DOC: MonthBegin suggestion to get start of current month not quite right?

[MarcoGorelli]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

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)

[MarcoGorelli]

Same story in #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

[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?

[pranav-ravuri]

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

[MarcoGorelli]

go ahead, thanks!

[pranav-ravuri]

take

[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

[MarcoGorelli]

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

[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?

[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)