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This is a short introduction to pandas, geared mainly for new users. You can see more complex recipes in the :ref:`Cookbook<cookbook>`.
Customarily, we import as follows:
.. ipython:: python import numpy as np import pandas as pd
See the :ref:`Data Structure Intro section <dsintro>`.
Creating a :class:`Series` by passing a list of values, letting pandas create a default integer index:
.. ipython:: python s = pd.Series([1, 3, 5, np.nan, 6, 8]) s
Creating a :class:`DataFrame` by passing a NumPy array, with a datetime index and labeled columns:
.. ipython:: python
dates = pd.date_range('20130101', periods=6)
dates
df = pd.DataFrame(np.random.randn(6, 4), index=dates, columns=list('ABCD'))
df
Creating a DataFrame by passing a dict of objects that can be converted to series-like.
.. ipython:: python
df2 = pd.DataFrame({'A': 1.,
'B': pd.Timestamp('20130102'),
'C': pd.Series(1, index=list(range(4)), dtype='float32'),
'D': np.array([3] * 4, dtype='int32'),
'E': pd.Categorical(["test", "train", "test", "train"]),
'F': 'foo'})
df2
The columns of the resulting DataFrame have different
:ref:`dtypes <basics.dtypes>`.
.. ipython:: python df2.dtypes
If you're using IPython, tab completion for column names (as well as public attributes) is automatically enabled. Here's a subset of the attributes that will be completed:
.. ipython:: @verbatim In [1]: df2.<TAB> # noqa: E225, E999 df2.A df2.bool df2.abs df2.boxplot df2.add df2.C df2.add_prefix df2.clip df2.add_suffix df2.clip_lower df2.align df2.clip_upper df2.all df2.columns df2.any df2.combine df2.append df2.combine_first df2.apply df2.compound df2.applymap df2.consolidate df2.D
As you can see, the columns A, B, C, and D are automatically
tab completed. E is there as well; the rest of the attributes have been
truncated for brevity.
See the :ref:`Basics section <basics>`.
Here is how to view the top and bottom rows of the frame:
.. ipython:: python df.head() df.tail(3)
Display the index, columns:
.. ipython:: python df.index df.columns
:meth:`DataFrame.to_numpy` gives a NumPy representation of the underlying data.
Note that this can be an expensive operation when your :class:`DataFrame` has
columns with different data types, which comes down to a fundamental difference
between pandas and NumPy: NumPy arrays have one dtype for the entire array,
while pandas DataFrames have one dtype per column. When you call
:meth:`DataFrame.to_numpy`, pandas will find the NumPy dtype that can hold all
of the dtypes in the DataFrame. This may end up being object, which requires
casting every value to a Python object.
For df, our :class:`DataFrame` of all floating-point values,
:meth:`DataFrame.to_numpy` is fast and doesn't require copying data.
.. ipython:: python df.to_numpy()
For df2, the :class:`DataFrame` with multiple dtypes,
:meth:`DataFrame.to_numpy` is relatively expensive.
.. ipython:: python df2.to_numpy()
Note
:meth:`DataFrame.to_numpy` does not include the index or column labels in the output.
:func:`~DataFrame.describe` shows a quick statistic summary of your data:
.. ipython:: python df.describe()
Transposing your data:
.. ipython:: python df.T
Sorting by an axis:
.. ipython:: python df.sort_index(axis=1, ascending=False)
Sorting by values:
.. ipython:: python df.sort_values(by='B')
Note
While standard Python / Numpy expressions for selecting and setting are
intuitive and come in handy for interactive work, for production code, we
recommend the optimized pandas data access methods, .at, .iat,
.loc and .iloc.
See the indexing documentation :ref:`Indexing and Selecting Data <indexing>` and :ref:`MultiIndex / Advanced Indexing <advanced>`.
Selecting a single column, which yields a Series,
equivalent to df.A:
.. ipython:: python df['A']
Selecting via [], which slices the rows.
.. ipython:: python df[0:3] df['20130102':'20130104']
See more in :ref:`Selection by Label <indexing.label>`.
For getting a cross section using a label:
.. ipython:: python df.loc[dates[0]]
Selecting on a multi-axis by label:
.. ipython:: python df.loc[:, ['A', 'B']]
Showing label slicing, both endpoints are included:
.. ipython:: python df.loc['20130102':'20130104', ['A', 'B']]
Reduction in the dimensions of the returned object:
.. ipython:: python df.loc['20130102', ['A', 'B']]
For getting a scalar value:
.. ipython:: python df.loc[dates[0], 'A']
For getting fast access to a scalar (equivalent to the prior method):
.. ipython:: python df.at[dates[0], 'A']
See more in :ref:`Selection by Position <indexing.integer>`.
Select via the position of the passed integers:
.. ipython:: python df.iloc[3]
By integer slices, acting similar to numpy/python:
.. ipython:: python df.iloc[3:5, 0:2]
By lists of integer position locations, similar to the numpy/python style:
.. ipython:: python df.iloc[[1, 2, 4], [0, 2]]
For slicing rows explicitly:
.. ipython:: python df.iloc[1:3, :]
For slicing columns explicitly:
.. ipython:: python df.iloc[:, 1:3]
For getting a value explicitly:
.. ipython:: python df.iloc[1, 1]
For getting fast access to a scalar (equivalent to the prior method):
.. ipython:: python df.iat[1, 1]
Using a single column's values to select data.
.. ipython:: python df[df.A > 0]
Selecting values from a DataFrame where a boolean condition is met.
.. ipython:: python df[df > 0]
Using the :func:`~Series.isin` method for filtering:
.. ipython:: python df2 = df.copy() df2['E'] = ['one', 'one', 'two', 'three', 'four', 'three'] df2 df2[df2['E'].isin(['two', 'four'])]
Setting a new column automatically aligns the data by the indexes.
.. ipython:: python
s1 = pd.Series([1, 2, 3, 4, 5, 6], index=pd.date_range('20130102', periods=6))
s1
df['F'] = s1
Setting values by label:
.. ipython:: python df.at[dates[0], 'A'] = 0
Setting values by position:
.. ipython:: python df.iat[0, 1] = 0
Setting by assigning with a NumPy array:
.. ipython:: python df.loc[:, 'D'] = np.array([5] * len(df))
The result of the prior setting operations.
.. ipython:: python df
A where operation with setting.
.. ipython:: python df2 = df.copy() df2[df2 > 0] = -df2 df2
pandas primarily uses the value np.nan to represent missing data. It is by
default not included in computations. See the :ref:`Missing Data section
<missing_data>`.
Reindexing allows you to change/add/delete the index on a specified axis. This returns a copy of the data.
.. ipython:: python df1 = df.reindex(index=dates[0:4], columns=list(df.columns) + ['E']) df1.loc[dates[0]:dates[1], 'E'] = 1 df1
To drop any rows that have missing data.
.. ipython:: python df1.dropna(how='any')
Filling missing data.
.. ipython:: python df1.fillna(value=5)
To get the boolean mask where values are nan.
.. ipython:: python pd.isna(df1)
See the :ref:`Basic section on Binary Ops <basics.binop>`.
Operations in general exclude missing data.
Performing a descriptive statistic:
.. ipython:: python df.mean()
Same operation on the other axis:
.. ipython:: python df.mean(1)
Operating with objects that have different dimensionality and need alignment. In addition, pandas automatically broadcasts along the specified dimension.
.. ipython:: python s = pd.Series([1, 3, 5, np.nan, 6, 8], index=dates).shift(2) s df.sub(s, axis='index')
Applying functions to the data:
.. ipython:: python df.apply(np.cumsum) df.apply(lambda x: x.max() - x.min())
See more at :ref:`Histogramming and Discretization <basics.discretization>`.
.. ipython:: python s = pd.Series(np.random.randint(0, 7, size=10)) s s.value_counts()
Series is equipped with a set of string processing methods in the str attribute that make it easy to operate on each element of the array, as in the code snippet below. Note that pattern-matching in str generally uses regular expressions by default (and in some cases always uses them). See more at :ref:`Vectorized String Methods <text.string_methods>`.
.. ipython:: python s = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan, 'CABA', 'dog', 'cat']) s.str.lower()
pandas provides various facilities for easily combining together Series and DataFrame objects with various kinds of set logic for the indexes and relational algebra functionality in the case of join / merge-type operations.
See the :ref:`Merging section <merging>`.
Concatenating pandas objects together with :func:`concat`:
.. ipython:: python df = pd.DataFrame(np.random.randn(10, 4)) df # break it into pieces pieces = [df[:3], df[3:7], df[7:]] pd.concat(pieces)
SQL style merges. See the :ref:`Database style joining <merging.join>` section.
.. ipython:: python
left = pd.DataFrame({'key': ['foo', 'foo'], 'lval': [1, 2]})
right = pd.DataFrame({'key': ['foo', 'foo'], 'rval': [4, 5]})
left
right
pd.merge(left, right, on='key')
Another example that can be given is:
.. ipython:: python
left = pd.DataFrame({'key': ['foo', 'bar'], 'lval': [1, 2]})
right = pd.DataFrame({'key': ['foo', 'bar'], 'rval': [4, 5]})
left
right
pd.merge(left, right, on='key')
Append rows to a dataframe. See the :ref:`Appending <merging.concatenation>` section.
.. ipython:: python df = pd.DataFrame(np.random.randn(8, 4), columns=['A', 'B', 'C', 'D']) df s = df.iloc[3] df.append(s, ignore_index=True)
By "group by" we are referring to a process involving one or more of the following steps:
- Splitting the data into groups based on some criteria
- Applying a function to each group independently
- Combining the results into a data structure
See the :ref:`Grouping section <groupby>`.
.. ipython:: python
df = pd.DataFrame({'A': ['foo', 'bar', 'foo', 'bar',
'foo', 'bar', 'foo', 'foo'],
'B': ['one', 'one', 'two', 'three',
'two', 'two', 'one', 'three'],
'C': np.random.randn(8),
'D': np.random.randn(8)})
df
Grouping and then applying the :meth:`~DataFrame.sum` function to the resulting groups.
.. ipython:: python
df.groupby('A').sum()
Grouping by multiple columns forms a hierarchical index, and again we can
apply the sum function.
.. ipython:: python df.groupby(['A', 'B']).sum()
See the sections on :ref:`Hierarchical Indexing <advanced.hierarchical>` and :ref:`Reshaping <reshaping.stacking>`.
.. ipython:: python
tuples = list(zip(*[['bar', 'bar', 'baz', 'baz',
'foo', 'foo', 'qux', 'qux'],
['one', 'two', 'one', 'two',
'one', 'two', 'one', 'two']]))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
df = pd.DataFrame(np.random.randn(8, 2), index=index, columns=['A', 'B'])
df2 = df[:4]
df2
The :meth:`~DataFrame.stack` method "compresses" a level in the DataFrame's columns.
.. ipython:: python stacked = df2.stack() stacked
With a "stacked" DataFrame or Series (having a MultiIndex as the
index), the inverse operation of :meth:`~DataFrame.stack` is
:meth:`~DataFrame.unstack`, which by default unstacks the last level:
.. ipython:: python stacked.unstack() stacked.unstack(1) stacked.unstack(0)
See the section on :ref:`Pivot Tables <reshaping.pivot>`.
.. ipython:: python
df = pd.DataFrame({'A': ['one', 'one', 'two', 'three'] * 3,
'B': ['A', 'B', 'C'] * 4,
'C': ['foo', 'foo', 'foo', 'bar', 'bar', 'bar'] * 2,
'D': np.random.randn(12),
'E': np.random.randn(12)})
df
We can produce pivot tables from this data very easily:
.. ipython:: python pd.pivot_table(df, values='D', index=['A', 'B'], columns=['C'])
pandas has 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. See the :ref:`Time Series section <timeseries>`.
.. 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()
Time zone representation:
.. ipython:: python
rng = pd.date_range('3/6/2012 00:00', periods=5, freq='D')
ts = pd.Series(np.random.randn(len(rng)), rng)
ts
ts_utc = ts.tz_localize('UTC')
ts_utc
Converting to another time zone:
.. ipython:: python
ts_utc.tz_convert('US/Eastern')
Converting between time span representations:
.. 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()
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()
pandas can include categorical data in a DataFrame. For full docs, see the
:ref:`categorical introduction <categorical>` and the :ref:`API documentation <api.arrays.categorical>`.
.. ipython:: python
df = pd.DataFrame({"id": [1, 2, 3, 4, 5, 6],
"raw_grade": ['a', 'b', 'b', 'a', 'a', 'e']})
Convert the raw grades to a categorical data type.
.. ipython:: python
df["grade"] = df["raw_grade"].astype("category")
df["grade"]
Rename the categories to more meaningful names (assigning to
Series.cat.categories is inplace!).
.. ipython:: python
df["grade"].cat.categories = ["very good", "good", "very bad"]
Reorder the categories and simultaneously add the missing categories (methods under Series
.cat return a new Series by default).
.. ipython:: python
df["grade"] = df["grade"].cat.set_categories(["very bad", "bad", "medium",
"good", "very good"])
df["grade"]
Sorting is per order in the categories, not lexical order.
.. ipython:: python
df.sort_values(by="grade")
Grouping by a categorical column also shows empty categories.
.. ipython:: python
df.groupby("grade").size()
See the :ref:`Plotting <visualization>` docs.
.. ipython:: python
:suppress:
import matplotlib.pyplot as plt
plt.close('all')
.. ipython:: python
ts = pd.Series(np.random.randn(1000),
index=pd.date_range('1/1/2000', periods=1000))
ts = ts.cumsum()
@savefig series_plot_basic.png
ts.plot()
On a DataFrame, the :meth:`~DataFrame.plot` method is a convenience to plot all of the columns with labels:
.. ipython:: python
df = pd.DataFrame(np.random.randn(1000, 4), index=ts.index,
columns=['A', 'B', 'C', 'D'])
df = df.cumsum()
plt.figure()
df.plot()
@savefig frame_plot_basic.png
plt.legend(loc='best')
:ref:`Writing to a csv file. <io.store_in_csv>`
.. ipython:: python
df.to_csv('foo.csv')
:ref:`Reading from a csv file. <io.read_csv_table>`
.. ipython:: python
pd.read_csv('foo.csv')
.. ipython:: python
:suppress:
import os
os.remove('foo.csv')
Reading and writing to :ref:`HDFStores <io.hdf5>`.
Writing to a HDF5 Store.
.. ipython:: python
df.to_hdf('foo.h5', 'df')
Reading from a HDF5 Store.
.. ipython:: python
pd.read_hdf('foo.h5', 'df')
.. ipython:: python
:suppress:
os.remove('foo.h5')
Reading and writing to :ref:`MS Excel <io.excel>`.
Writing to an excel file.
.. ipython:: python
df.to_excel('foo.xlsx', sheet_name='Sheet1')
Reading from an excel file.
.. ipython:: python
pd.read_excel('foo.xlsx', 'Sheet1', index_col=None, na_values=['NA'])
.. ipython:: python
:suppress:
os.remove('foo.xlsx')
If you are attempting to perform an operation you might see an exception like:
>>> if pd.Series([False, True, False]):
... print("I was true")
Traceback
...
ValueError: The truth value of an array is ambiguous. Use a.empty, a.any() or a.all().See :ref:`Comparisons<basics.compare>` for an explanation and what to do.
See :ref:`Gotchas<gotchas>` as well.