ENH: return .dt.weekday/isoweekday/month_name/day_name as ordered categoricals

[jreback]

#12803 added .dt.weekday_name. I think its appropriate to return this (and .weekday) as ordered categoricals

In [1]: s = Series(pd.date_range('20130101',periods=10))

In [2]: s.dt.weekday
Out[2]: 
0    1
1    2
2    3
3    4
4    5
5    6
6    0
7    1
8    2
9    3
dtype: int64

In [3]: s.dt.weekday_name
Out[3]: 
0      Tuesday
1    Wednesday
2     Thursday
3       Friday
4     Saturday
5       Sunday
6       Monday
7      Tuesday
8    Wednesday
9     Thursday
dtype: object

[jreback]

xfref #12806

cc @BastiaanBergman

I realized as merging #12803 that we didn't actually have to do this in cython and instead is a trivial map operation.

In [7]: s.dt.weekday.map(dict(enumerate(['Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sunday'])))
Out[7]: 
0      Tuesday
1    Wednesday
2     Thursday
3       Friday
4     Saturday
5       Sunday
6       Monday
7      Tuesday
8    Wednesday
9     Thursday
dtype: object

[jreback]

And if you categorize its even easier (and way more efficient)

In [18]: cats
Out[18]: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

In [19]: s.dt.weekday.astype('category',ordered=True).cat.rename_categories(cats)
Out[19]: 
0      Tuesday
1    Wednesday
2     Thursday
3       Friday
4     Saturday
5       Sunday
6       Monday
7      Tuesday
8    Wednesday
9     Thursday
dtype: category
Categories (7, object): [Monday < Tuesday < Wednesday < Thursday < Friday < Saturday < Sunday]

[BastiaanBergman]

I don't know what the speed implications are for big dataframes. In any
case, implementing alongside the existing Cython code wasn't exactly
un-trivial.

On Tue, Apr 26, 2016 at 6:52 AM, Jeff Reback [email protected]
wrote:

And if you categorize its even easier.

In [18]: cats
Out[18]: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

In [19]: s.dt.weekday.astype('category',ordered=True).cat.rename_categories(cats)
Out[19]:
0 Tuesday
1 Wednesday
2 Thursday
3 Friday
4 Saturday
5 Sunday
6 Monday
7 Tuesday
8 Wednesday
9 Thursday
dtype: category
Categories (7, object): [Monday < Tuesday < Wednesday < Thursday < Friday < Saturday < Sunday]

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#12993 (comment)

[jreback]

@BastiaanBergman no, what I mean is that THIS impl is trivial. Of course the cython is not :<

[kawochen]

I would think they shouldn't be ordered (because it's cyclic). An order would probably only enable .max(), and .min(), right?

[jreback]

well also allows comparisons, e.g.

In [4]: os = s.dt.weekday.astype('category',ordered=True).cat.rename_categories(cats)

In [5]: os
Out[5]: 
0      Tuesday
1    Wednesday
2     Thursday
3       Friday
4     Saturday
5       Sunday
6       Monday
7      Tuesday
8    Wednesday
9     Thursday
dtype: category
Categories (7, object): [Monday < Tuesday < Wednesday < Thursday < Friday < Saturday < Sunday]

In [9]: os
Out[9]: 
0      Tuesday
1    Wednesday
2     Thursday
3       Friday
4     Saturday
5       Sunday
6       Monday
7      Tuesday
8    Wednesday
9     Thursday
dtype: category
Categories (7, object): [Monday < Tuesday < Wednesday < Thursday < Friday < Saturday < Sunday]

In [10]: os.min()
Out[10]: 'Monday'

In [11]: os<'Wednesday'
Out[11]: 
0     True
1    False
2    False
3    False
4    False
5    False
6     True
7     True
8    False
9    False
dtype: bool

[sivakar12]

I'd like to give this a try. Can I work on this?

[mroeschke]

Go for it @sivakar12! Some of the files you may want to edit are in this recent PR https://github.com/pandas-dev/pandas/pull/18164/files

[sivakar12]

I found that categorical is not defined in the Cython code. So I focused on the DatetimeIndex class, tried calling as_type, returning a CategoricalIndex from the _field_accessor method there. They are not working and I always end up getting dtype: object. What am I missing?

[mroeschke]

After the index is created, you can either use the map function or astype with predefined categories as described in these comments: #12993 (comment) or #12993 (comment)

[sivakar12]

I made DatetimeIndex class return a CategoricalIndex when weekday_name property is accessed. But the output of s.dt.weekday_name returns a DatetimeProperties object which seems to convert it back to object type.
The code in the comments apply map or astype on an instance of DatetimeProperties not on DatetimeIndex which works fine.
I can't figure out what's going on inside DatetimeProperties

[mroeschke]

Feel free to open a pull request (you can mark it as a work in progress) with your initial changes. It will be easier for us to review and help debug the issue.

[jbrockmendel]

Not wild about making DatetimeArray have a dependency on Categorical (which in turn has dependency on Index)

[jreback]

this would be an indirect dependency and is for user convenience