jul 31, 2009', '2010-01-10', None]))
pd.to_datetime(['2005/11/23', '2010.12.31'])
If you use dates which start with the day first (i.e. European style),
you can pass the dayfirst flag:
.. ipython:: python
pd.to_datetime(['04-01-2012 10:00'], dayfirst=True)
pd.to_datetime(['14-01-2012', '01-14-2012'], dayfirst=True)
Warning
You see in the above example that dayfirst isn't strict, so if a date
can't be parsed with the day being first it will be parsed as if
dayfirst were False.
If you pass a single string to to_datetime, it returns a single Timestamp.
Timestamp can also accept string input, but it doesn't accept string parsing
options like dayfirst or format, so use to_datetime if these are required.
.. ipython:: python
pd.to_datetime('2010/11/12')
pd.Timestamp('2010/11/12')
You can also use the DatetimeIndex constructor directly:
.. ipython:: python
pd.DatetimeIndex(['2018-01-01', '2018-01-03', '2018-01-05'])
The string 'infer' can be passed in order to set the frequency of the index as the inferred frequency upon creation:
.. ipython:: python
pd.DatetimeIndex(['2018-01-01', '2018-01-03', '2018-01-05'], freq='infer')
In addition to the required datetime string, a format argument can be passed to ensure specific parsing.
This could also potentially speed up the conversion considerably.
.. ipython:: python
pd.to_datetime('2010/11/12', format='%Y/%m/%d')
pd.to_datetime('12-11-2010 00:00', format='%d-%m-%Y %H:%M')
For more information on the choices available when specifying the format
option, see the Python datetime documentation.
.. versionadded:: 0.18.1
You can also pass a DataFrame of integer or string columns to assemble into a Series of Timestamps.
.. ipython:: python
df = pd.DataFrame({'year': [2015, 2016],
'month': [2, 3],
'day': [4, 5],
'hour': [2, 3]})
pd.to_datetime(df)
You can pass only the columns that you need to assemble.
.. ipython:: python pd.to_datetime(df[['year', 'month', 'day']])
pd.to_datetime looks for standard designations of the datetime component in the column names, including:
- required:
year,month,day - optional:
hour,minute,second,millisecond,microsecond,nanosecond
The default behavior, errors='raise', is to raise when unparseable:
In [2]: pd.to_datetime(['2009/07/31', 'asd'], errors='raise')
ValueError: Unknown string format
Pass errors='ignore' to return the original input when unparseable:
.. ipython:: python pd.to_datetime(['2009/07/31', 'asd'], errors='ignore')
Pass errors='coerce' to convert unparseable data to NaT (not a time):
.. ipython:: python pd.to_datetime(['2009/07/31', 'asd'], errors='coerce')
pandas supports converting integer or float epoch times to Timestamp and
DatetimeIndex. The default unit is nanoseconds, since that is how Timestamp
objects are stored internally. However, epochs are often stored in another unit
which can be specified. These are computed from the starting point specified by the
origin parameter.
.. ipython:: python
pd.to_datetime([1349720105, 1349806505, 1349892905,
1349979305, 1350065705], unit='s')
pd.to_datetime([1349720105100, 1349720105200, 1349720105300,
1349720105400, 1349720105500], unit='ms')
Constructing a :class:`Timestamp` or :class:`DatetimeIndex` with an epoch timestamp
with the tz argument specified will localize the epoch timestamps to UTC
first then convert the result to the specified time zone.
.. ipython:: python pd.Timestamp(1262347200000000000, tz='US/Pacific') pd.DatetimeIndex([1262347200000000000], tz='US/Pacific')
Note
Epoch times will be rounded to the nearest nanosecond.
Warning
Conversion of float epoch times can lead to inaccurate and unexpected results.
:ref:`Python floats <python:tut-fp-issues>` have about 15 digits precision in
decimal. Rounding during conversion from float to high precision Timestamp is
unavoidable. The only way to achieve exact precision is to use a fixed-width
types (e.g. an int64).
.. ipython:: python pd.to_datetime([1490195805.433, 1490195805.433502912], unit='s') pd.to_datetime(1490195805433502912, unit='ns')
.. seealso:: :ref:`timeseries.origin`
To invert the operation from above, namely, to convert from a Timestamp to a 'unix' epoch:
.. ipython:: python
stamps = pd.date_range('2012-10-08 18:15:05', periods=4, freq='D')
stamps
We subtract the epoch (midnight at January 1, 1970 UTC) and then floor divide by the "unit" (1 second).
.. ipython:: python
(stamps - pd.Timestamp("1970-01-01")) // pd.Timedelta('1s')
.. versionadded:: 0.20.0
Using the origin parameter, one can specify an alternative starting point for creation
of a DatetimeIndex. For example, to use 1960-01-01 as the starting date:
.. ipython:: python
pd.to_datetime([1, 2, 3], unit='D', origin=pd.Timestamp('1960-01-01'))
The default is set at origin='unix', which defaults to 1970-01-01 00:00:00.
Commonly called 'unix epoch' or POSIX time.
.. ipython:: python pd.to_datetime([1, 2, 3], unit='D')
To generate an index with timestamps, you can use either the DatetimeIndex or
Index constructor and pass in a list of datetime objects:
.. ipython:: python
dates = [datetime.datetime(2012, 5, 1),
datetime.datetime(2012, 5, 2),
datetime.datetime(2012, 5, 3)]
# Note the frequency information
index = pd.DatetimeIndex(dates)
index
# Automatically converted to DatetimeIndex
index = pd.Index(dates)
index
In practice this becomes very cumbersome because we often need a very long
index with a large number of timestamps. If we need timestamps on a regular
frequency, we can use the :func:`date_range` and :func:`bdate_range` functions
to create a DatetimeIndex. The default frequency for date_range is a
calendar day while the default for bdate_range is a business day:
.. ipython:: python start = datetime.datetime(2011, 1, 1) end = datetime.datetime(2012, 1, 1) index = pd.date_range(start, end) index index = pd.bdate_range(start, end) index
Convenience functions like date_range and bdate_range can utilize a
variety of :ref:`frequency aliases <timeseries.offset_aliases>`:
.. ipython:: python pd.date_range(start, periods=1000, freq='M') pd.bdate_range(start, periods=250, freq='BQS')
date_range and bdate_range make it easy to generate a range of dates
using various combinations of parameters like start, end, periods,
and freq. The start and end dates are strictly inclusive, so dates outside
of those specified will not be generated:
.. ipython:: python pd.date_range(start, end, freq='BM') pd.date_range(start, end, freq='W') pd.bdate_range(end=end, periods=20) pd.bdate_range(start=start, periods=20)
.. versionadded:: 0.23.0
Specifying start, end, and periods will generate a range of evenly spaced
dates from start to end inclusively, with periods number of elements in the
resulting DatetimeIndex:
.. ipython:: python
pd.date_range('2018-01-01', '2018-01-05', periods=5)
pd.date_range('2018-01-01', '2018-01-05', periods=10)
Warning
This functionality was originally exclusive to cdate_range, which is
deprecated as of version 0.21.0 in favor of bdate_range. Note that
cdate_range only utilizes the weekmask and holidays parameters
when custom business day, 'C', is passed as the frequency string. Support has
been expanded with bdate_range to work with any custom frequency string.
.. versionadded:: 0.21.0
bdate_range can also generate a range of custom frequency dates by using
the weekmask and holidays parameters. These parameters will only be
used if a custom frequency string is passed.
.. ipython:: python weekmask = 'Mon Wed Fri' holidays = [datetime.datetime(2011, 1, 5), datetime.datetime(2011, 3, 14)] pd.bdate_range(start, end, freq='C', weekmask=weekmask, holidays=holidays) pd.bdate_range(start, end, freq='CBMS', weekmask=weekmask)
.. seealso:: :ref:`timeseries.custombusinessdays`
Since pandas represents timestamps in nanosecond resolution, the time span that can be represented using a 64-bit integer is limited to approximately 584 years:
.. ipython:: python pd.Timestamp.min pd.Timestamp.max
.. seealso:: :ref:`timeseries.oob`
One of the main uses for DatetimeIndex is as an index for pandas objects.
The DatetimeIndex class contains many time series related optimizations:
- A large range of dates for various offsets are pre-computed and cached under the hood in order to make generating subsequent date ranges very fast (just have to grab a slice).
- Fast shifting using the
shiftandtshiftmethod on pandas objects. - Unioning of overlapping
DatetimeIndexobjects with the same frequency is very fast (important for fast data alignment). - Quick access to date fields via properties such as
year,month, etc. - Regularization functions like
snapand very fastasoflogic.
DatetimeIndex objects have all the basic functionality of regular Index
objects, and a smorgasbord of advanced time series specific methods for easy
frequency processing.
.. seealso::
:ref:`Reindexing methods <basics.reindexing>`
Note
While pandas does not force you to have a sorted date index, some of these methods may have unexpected or incorrect behavior if the dates are unsorted.
DatetimeIndex can be used like a regular index and offers all of its
intelligent functionality like selection, slicing, etc.
.. ipython:: python rng = pd.date_range(start, end, freq='BM') ts = pd.Series(np.random.randn(len(rng)), index=rng) ts.index ts[:5].index ts[::2].index
Dates and strings that parse to timestamps can be passed as indexing parameters:
.. ipython:: python ts['1/31/2011'] ts[datetime.datetime(2011, 12, 25):] ts['10/31/2011':'12/31/2011']
To provide convenience for accessing longer time series, you can also pass in the year or year and month as strings:
.. ipython:: python ts['2011'] ts['2011-6']
This type of slicing will work on a DataFrame with a DatetimeIndex as well. Since the
partial string selection is a form of label slicing, the endpoints will be included. This
would include matching times on an included date:
.. ipython:: python
dft = pd.DataFrame(np.random.randn(100000, 1), columns=['A'],
index=pd.date_range('20130101', periods=100000, freq='T'))
dft
dft['2013']
This starts on the very first time in the month, and includes the last date and time for the month:
.. ipython:: python dft['2013-1':'2013-2']
This specifies a stop time that includes all of the times on the last day:
.. ipython:: python dft['2013-1':'2013-2-28']
This specifies an exact stop time (and is not the same as the above):
.. ipython:: python dft['2013-1':'2013-2-28 00:00:00']
We are stopping on the included end-point as it is part of the index:
.. ipython:: python dft['2013-1-15':'2013-1-15 12:30:00']
.. versionadded:: 0.18.0
DatetimeIndex partial string indexing also works on a DataFrame with a MultiIndex:
.. ipython:: python
dft2 = pd.DataFrame(np.random.randn(20, 1),
columns=['A'],
index=pd.MultiIndex.from_product(
[pd.date_range('20130101', periods=10, freq='12H'),
['a', 'b']]))
dft2
dft2.loc['2013-01-05']
idx = pd.IndexSlice
dft2 = dft2.swaplevel(0, 1).sort_index()
dft2.loc[idx[:, '2013-01-05'], :]
.. versionchanged:: 0.20.0
The same string used as an indexing parameter can be treated either as a slice or as an exact match depending on the resolution of the index. If the string is less accurate than the index, it will be treated as a slice, otherwise as an exact match.
Consider a Series object with a minute resolution index:
.. ipython:: python
series_minute = pd.Series([1, 2, 3],
pd.DatetimeIndex(['2011-12-31 23:59:00',
'2012-01-01 00:00:00',
'2012-01-01 00:02:00']))
series_minute.index.resolution
A timestamp string less accurate than a minute gives a Series object.
.. ipython:: python
series_minute['2011-12-31 23']
A timestamp string with minute resolution (or more accurate), gives a scalar instead, i.e. it is not casted to a slice.
.. ipython:: python
series_minute['2011-12-31 23:59']
series_minute['2011-12-31 23:59:00']
If index resolution is second, then the minute-accurate timestamp gives a
Series.
.. ipython:: python
series_second = pd.Series([1, 2, 3],
pd.DatetimeIndex(['2011-12-31 23:59:59',
'2012-01-01 00:00:00',
'2012-01-01 00:00:01']))
series_second.index.resolution
series_second['2011-12-31 23:59']
If the timestamp string is treated as a slice, it can be used to index DataFrame with [] as well.
.. ipython:: python
dft_minute = pd.DataFrame({'a': [1, 2, 3], 'b': [4, 5, 6]},
index=series_minute.index)
dft_minute['2011-12-31 23']
Warning
However, if the string is treated as an exact match, the selection in DataFrame's [] will be column-wise and not row-wise, see :ref:`Indexing Basics <indexing.basics>`. For example dft_minute['2011-12-31 23:59'] will raise KeyError as '2012-12-31 23:59' has the same resolution as the index and there is no column with such name:
To always have unambiguous selection, whether the row is treated as a slice or a single selection, use .loc.
.. ipython:: python dft_minute.loc['2011-12-31 23:59']
Note also that DatetimeIndex resolution cannot be less precise than day.
.. ipython:: python
series_monthly = pd.Series([1, 2, 3],
pd.DatetimeIndex(['2011-12', '2012-01', '2012-02']))
series_monthly.index.resolution
series_monthly['2011-12'] # returns Series
As discussed in previous section, indexing a DatetimeIndex with a partial string depends on the "accuracy" of the period, in other words how specific the interval is in relation to the resolution of the index. In contrast, indexing with Timestamp or datetime objects is exact, because the objects have exact meaning. These also follow the semantics of including both endpoints.
These Timestamp and datetime objects have exact hours, minutes, and seconds, even though they were not explicitly specified (they are 0).
.. ipython:: python dft[datetime.datetime(2013, 1, 1):datetime.datetime(2013, 2, 28)]
With no defaults.
.. ipython:: python
dft[datetime.datetime(2013, 1, 1, 10, 12, 0):
datetime.datetime(2013, 2, 28, 10, 12, 0)]
A :meth:`~DataFrame.truncate` convenience function is provided that is similar
to slicing. Note that truncate assumes a 0 value for any unspecified date
component in a DatetimeIndex in contrast to slicing which returns any
partially matching dates:
.. ipython:: python
rng2 = pd.date_range('2011-01-01', '2012-01-01', freq='W')
ts2 = pd.Series(np.random.randn(len(rng2)), index=rng2)
ts2.truncate(before='2011-11', after='2011-12')
ts2['2011-11':'2011-12']
Even complicated fancy indexing that breaks the DatetimeIndex frequency
regularity will result in a DatetimeIndex, although frequency is lost:
.. ipython:: python ts2[[0, 2, 6]].index
With the Resampler object in hand, iterating through the grouped data is very
natural and functions similarly to :py:func:`itertools.groupby`:
.. ipython:: python
small = pd.Series(
range(6),
index=pd.to_datetime(['2017-01-01T00:00:00',
'2017-01-01T00:30:00',
'2017-01-01T00:31:00',
'2017-01-01T01:00:00',
'2017-01-01T03:00:00',
'2017-01-01T03:05:00'])
)
resampled = small.resample('H')
for name, group in resampled:
print("Group: ", name)
print("-" * 27)
print(group, end="\n\n")
See :ref:`groupby.iterating-label` or :class:`Resampler.__iter__` for more.
There are several time/date properties that one can access from Timestamp or a collection of timestamps like a DatetimeIndex.
| Property | Description |
|---|---|
| year | The year of the datetime |
| month | The month of the datetime |
| day | The days of the datetime |
| hour | The hour of the datetime |
| minute | The minutes of the datetime |
| second | The seconds of the datetime |
| microsecond | The microseconds of the datetime |
| nanosecond | The nanoseconds of the datetime |
| date | Returns datetime.date (does not contain timezone information) |
| time | Returns datetime.time (does not contain timezone information) |
| timetz | Returns datetime.time as local time with timezone information |
| dayofyear | The ordinal day of year |
| weekofyear | The week ordinal of the year |
| week | The week ordinal of the year |
| dayofweek | The number of the day of the week with Monday=0, Sunday=6 |
| weekday | The number of the day of the week with Monday=0, Sunday=6 |
| weekday_name | The name of the day in a week (ex: Friday) |
| quarter | Quarter of the date: Jan-Mar = 1, Apr-Jun = 2, etc. |
| days_in_month | The number of days in the month of the datetime |
| is_month_start | Logical indicating if first day of month (defined by frequency) |
| is_month_end | Logical indicating if last day of month (defined by frequency) |
| is_quarter_start | Logical indicating if first day of quarter (defined by frequency) |
| is_quarter_end | Logical indicating if last day of quarter (defined by frequency) |
| is_year_start | Logical indicating if first day of year (defined by frequency) |
| is_year_end | Logical indicating if last day of year (defined by frequency) |
| is_leap_year | Logical indicating if the date belongs to a leap year |
Furthermore, if you have a Series with datetimelike values, then you can
access these properties via the .dt accessor, as detailed in the section
on :ref:`.dt accessors<basics.dt_accessors>`.
In the preceding examples, frequency strings (e.g. 'D') were used to specify
a frequency that defined:
- how the date times in :class:`DatetimeIndex` were spaced when using :meth:`date_range`
- the frequency of a :class:`Period` or :class:`PeriodIndex`
These frequency strings map to a :class:`DateOffset` object and its subclasses. A :class:`DateOffset`
is similar to a :class:`Timedelta` that represents a duration of time but follows specific calendar duration rules.
For example, a :class:`Timedelta` day will always increment datetimes by 24 hours, while a :class:`DateOffset` day
will increment datetimes to the same time the next day whether a day represents 23, 24 or 25 hours due to daylight
savings time. However, all :class:`DateOffset` subclasses that are an hour or smaller
(Hour, Minute, Second, Milli, Micro, Nano) behave like
:class:`Timedelta` and respect absolute time.
The basic :class:`DateOffset` acts similar to dateutil.relativedelta (relativedelta documentation)
that shifts a date time by the corresponding calendar duration specified. The
arithmetic operator (+) or the apply method can be used to perform the shift.
.. ipython:: python
# This particular day contains a day light savings time transition
ts = pd.Timestamp('2016-10-30 00:00:00', tz='Europe/Helsinki')
# Respects absolute time
ts + pd.Timedelta(days=1)
# Respects calendar time
ts + pd.DateOffset(days=1)
friday = pd.Timestamp('2018-01-05')
friday.day_name()
# Add 2 business days (Friday --> Tuesday)
two_business_days = 2 * pd.offsets.BDay()
two_business_days.apply(friday)
friday + two_business_days
(friday + two_business_days).day_name()
Most DateOffsets have associated frequencies strings, or offset aliases, that can be passed
into freq keyword arguments. The available date offsets and associated frequency strings can be found below:
DateOffsets additionally have :meth:`rollforward` and :meth:`rollback`
methods for moving a date forward or backward respectively to a valid offset
date relative to the offset. For example, business offsets will roll dates
that land on the weekends (Saturday and Sunday) forward to Monday since
business offsets operate on the weekdays.
.. ipython:: python
ts = pd.Timestamp('2018-01-06 00:00:00')
ts.day_name()
# BusinessHour's valid offset dates are Monday through Friday
offset = pd.offsets.BusinessHour(start='09:00')
# Bring the date to the closest offset date (Monday)
offset.rollforward(ts)
# Date is brought to the closest offset date first and then the hour is added
ts + offset
These operations preserve time (hour, minute, etc) information by default. To reset time to midnight, use :meth:`normalize` before or after applying the operation (depending on whether you want the time information included in the operation).
.. ipython:: python
ts = pd.Timestamp('2014-01-01 09:00')
day = pd.offsets.Day()
day.apply(ts)
day.apply(ts).normalize()
ts = pd.Timestamp('2014-01-01 22:00')
hour = pd.offsets.Hour()
hour.apply(ts)
hour.apply(ts).normalize()
hour.apply(pd.Timestamp("2014-01-01 23:30")).normalize()
Some of the offsets can be "parameterized" when created to result in different
behaviors. For example, the Week offset for generating weekly data accepts a
weekday parameter which results in the generated dates always lying on a
particular day of the week:
.. ipython:: python d = datetime.datetime(2008, 8, 18, 9, 0) d d + pd.offsets.Week() d + pd.offsets.Week(weekday=4) (d + pd.offsets.Week(weekday=4)).weekday() d - pd.offsets.Week()
The normalize option will be effective for addition and subtraction.
.. ipython:: python d + pd.offsets.Week(normalize=True) d - pd.offsets.Week(normalize=True)
Another example is parameterizing YearEnd with the specific ending month:
.. ipython:: python d + pd.offsets.YearEnd() d + pd.offsets.YearEnd(month=6)
Offsets can be used with either a Series or DatetimeIndex to
apply the offset to each element.
.. ipython:: python
rng = pd.date_range('2012-01-01', '2012-01-03')
s = pd.Series(rng)
rng
rng + pd.DateOffset(months=2)
s + pd.DateOffset(months=2)
s - pd.DateOffset(months=2)
If the offset class maps directly to a Timedelta (Day, Hour,
Minute, Second, Micro, Milli, Nano) it can be
used exactly like a Timedelta - see the
:ref:`Timedelta section<timedeltas.operations>` for more examples.
.. ipython:: python
s - pd.offsets.Day(2)
td = s - pd.Series(pd.date_range('2011-12-29', '2011-12-31'))
td
td + pd.offsets.Minute(15)
Note that some offsets (such as BQuarterEnd) do not have a
vectorized implementation. They can still be used but may
calculate significantly slower and will show a PerformanceWarning
.. ipython:: python :okwarning: rng + pd.offsets.BQuarterEnd()
The CDay or CustomBusinessDay class provides a parametric
BusinessDay class which can be used to create customized business day
calendars which account for local holidays and local weekend conventions.
As an interesting example, let's look at Egypt where a Friday-Saturday weekend is observed.
.. ipython:: python
weekmask_egypt = 'Sun Mon Tue Wed Thu'
# They also observe International Workers' Day so let's
# add that for a couple of years
holidays = ['2012-05-01',
datetime.datetime(2013, 5, 1),
np.datetime64('2014-05-01')]
bday_egypt = pd.offsets.CustomBusinessDay(holidays=holidays,
weekmask=weekmask_egypt)
dt = datetime.datetime(2013, 4, 30)
dt + 2 * bday_egypt
Let's map to the weekday names:
.. ipython:: python
dts = pd.date_range(dt, periods=5, freq=bday_egypt)
pd.Series(dts.weekday, dts).map(
pd.Series('Mon Tue Wed Thu Fri Sat Sun'.split()))
Holiday calendars can be used to provide the list of holidays. See the :ref:`holiday calendar<timeseries.holiday>` section for more information.
.. ipython:: python
from pandas.tseries.holiday import USFederalHolidayCalendar
bday_us = pd.offsets.CustomBusinessDay(calendar=USFederalHolidayCalendar())
# Friday before MLK Day
dt = datetime.datetime(2014, 1, 17)
# Tuesday after MLK Day (Monday is skipped because it's a holiday)
dt + bday_us
Monthly offsets that respect a certain holiday calendar can be defined in the usual way.
.. ipython:: python
bmth_us = pd.offsets.CustomBusinessMonthBegin(
calendar=USFederalHolidayCalendar())
# Skip new years
dt = datetime.datetime(2013, 12, 17)
dt + bmth_us
# Define date index with custom offset
pd.date_range(start='20100101', end='20120101', freq=bmth_us)
Note
The frequency string 'C' is used to indicate that a CustomBusinessDay DateOffset is used, it is important to note that since CustomBusinessDay is a parameterised type, instances of CustomBusinessDay may differ and this is not detectable from the 'C' frequency string. The user therefore needs to ensure that the 'C' frequency string is used consistently within the user's application.
The BusinessHour class provides a business hour representation on BusinessDay,
allowing to use specific start and end times.
By default, BusinessHour uses 9:00 - 17:00 as business hours.
Adding BusinessHour will increment Timestamp by hourly frequency.
If target Timestamp is out of business hours, move to the next business hour
then increment it. If the result exceeds the business hours end, the remaining
hours are added to the next business day.
.. ipython:: python
bh = pd.offsets.BusinessHour()
bh
# 2014-08-01 is Friday
pd.Timestamp('2014-08-01 10:00').weekday()
pd.Timestamp('2014-08-01 10:00') + bh
# Below example is the same as: pd.Timestamp('2014-08-01 09:00') + bh
pd.Timestamp('2014-08-01 08:00') + bh
# If the results is on the end time, move to the next business day
pd.Timestamp('2014-08-01 16:00') + bh
# Remainings are added to the next day
pd.Timestamp('2014-08-01 16:30') + bh
# Adding 2 business hours
pd.Timestamp('2014-08-01 10:00') + pd.offsets.BusinessHour(2)
# Subtracting 3 business hours
pd.Timestamp('2014-08-01 10:00') + pd.offsets.BusinessHour(-3)
You can also specify start and end time by keywords. The argument must
be a str with an hour:minute representation or a datetime.time
instance. Specifying seconds, microseconds and nanoseconds as business hour
results in ValueError.
.. ipython:: python
bh = pd.offsets.BusinessHour(start='11:00', end=datetime.time(20, 0))
bh
pd.Timestamp('2014-08-01 13:00') + bh
pd.Timestamp('2014-08-01 09:00') + bh
pd.Timestamp('2014-08-01 18:00') + bh
Passing start time later than end represents midnight business hour.
In this case, business hour exceeds midnight and overlap to the next day.
Valid business hours are distinguished by whether it started from valid BusinessDay.
.. ipython:: python
bh = pd.offsets.BusinessHour(start='17:00', end='09:00')
bh
pd.Timestamp('2014-08-01 17:00') + bh
pd.Timestamp('2014-08-01 23:00') + bh
# Although 2014-08-02 is Satuaday,
# it is valid because it starts from 08-01 (Friday).
pd.Timestamp('2014-08-02 04:00') + bh
# Although 2014-08-04 is Monday,
# it is out of business hours because it starts from 08-03 (Sunday).
pd.Timestamp('2014-08-04 04:00') + bh
Applying BusinessHour.rollforward and rollback to out of business hours results in
the next business hour start or previous day's end. Different from other offsets, BusinessHour.rollforward
may output different results from apply by definition.
This is because one day's business hour end is equal to next day's business hour start. For example,
under the default business hours (9:00 - 17:00), there is no gap (0 minutes) between 2014-08-01 17:00 and
2014-08-04 09:00.
.. ipython:: python
# This adjusts a Timestamp to business hour edge
pd.offsets.BusinessHour().rollback(pd.Timestamp('2014-08-02 15:00'))
pd.offsets.BusinessHour().rollforward(pd.Timestamp('2014-08-02 15:00'))
# It is the same as BusinessHour().apply(pd.Timestamp('2014-08-01 17:00')).
# And it is the same as BusinessHour().apply(pd.Timestamp('2014-08-04 09:00'))
pd.offsets.BusinessHour().apply(pd.Timestamp('2014-08-02 15:00'))
# BusinessDay results (for reference)
pd.offsets.BusinessHour().rollforward(pd.Timestamp('2014-08-02'))
# It is the same as BusinessDay().apply(pd.Timestamp('2014-08-01'))
# The result is the same as rollworward because BusinessDay never overlap.
pd.offsets.BusinessHour().apply(pd.Timestamp('2014-08-02'))
BusinessHour regards Saturday and Sunday as holidays. To use arbitrary
holidays, you can use CustomBusinessHour offset, as explained in the
following subsection.
.. versionadded:: 0.18.1
The CustomBusinessHour is a mixture of BusinessHour and CustomBusinessDay which
allows you to specify arbitrary holidays. CustomBusinessHour works as the same
as BusinessHour except that it skips specified custom holidays.
.. ipython:: python
from pandas.tseries.holiday import USFederalHolidayCalendar
bhour_us = pd.offsets.CustomBusinessHour(calendar=USFederalHolidayCalendar())
# Friday before MLK Day
dt = datetime.datetime(2014, 1, 17, 15)
dt + bhour_us
# Tuesday after MLK Day (Monday is skipped because it's a holiday)
dt + bhour_us * 2
You can use keyword arguments supported by either BusinessHour and CustomBusinessDay.
.. ipython:: python
bhour_mon = pd.offsets.CustomBusinessHour(start='10:00',
weekmask='Tue Wed Thu Fri')
# Monday is skipped because it's a holiday, business hour starts from 10:00
dt + bhour_mon * 2
A number of string aliases are given to useful common time series frequencies. We will refer to these aliases as offset aliases.
| Alias | Description |
|---|---|
| B | business day frequency |
| C | custom business day frequency |
| D | calendar day frequency |
| W | weekly frequency |
| M | month end frequency |
| SM | semi-month end frequency (15th and end of month) |
| BM | business month end frequency |
| CBM | custom business month end frequency |
| MS | month start frequency |
| SMS | semi-month start frequency (1st and 15th) |
| BMS | business month start frequency |
| CBMS | custom business month start frequency |
| Q | quarter end frequency |
| BQ | business quarter end frequency |
| QS | quarter start frequency |
| BQS | business quarter start frequency |
| A, Y | year end frequency |
| BA, BY | business year end frequency |
| AS, YS | year start frequency |
| BAS, BYS | business year start frequency |
| BH | business hour frequency |
| H | hourly frequency |
| T, min | minutely frequency |
| S | secondly frequency |
| L, ms | milliseconds |
| U, us | microseconds |
| N | nanoseconds |
As we have seen previously, the alias and the offset instance are fungible in most functions:
.. ipython:: python pd.date_range(start, periods=5, freq='B') pd.date_range(start, periods=5, freq=pd.offsets.BDay())
You can combine together day and intraday offsets:
.. ipython:: python pd.date_range(start, periods=10, freq='2h20min') pd.date_range(start, periods=10, freq='1D10U')
For some frequencies you can specify an anchoring suffix:
| Alias | Description |
|---|---|
| W-SUN | weekly frequency (Sundays). Same as 'W' |
| W-MON | weekly frequency (Mondays) |
| W-TUE | weekly frequency (Tuesdays) |
| W-WED | weekly frequency (Wednesdays) |
| W-THU | weekly frequency (Thursdays) |
| W-FRI | weekly frequency (Fridays) |
| W-SAT | weekly frequency (Saturdays) |
| (B)Q(S)-DEC | quarterly frequency, year ends in December. Same as 'Q' |
| (B)Q(S)-JAN | quarterly frequency, year ends in January |
| (B)Q(S)-FEB | quarterly frequency, year ends in February |
| (B)Q(S)-MAR | quarterly frequency, year ends in March |
| (B)Q(S)-APR | quarterly frequency, year ends in April |
| (B)Q(S)-MAY | quarterly frequency, year ends in May |
| (B)Q(S)-JUN | quarterly frequency, year ends in June |
| (B)Q(S)-JUL | quarterly frequency, year ends in July |
| (B)Q(S)-AUG | quarterly frequency, year ends in August |
| (B)Q(S)-SEP | quarterly frequency, year ends in September |
| (B)Q(S)-OCT | quarterly frequency, year ends in October |
| (B)Q(S)-NOV | quarterly frequency, year ends in November |
| (B)A(S)-DEC | annual frequency, anchored end of December. Same as 'A' |
| (B)A(S)-JAN | annual frequency, anchored end of January |
| (B)A(S)-FEB | annual frequency, anchored end of February |
| (B)A(S)-MAR | annual frequency, anchored end of March |
| (B)A(S)-APR | annual frequency, anchored end of April |
| (B)A(S)-MAY | annual frequency, anchored end of May |
| (B)A(S)-JUN | annual frequency, anchored end of June |
| (B)A(S)-JUL | annual frequency, anchored end of July |
| (B)A(S)-AUG | annual frequency, anchored end of August |
| (B)A(S)-SEP | annual frequency, anchored end of September |
| (B)A(S)-OCT | annual frequency, anchored end of October |
| (B)A(S)-NOV | annual frequency, anchored end of November |
These can be used as arguments to date_range, bdate_range, constructors
for DatetimeIndex, as well as various other timeseries-related functions
in pandas.
For those offsets that are anchored to the start or end of specific
frequency (MonthEnd, MonthBegin, WeekEnd, etc), the following
rules apply to rolling forward and backwards.
When n is not 0, if the given date is not on an anchor point, it snapped to the next(previous)
anchor point, and moved |n|-1 additional steps forwards or backwards.
.. ipython:: python
pd.Timestamp('2014-01-02') + pd.offsets.MonthBegin(n=1)
pd.Timestamp('2014-01-02') + pd.offsets.MonthEnd(n=1)
pd.Timestamp('2014-01-02') - pd.offsets.MonthBegin(n=1)
pd.Timestamp('2014-01-02') - pd.offsets.MonthEnd(n=1)
pd.Timestamp('2014-01-02') + pd.offsets.MonthBegin(n=4)
pd.Timestamp('2014-01-02') - pd.offsets.MonthBegin(n=4)
If the given date is on an anchor point, it is moved |n| points forwards
or backwards.
.. ipython:: python
pd.Timestamp('2014-01-01') + pd.offsets.MonthBegin(n=1)
pd.Timestamp('2014-01-31') + pd.offsets.MonthEnd(n=1)
pd.Timestamp('2014-01-01') - pd.offsets.MonthBegin(n=1)
pd.Timestamp('2014-01-31') - pd.offsets.MonthEnd(n=1)
pd.Timestamp('2014-01-01') + pd.offsets.MonthBegin(n=4)
pd.Timestamp('2014-01-31') - pd.offsets.MonthBegin(n=4)
For the case when n=0, the date is not moved if on an anchor point, otherwise
it is rolled forward to the next anchor point.
.. ipython:: python
pd.Timestamp('2014-01-02') + pd.offsets.MonthBegin(n=0)
pd.Timestamp('2014-01-02') + pd.offsets.MonthEnd(n=0)
pd.Timestamp('2014-01-01') + pd.offsets.MonthBegin(n=0)
pd.Timestamp('2014-01-31') + pd.offsets.MonthEnd(n=0)
Holidays and calendars provide a simple way to define holiday rules to be used
with CustomBusinessDay or in other analysis that requires a predefined
set of holidays. The AbstractHolidayCalendar class provides all the necessary
methods to return a list of holidays and only rules need to be defined
in a specific holiday calendar class. Furthermore, the start_date and end_date
class attributes determine over what date range holidays are generated. These
should be overwritten on the AbstractHolidayCalendar class to have the range
apply to all calendar subclasses. USFederalHolidayCalendar is the
only calendar that exists and primarily serves as an example for developing
other calendars.
For holidays that occur on fixed dates (e.g., US Memorial Day or July 4th) an observance rule determines when that holiday is observed if it falls on a weekend or some other non-observed day. Defined observance rules are:
| Rule | Description |
|---|---|
| nearest_workday | move Saturday to Friday and Sunday to Monday |
| sunday_to_monday | move Sunday to following Monday |
| next_monday_or_tuesday | move Saturday to Monday and Sunday/Monday to Tuesday |
| previous_friday | move Saturday and Sunday to previous Friday" |
| next_monday | move Saturday and Sunday to following Monday |
An example of how holidays and holiday calendars are defined:
.. ipython:: python
from pandas.tseries.holiday import Holiday, USMemorialDay,\
AbstractHolidayCalendar, nearest_workday, MO
class ExampleCalendar(AbstractHolidayCalendar):
rules = [
USMemorialDay,
Holiday('July 4th', month=7, day=4, observance=nearest_workday),
Holiday('Columbus Day', month=10, day=1,
offset=pd.DateOffset(weekday=MO(2)))]
cal = ExampleCalendar()
cal.holidays(datetime.datetime(2012, 1, 1), datetime.datetime(2012, 12, 31))
| hint: | weekday=MO(2) is same as 2 * Week(weekday=2) |
|---|
Using this calendar, creating an index or doing offset arithmetic skips weekends
and holidays (i.e., Memorial Day/July 4th). For example, the below defines
a custom business day offset using the ExampleCalendar. Like any other offset,
it can be used to create a DatetimeIndex or added to datetime
or Timestamp objects.
.. ipython:: python
pd.date_range(start='7/1/2012', end='7/10/2012',
freq=pd.offsets.CDay(calendar=cal)).to_pydatetime()
offset = pd.offsets.CustomBusinessDay(calendar=cal)
datetime.datetime(2012, 5, 25) + offset
datetime.datetime(2012, 7, 3) + offset
datetime.datetime(2012, 7, 3) + 2 * offset
datetime.datetime(2012, 7, 6) + offset
Ranges are defined by the start_date and end_date class attributes
of AbstractHolidayCalendar. The defaults are shown below.
.. ipython:: python
AbstractHolidayCalendar.start_date
AbstractHolidayCalendar.end_date
These dates can be overwritten by setting the attributes as datetime/Timestamp/string.
.. ipython:: python
AbstractHolidayCalendar.start_date = datetime.datetime(2012, 1, 1)
AbstractHolidayCalendar.end_date = datetime.datetime(2012, 12, 31)
cal.holidays()
Every calendar class is accessible by name using the get_calendar function
which returns a holiday class instance. Any imported calendar class will
automatically be available by this function. Also, HolidayCalendarFactory
provides an easy interface to create calendars that are combinations of calendars
or calendars with additional rules.
.. ipython:: python
from pandas.tseries.holiday import get_calendar, HolidayCalendarFactory,\
USLaborDay
cal = get_calendar('ExampleCalendar')
cal.rules
new_cal = HolidayCalendarFactory('NewExampleCalendar', cal, USLaborDay)
new_cal.rules
One may want to shift or lag the values in a time series back and forward in time. The method for this is :meth:`~Series.shift`, which is available on all of the pandas objects.
.. ipython:: python ts = pd.Series(range(len(rng)), index=rng) ts = ts[:5] ts.shift(1)
The shift method accepts an freq argument which can accept a
DateOffset class or other timedelta-like object or also an
:ref:`offset alias <timeseries.offset_aliases>`:
.. ipython:: python ts.shift(5, freq=pd.offsets.BDay()) ts.shift(5, freq='BM')
Rather than changing the alignment of the data and the index, DataFrame and
Series objects also have a :meth:`~Series.tshift` convenience method that
changes all the dates in the index by a specified number of offsets:
.. ipython:: python ts.tshift(5, freq='D')
Note that with tshift, the leading entry is no longer NaN because the data
is not being realigned.
The primary function for changing frequencies is the :meth:`~Series.asfreq`
method. For a DatetimeIndex, this is basically just a thin, but convenient
wrapper around :meth:`~Series.reindex` which generates a date_range and
calls reindex.
.. ipython:: python
dr = pd.date_range('1/1/2010', periods=3, freq=3 * pd.offsets.BDay())
ts = pd.Series(np.random.randn(3), index=dr)
ts
ts.asfreq(pd.offsets.BDay())
asfreq provides a further convenience so you can specify an interpolation
method for any gaps that may appear after the frequency conversion.
.. ipython:: python ts.asfreq(pd.offsets.BDay(), method='pad')
Related to asfreq and reindex is :meth:`~Series.fillna`, which is
documented in the :ref:`missing data section <missing_data.fillna>`.
DatetimeIndex can be converted to an array of Python native
:py:class:`datetime.datetime` objects using the to_pydatetime method.
Warning
The interface to .resample has changed in 0.18.0 to be more groupby-like and hence more flexible.
See the :ref:`whatsnew docs <whatsnew_0180.breaking.resample>` for a comparison with prior versions.
Pandas has a simple, powerful, and efficient functionality for performing resampling operations during frequency conversion (e.g., converting secondly data into 5-minutely data). This is extremely common in, but not limited to, financial applications.
:meth:`~Series.resample` is a time-based groupby, followed by a reduction method on each of its groups. See some :ref:`cookbook examples <cookbook.resample>` for some advanced strategies.
Starting in version 0.18.1, the resample() function can be used directly from
DataFrameGroupBy objects, see the :ref:`groupby docs <groupby.transform.window_resample>`.
Note
.resample() is similar to using a :meth:`~Series.rolling` operation with
a time-based offset, see a discussion :ref:`here <stats.moments.ts-versus-resampling>`.
.. ipython:: python
rng = pd.date_range('1/1/2012', periods=100, freq='S')
ts = pd.Series(np.random.randint(0, 500, len(rng)), index=rng)
ts.resample('5Min').sum()
The resample function is very flexible and allows you to specify many
different parameters to control the frequency conversion and resampling
operation.
Any function available via :ref:`dispatching <groupby.dispatch>` is available as
a method of the returned object, including sum, mean, std, sem,
max, min, median, first, last, ohlc:
.. ipython:: python
ts.resample('5Min').mean()
ts.resample('5Min').ohlc()
ts.resample('5Min').max()
For downsampling, closed can be set to 'left' or 'right' to specify which
end of the interval is closed:
.. ipython:: python
ts.resample('5Min', closed='right').mean()
ts.resample('5Min', closed='left').mean()
Parameters like label and loffset are used to manipulate the resulting
labels. label specifies whether the result is labeled with the beginning or
the end of the interval. loffset performs a time adjustment on the output
labels.
.. ipython:: python
ts.resample('5Min').mean() # by default label='left'
ts.resample('5Min', label='left').mean()
ts.resample('5Min', label='left', loffset='1s').mean()
Note
The default values for label and closed is 'left' for all
frequency offsets except for 'M', 'A', 'Q', 'BM', 'BA', 'BQ', and 'W'
which all have a default of 'right'.
.. ipython:: python
rng2 = pd.date_range('1/1/2012', end='3/31/2012', freq='D')
ts2 = pd.Series(range(len(rng2)), index=rng2)
# default: label='right', closed='right'
ts2.resample('M').max()
# default: label='left', closed='left'
ts2.resample('SM').max()
ts2.resample('SM', label='right', closed='right').max()
The axis parameter can be set to 0 or 1 and allows you to resample the
specified axis for a DataFrame.
kind can be set to 'timestamp' or 'period' to convert the resulting index
to/from timestamp and time span representations. By default resample
retains the input representation.
convention can be set to 'start' or 'end' when resampling period data
(detail below). It specifies how low frequency periods are converted to higher
frequency periods.
For upsampling, you can specify a way to upsample and the limit parameter to interpolate over the gaps that are created:
.. ipython:: python
# from secondly to every 250 milliseconds
ts[:2].resample('250L').asfreq()
ts[:2].resample('250L').ffill()
ts[:2].resample('250L').ffill(limit=2)
Sparse timeseries are the ones where you have a lot fewer points relative
to the amount of time you are looking to resample. Naively upsampling a sparse
series can potentially generate lots of intermediate values. When you don't want
to use a method to fill these values, e.g. fill_method is None, then
intermediate values will be filled with NaN.
Since resample is a time-based groupby, the following is a method to efficiently
resample only the groups that are not all NaN.
.. ipython:: python
rng = pd.date_range('2014-1-1', periods=100, freq='D') + pd.Timedelta('1s')
ts = pd.Series(range(100), index=rng)
If we want to resample to the full range of the series:
.. ipython:: python
ts.resample('3T').sum()
We can instead only resample those groups where we have points as follows:
.. ipython:: python
from functools import partial
from pandas.tseries.frequencies import to_offset
def round(t, freq):
# round a Timestamp to a specified freq
freq = to_offset(freq)
return pd.Timestamp((t.value // freq.delta.value) * freq.delta.value)
ts.groupby(partial(round, freq='3T')).sum()
Similar to the :ref:`aggregating API <basics.aggregate>`, :ref:`groupby API <groupby.aggregate>`, and the :ref:`window functions API <stats.aggregate>`,
a Resampler can be selectively resampled.
Resampling a DataFrame, the default will be to act on all columns with the same function.
.. ipython:: python
df = pd.DataFrame(np.random.randn(1000, 3),
index=pd.date_range('1/1/2012', freq='S', periods=1000),
columns=['A', 'B', 'C'])
r = df.resample('3T')
r.mean()
We can select a specific column or columns using standard getitem.
.. ipython:: python r['A'].mean() r[['A', 'B']].mean()
You can pass a list or dict of functions to do aggregation with, outputting a DataFrame:
.. ipython:: python r['A'].agg([np.sum, np.mean, np.std])
On a resampled DataFrame, you can pass a list of functions to apply to each
column, which produces an aggregated result with a hierarchical index:
.. ipython:: python r.agg([np.sum, np.mean])
By passing a dict to aggregate you can apply a different aggregation to the
columns of a DataFrame:
.. ipython:: python
:okexcept:
r.agg({'A': np.sum,
'B': lambda x: np.std(x, ddof=1)})
The function names can also be strings. In order for a string to be valid it must be implemented on the resampled object:
.. ipython:: python
r.agg({'A': 'sum', 'B': 'std'})
Furthermore, you can also specify multiple aggregation functions for each column separately.
.. ipython:: python
r.agg({'A': ['sum', 'std'], 'B': ['mean', 'std']})
If a DataFrame does not have a datetimelike index, but instead you want
to resample based on datetimelike column in the frame, it can passed to the
on keyword.
.. ipython:: python
df = pd.DataFrame({'date': pd.date_range('2015-01-01', freq='W', periods=5),
'a': np.arange(5)},
index=pd.MultiIndex.from_arrays([
[1, 2, 3, 4, 5],
pd.date_range('2015-01-01', freq='W', periods=5)],
names=['v', 'd']))
df
df.resample('M', on='date').sum()
Similarly, if you instead want to resample by a datetimelike
level of MultiIndex, its name or location can be passed to the
level keyword.
.. ipython:: python
df.resample('M', level='d').sum()
Regular intervals of time are represented by Period objects in pandas while
sequences of Period objects are collected in a PeriodIndex, which can
be created with the convenience function period_range.
A Period represents a span of time (e.g., a day, a month, a quarter, etc).
You can specify the span via freq keyword using a frequency alias like below.
Because freq represents a span of Period, it cannot be negative like "-3D".
.. ipython:: python
pd.Period('2012', freq='A-DEC')
pd.Period('2012-1-1', freq='D')
pd.Period('2012-1-1 19:00', freq='H')
pd.Period('2012-1-1 19:00', freq='5H')
Adding and subtracting integers from periods shifts the period by its own
frequency. Arithmetic is not allowed between Period with different freq (span).
.. ipython:: python
p = pd.Period('2012', freq='A-DEC')
p + 1
p - 3
p = pd.Period('2012-01', freq='2M')
p + 2
p - 1
@okexcept
p == pd.Period('2012-01', freq='3M')
If Period freq is daily or higher (D, H, T, S, L, U, N), offsets and timedelta-like can be added if the result can have the same freq. Otherwise, ValueError will be raised.
.. ipython:: python
p = pd.Period('2014-07-01 09:00', freq='H')
p + pd.offsets.Hour(2)
p + datetime.timedelta(minutes=120)
p + np.timedelta64(7200, 's')
In [1]: p + pd.offsets.Minute(5)
Traceback
...
ValueError: Input has different freq from Period(freq=H)
If Period has other frequencies, only the same offsets can be added. Otherwise, ValueError will be raised.
.. ipython:: python
p = pd.Period('2014-07', freq='M')
p + pd.offsets.MonthEnd(3)
In [1]: p + pd.offsets.MonthBegin(3)
Traceback
...
ValueError: Input has different freq from Period(freq=M)
Taking the difference of Period instances with the same frequency will
return the number of frequency units between them:
.. ipython:: python
pd.Period('2012', freq='A-DEC') - pd.Period('2002', freq='A-DEC')
Regular sequences of Period objects can be collected in a PeriodIndex,
which can be constructed using the period_range convenience function:
.. ipython:: python
prng = pd.period_range('1/1/2011', '1/1/2012', freq='M')
prng
The PeriodIndex constructor can also be used directly:
.. ipython:: python pd.PeriodIndex(['2011-1', '2011-2', '2011-3'], freq='M')
Passing multiplied frequency outputs a sequence of Period which
has multiplied span.
.. ipython:: python pd.period_range(start='2014-01', freq='3M', periods=4)
If start or end are Period objects, they will be used as anchor
endpoints for a PeriodIndex with frequency matching that of the
PeriodIndex constructor.
.. ipython:: python
pd.period_range(start=pd.Period('2017Q1', freq='Q'),
end=pd.Period('2017Q2', freq='Q'), freq='M')
Just like DatetimeIndex, a PeriodIndex can also be used to index pandas
objects:
.. ipython:: python ps = pd.Series(np.random.randn(len(prng)), prng) ps
PeriodIndex supports addition and subtraction with the same rule as Period.
.. ipython:: python
idx = pd.period_range('2014-07-01 09:00', periods=5, freq='H')
idx
idx + pd.offsets.Hour(2)
idx = pd.period_range('2014-07', periods=5, freq='M')
idx
idx + pd.offsets.MonthEnd(3)
PeriodIndex has its own dtype named period, refer to :ref:`Period Dtypes <timeseries.period_dtype>`.
.. versionadded:: 0.19.0
PeriodIndex has a custom period dtype. This is a pandas extension
dtype similar to the :ref:`timezone aware dtype <timeseries.timezone_series>` (datetime64[ns, tz]).
The period dtype holds the freq attribute and is represented with
period[freq] like period[D] or period[M], using :ref:`frequency strings <timeseries.offset_aliases>`.
.. ipython:: python
pi = pd.period_range('2016-01-01', periods=3, freq='M')
pi
pi.dtype
The period dtype can be used in .astype(...). It allows one to change the
freq of a PeriodIndex like .asfreq() and convert a
DatetimeIndex to PeriodIndex like to_period():
.. ipython:: python
# change monthly freq to daily freq
pi.astype('period[D]')
# convert to DatetimeIndex
pi.astype('datetime64[ns]')
# convert to PeriodIndex
dti = pd.date_range('2011-01-01', freq='M', periods=3)
dti
dti.astype('period[M]')
You can pass in dates and strings to Series and DataFrame with PeriodIndex, in the same manner as DatetimeIndex. For details, refer to :ref:`DatetimeIndex Partial String Indexing <timeseries.partialindexing>`.
.. ipython:: python ps['2011-01'] ps[datetime.datetime(2011, 12, 25):] ps['10/31/2011':'12/31/2011']
Passing a string representing a lower frequency than PeriodIndex returns partial sliced data.
.. ipython:: python
ps['2011']
dfp = pd.DataFrame(np.random.randn(600, 1),
columns=['A'],
index=pd.period_range('2013-01-01 9:00',
periods=600,
freq='T'))
dfp
dfp['2013-01-01 10H']
As with DatetimeIndex, the endpoints will be included in the result. The example below slices data starting from 10:00 to 11:59.
.. ipython:: python dfp['2013-01-01 10H':'2013-01-01 11H']
The frequency of Period and PeriodIndex can be converted via the asfreq
method. Let's start with the fiscal year 2011, ending in December:
.. ipython:: python
p = pd.Period('2011', freq='A-DEC')
p
We can convert it to a monthly frequency. Using the how parameter, we can
specify whether to return the starting or ending month:
.. ipython:: python
p.asfreq('M', how='start')
p.asfreq('M', how='end')
The shorthands 's' and 'e' are provided for convenience:
.. ipython:: python
p.asfreq('M', 's')
p.asfreq('M', 'e')
Converting to a "super-period" (e.g., annual frequency is a super-period of quarterly frequency) automatically returns the super-period that includes the input period:
.. ipython:: python
p = pd.Period('2011-12', freq='M')
p.asfreq('A-NOV')
Note that since we converted to an annual frequency that ends the year in November, the monthly period of December 2011 is actually in the 2012 A-NOV period.
Period conversions with anchored frequencies are particularly useful for
working with various quarterly data common to economics, business, and other
fields. Many organizations define quarters relative to the month in which their
fiscal year starts and ends. Thus, first quarter of 2011 could start in 2010 or
a few months into 2011. Via anchored frequencies, pandas works for all quarterly
frequencies Q-JAN through Q-DEC.
Q-DEC define regular calendar quarters:
.. ipython:: python
p = pd.Period('2012Q1', freq='Q-DEC')
p.asfreq('D', 's')
p.asfreq('D', 'e')
Q-MAR defines fiscal year end in March:
.. ipython:: python
p = pd.Period('2011Q4', freq='Q-MAR')
p.asfreq('D', 's')
p.asfreq('D', 'e')
Timestamped data can be converted to PeriodIndex-ed data using to_period
and vice-versa using to_timestamp:
.. ipython:: python
rng = pd.date_range('1/1/2012', periods=5, freq='M')
ts = pd.Series(np.random.randn(len(rng)), index=rng)
ts
ps = ts.to_period()
ps
ps.to_timestamp()
Remember that 's' and 'e' can be used to return the timestamps at the start or end of the period:
.. ipython:: python
ps.to_timestamp('D', how='s')
Converting between period and timestamp enables some convenient arithmetic functions to be used. In the following example, we convert a quarterly frequency with year ending in November to 9am of the end of the month following the quarter end:
.. ipython:: python
prng = pd.period_range('1990Q1', '2000Q4', freq='Q-NOV')
ts = pd.Series(np.random.randn(len(prng)), prng)
ts.index = (prng.asfreq('M', 'e') + 1).asfreq('H', 's') + 9
ts.head()
If you have data that is outside of the Timestamp bounds, see :ref:`Timestamp limitations <timeseries.timestamp-limits>`,
then you can use a PeriodIndex and/or Series of Periods to do computations.
.. ipython:: python
span = pd.period_range('1215-01-01', '1381-01-01', freq='D')
span
To convert from an int64 based YYYYMMDD representation.
.. ipython:: python
s = pd.Series([20121231, 20141130, 99991231])
s
def conv(x):
return pd.Period(year=x // 10000, month=x // 100 % 100,
day=x % 100, freq='D')
s.apply(conv)
s.apply(conv)[2]
These can easily be converted to a PeriodIndex:
.. ipython:: python span = pd.PeriodIndex(s.apply(conv)) span
pandas provides rich support for working with timestamps in different time
zones using the pytz and dateutil libraries.
Note
pandas does not yet support datetime.timezone objects from the standard
library.
By default, pandas objects are time zone unaware:
.. ipython:: python
rng = pd.date_range('3/6/2012 00:00', periods=15, freq='D')
rng.tz is None
To localize these dates to a time zone (assign a particular time zone to a naive date),
you can use the tz_localize method or the tz keyword argument in
:func:`date_range`, :class:`Timestamp`, or :class:`DatetimeIndex`.
You can either pass pytz or dateutil time zone objects or Olson time zone database strings.
Olson time zone strings will return pytz time zone objects by default.
To return dateutil time zone objects, append dateutil/ before the string.
- In
pytzyou can find a list of common (and less common) time zones usingfrom pytz import common_timezones, all_timezones. dateutiluses the OS time zones so there isn't a fixed list available. For common zones, the names are the same aspytz.
.. ipython:: python
import dateutil
# pytz
rng_pytz = pd.date_range('3/6/2012 00:00', periods=3, freq='D',
tz='Europe/London')
rng_pytz.tz
# dateutil
rng_dateutil = pd.date_range('3/6/2012 00:00', periods=3, freq='D')
rng_dateutil = rng_dateutil.tz_localize('dateutil/Europe/London')
rng_dateutil.tz
# dateutil - utc special case
rng_utc = pd.date_range('3/6/2012 00:00', periods=3, freq='D',
tz=dateutil.tz.tzutc())
rng_utc.tz
Note that the UTC time zone is a special case in dateutil and should be constructed explicitly
as an instance of dateutil.tz.tzutc. You can also construct other time
zones objects explicitly first.
.. ipython:: python
import pytz
# pytz
tz_pytz = pytz.timezone('Europe/London')
rng_pytz = pd.date_range('3/6/2012 00:00', periods=3, freq='D')
rng_pytz = rng_pytz.tz_localize(tz_pytz)
rng_pytz.tz == tz_pytz
# dateutil
tz_dateutil = dateutil.tz.gettz('Europe/London')
rng_dateutil = pd.date_range('3/6/2012 00:00', periods=3, freq='D',
tz=tz_dateutil)
rng_dateutil.tz == tz_dateutil
To convert a time zone aware pandas object from one time zone to another,
you can use the tz_convert method.
.. ipython:: python
rng_pytz.tz_convert('US/Eastern')
Note
When using pytz time zones, :class:`DatetimeIndex` will construct a different
time zone object than a :class:`Timestamp` for the same time zone input. A :class:`DatetimeIndex`
can hold a collection of :class:`Timestamp` objects that may have different UTC offsets and cannot be
succinctly represented by one pytz time zone instance while one :class:`Timestamp`
represents one point in time with a specific UTC offset.
.. ipython:: python
dti = pd.date_range('2019-01-01', periods=3, freq='D', tz='US/Pacific')
dti.tz
ts = pd.Timestamp('2019-01-01', tz='US/Pacific')
ts.tz
Warning
Be wary of conversions between libraries. For some time zones, pytz and dateutil have different
definitions of the zone. This is more of a problem for unusual time zones than for
'standard' zones like US/Eastern.
Warning
Be aware that a time zone definition across versions of time zone libraries may not be considered equal. This may cause problems when working with stored data that is localized using one version and operated on with a different version. See :ref:`here<io.hdf5-notes>` for how to handle such a situation.
Warning
For pytz time zones, it is incorrect to pass a time zone object directly into
the datetime.datetime constructor
(e.g., datetime.datetime(2011, 1, 1, tz=pytz.timezone('US/Eastern')).
Instead, the datetime needs to be localized using the localize method
on the pytz time zone object.
Under the hood, all timestamps are stored in UTC. Values from a time zone aware :class:`DatetimeIndex` or :class:`Timestamp` will have their fields (day, hour, minute, etc.) localized to the time zone. However, timestamps with the same UTC value are still considered to be equal even if they are in different time zones:
.. ipython:: python
rng_eastern = rng_utc.tz_convert('US/Eastern')
rng_berlin = rng_utc.tz_convert('Europe/Berlin')
rng_eastern[2]
rng_berlin[2]
rng_eastern[2] == rng_berlin[2]
Operations between :class:`Series` in different time zones will yield UTC :class:`Series`, aligning the data on the UTC timestamps:
.. ipython:: python
ts_utc = pd.Series(range(3), pd.date_range('20130101', periods=3, tz='UTC'))
eastern = ts_utc.tz_convert('US/Eastern')
berlin = ts_utc.tz_convert('Europe/Berlin')
result = eastern + berlin
result
result.index
To remove time zone information, use tz_localize(None) or tz_convert(None).
tz_localize(None) will remove the time zone yielding the local time representation.
tz_convert(None) will remove the time zone after converting to UTC time.
.. ipython:: python
didx = pd.date_range(start='2014-08-01 09:00', freq='H',
periods=3, tz='US/Eastern')
didx
didx.tz_localize(None)
didx.tz_convert(None)
# tz_convert(None) is identical to tz_convert('UTC').tz_localize(None)
didx.tz_convert('UTC').tz_localize(None)
tz_localize may not be able to determine the UTC offset of a timestamp
because daylight savings time (DST) in a local time zone causes some times to occur
twice within one day ("clocks fall back"). The following options are available:
'raise': Raises apytz.AmbiguousTimeError(the default behavior)'infer': Attempt to determine the correct offset base on the monotonicity of the timestamps'NaT': Replaces ambiguous times withNaTbool:Truerepresents a DST time,Falserepresents non-DST time. An array-like ofboolvalues is supported for a sequence of times.
.. ipython:: python
rng_hourly = pd.DatetimeIndex(['11/06/2011 00:00', '11/06/2011 01:00',
'11/06/2011 01:00', '11/06/2011 02:00'])
This will fail as there are ambiguous times ('11/06/2011 01:00')
In [2]: rng_hourly.tz_localize('US/Eastern')
AmbiguousTimeError: Cannot infer dst time from Timestamp('2011-11-06 01:00:00'), try using the 'ambiguous' argument
Handle these ambiguous times by specifying the following.
.. ipython:: python
rng_hourly.tz_localize('US/Eastern', ambiguous='infer')
rng_hourly.tz_localize('US/Eastern', ambiguous='NaT')
rng_hourly.tz_localize('US/Eastern', ambiguous=[True, True, False, False])
A DST transition may also shift the local time ahead by 1 hour creating nonexistent
local times ("clocks spring forward"). The behavior of localizing a timeseries with nonexistent times
can be controlled by the nonexistent argument. The following options are available:
'raise': Raises apytz.NonExistentTimeError(the default behavior)'NaT': Replaces nonexistent times withNaT'shift_forward': Shifts nonexistent times forward to the closest real time'shift_backward': Shifts nonexistent times backward to the closest real time- timedelta object: Shifts nonexistent times by the timedelta duration
.. ipython:: python
dti = pd.date_range(start='2015-03-29 02:30:00', periods=3, freq='H')
# 2:30 is a nonexistent time
Localization of nonexistent times will raise an error by default.
In [2]: dti.tz_localize('Europe/Warsaw')
NonExistentTimeError: 2015-03-29 02:30:00
Transform nonexistent times to NaT or shift the times.
.. ipython:: python
dti
dti.tz_localize('Europe/Warsaw', nonexistent='shift_forward')
dti.tz_localize('Europe/Warsaw', nonexistent='shift_backward')
dti.tz_localize('Europe/Warsaw', nonexistent=pd.Timedelta(1, unit='H'))
dti.tz_localize('Europe/Warsaw', nonexistent='NaT')
A :class:`Series` with time zone naive values is
represented with a dtype of datetime64[ns].
.. ipython:: python
s_naive = pd.Series(pd.date_range('20130101', periods=3))
s_naive
A :class:`Series` with a time zone aware values is
represented with a dtype of datetime64[ns, tz] where tz is the time zone
.. ipython:: python
s_aware = pd.Series(pd.date_range('20130101', periods=3, tz='US/Eastern'))
s_aware
Both of these :class:`Series` time zone information
can be manipulated via the .dt accessor, see :ref:`the dt accessor section <basics.dt_accessors>`.
For example, to localize and convert a naive stamp to time zone aware.
.. ipython:: python
s_naive.dt.tz_localize('UTC').dt.tz_convert('US/Eastern')
Time zone information can also be manipulated using the astype method.
This method can localize and convert time zone naive timestamps or
convert time zone aware timestamps.
.. ipython:: python
# localize and convert a naive time zone
s_naive.astype('datetime64[ns, US/Eastern]')
# make an aware tz naive
s_aware.astype('datetime64[ns]')
# convert to a new time zone
s_aware.astype('datetime64[ns, CET]')
Note
Using :meth:`Series.to_numpy` on a Series, returns a NumPy array of the data.
NumPy does not currently support time zones (even though it is printing in the local time zone!),
therefore an object array of Timestamps is returned for time zone aware data:
.. ipython:: python s_naive.to_numpy() s_aware.to_numpy()
By converting to an object array of Timestamps, it preserves the time zone information. For example, when converting back to a Series:
.. ipython:: python pd.Series(s_aware.to_numpy())
However, if you want an actual NumPy datetime64[ns] array (with the values
converted to UTC) instead of an array of objects, you can specify the
dtype argument:
.. ipython:: python s_aware.to_numpy(dtype='datetime64[ns]')