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 notifications@github.com
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