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Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.tseries.offsets.MonthBegin.html?highlight=monthbegin#pandas.tseries.offsets.MonthBegin

Documentation problem

The docs for MonthEnd say

To get the end of the current month pass the parameter n equals 0.

This is correct:

In [51]: pd.Timestamp('2023-03-21') + pd.offsets.MonthEnd(n=0)
Out[51]: Timestamp('2023-03-31 00:00:00')

But for `MonthBegin, it says

To get the start of the current month pass the parameter n equals 0.

I think this is not quite right:

In [58]: pd.Timestamp('2023-03-21') + pd.offsets.MonthBegin(n=0)
Out[58]: Timestamp('2023-04-01 00:00:00')

Suggested fix for documentation

I would have expected - pd.offsets.MonthBegin(n=0) to go to the beginning of the current month, but it doesn't:

In [64]: pd.Timestamp('2023-03-21') - pd.offsets.MonthBegin(n=0)
Out[64]: Timestamp('2023-04-01 00:00:00')

I'm not sure if this is a bug, I've not looked closely enough.

I think I'd suggest: - removing the "To get the start of the current month pass the parameter n equals 0." suggestion - adding an example of using + pd.offsets.MonthBegin() - pd.offsets.MonthBegin() to get the start of the current month

cc @natmokval in case you wanted to make a PR (no blame of course! I should have noticed this when reviewing)

Comment From: MarcoGorelli

Same story in https://github.com/pandas-dev/pandas/issues/52105

I'll take a look this week about whether - MonthBegin(n=0) doesn't go to the beginning of the current month is a bug or not, that'll inform which change needs making to the docs

Comment From: MarcoGorelli

Right, the docs for DateOffset say

Zero presents a problem. Should it roll forward or back? We arbitrarily have it rollforward: date + BDay(0) == BDay.rollforward(date) Since 0 is a bit weird, we suggest avoiding its use.

So, we probably shouldn't recommend it in the docs.

How about we change the docs to say

To get the start of the current month use .rollback.

and have an example like

In [26]: pd.offsets.MonthBegin().rollback(pd.Timestamp('2023-03-21'))
Out[26]: Timestamp('2023-03-01 00:00:00')

as well as an example like

In [37]: pd.to_datetime(["2017-09-18", "2018-01-01"]).to_period('M').to_timestamp()
Out[37]: DatetimeIndex(['2017-09-01', '2018-01-01'], dtype='datetime64[ns]', freq=None)

to show how to get back to the beginning of the month if one has an array?

Comment From: pranav-ravuri

Hi, can I take up as my first issue to update the docs. Thankyou

Comment From: MarcoGorelli

go ahead, thanks!

Comment From: pranav-ravuri

take

Comment From: pranav-ravuri

Hi, @MarcoGorelli quick question, I have been reading the pandas contributor docs but I am unable to find a guideline to name my local (forked) branch to add this change, could you please help me with that. Thanks

Comment From: MarcoGorelli

hey - it doesn't really matter what you name your branch, up to you

Comment From: pranav-ravuri

Hi @MarcoGorelli , I updated the MonthBegin docstring to

    """
    DateOffset of one month at beginning.

    MonthBegin goes to the next date which is a start of the month.

    See Also
    --------
    :class:`~pandas.tseries.offsets.DateOffset` : Standard kind of date increment.

    Examples
    --------
    >>> ts = pd.Timestamp(2022, 11, 30)
    >>> ts + pd.offsets.MonthBegin()
    Timestamp('2022-12-01 00:00:00')

    >>> ts = pd.Timestamp(2022, 12, 1)
    >>> ts + pd.offsets.MonthBegin()
    Timestamp('2023-01-01 00:00:00')

    If you want to get the start of the current month:

    >>> ts = pd.Timestamp(2022, 12, 1)
    >>> pd.offsets.MonthBegin().rollback(ts)
    Timestamp('2022-12-01 00:00:00')
    """
    _prefix = "MS"
    _day_opt = "start"

and MonthEnd docstring to 

    """
    DateOffset of one month end.

    MonthEnd goes to the next date which is an end of the month.

    See Also
    --------
    :class:`~pandas.tseries.offsets.DateOffset` : Standard kind of date increment.

    Examples
    --------
    >>> ts = pd.Timestamp(2022, 1, 30)
    >>> ts + pd.offsets.MonthEnd()
    Timestamp('2022-01-31 00:00:00')

    >>> ts = pd.Timestamp(2022, 1, 31)
    >>> ts + pd.offsets.MonthEnd()
    Timestamp('2022-02-28 00:00:00')

    If you want to get the end of the current month pass the parameter n equals 0:

    >>> ts = pd.Timestamp(2022, 1, 31)
    >>> pd.offsets.MonthEnd().rollforward(ts)
    Timestamp('2022-01-31 00:00:00')
    """
    _period_dtype_code = PeriodDtypeCode.M
    _prefix = "M"
    _day_opt = "end"

So, is this okay and are there any more docstrings I need to update. On a side note out of curiosity what is 

_period_dtype_code = PeriodDtypeCode.M
    _prefix = "M"
    _day_opt = "end"

at the end of docstring?

Comment From: MarcoGorelli

If you want to get the end of the current month pass the parameter n equals 0:

This needs updating, but the rest looks good!

Those attributes are used internally in some other places (but there is an open PR to change 'M' to 'ME', as 'M' meaning "month end" is very confusing)

Comment From: pranav-ravuri

If you want to get the end of the current month pass the parameter n equals 0:

This needs updating, but the rest looks good!

Those attributes are used internally in some other places (but there is an open PR to change 'M' to 'ME', as 'M' meaning "month end" is very confusing)

so, are they like aliases for some sorts? could you please direct me to the concept being used here internally. On the other note, I have updated MonthBegin and MonthEnd doc string, but as a sanity, I checked whether, BMonthBegin and BMonthEnd had the same issue at it has the same issue

``` Python console

pd.Timestamp('2023-03-21') + pd.offsets.BMonthEnd(n=0) Timestamp('2023-03-31 00:00:00') # okay pd.Timestamp('2023-03-21') + pd.offsets.BMonthBegin(n=0) Timestamp('2023-04-03 00:00:00') # not okay ``` sould I update these docs also?

Comment From: MarcoGorelli

sould I update these docs also?

yes please! as well as any others which you find

it would be good to get this in in time for the 2.0 release, so please do open a pull request even if it's not 100% finished, I can finish it up if necessary