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DOC: Document datetime and timedelta to python's object type conversion #29557

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DOC: document datetime/timedelta to PyObject conversion
devmt04 Aug 13, 2025
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Merge branch 'numpy:main' into docs-datetime-pyobject-conversion
devmt04 Aug 13, 2025
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DOC: document datetime/timedelta to PyObject conversion
devmt04 Aug 13, 2025
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DOC: document datetime/timedelta to PyObject conversion
devmt04 Aug 13, 2025
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Merge branch 'numpy:main' into docs-datetime-pyobject-conversion
devmt04 Aug 21, 2025
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20 changes: 20 additions & 0 deletions doc/source/reference/arrays.datetime.rst
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Expand Up @@ -309,6 +309,26 @@ us / μs microsecond +/- 2.9e5 years [290301 BC, 294241 AD]
as attosecond +/- 9.2 seconds [ 1969 AD, 1970 AD]
======== ================ ======================= ==========================


Converting datetime and timedelta to Python Object
==================================================

NumPy follows a strict protocol when converting datetime and timedelta to Python Objects (e.g., tuple, list, datetime.datetime).

For conversion of datetime to a Python Object:

- Not-a-time is returned as None.
- For days or coarser, returns a datetime.date.
- For microseconds or coarser, returns a datetime.datetime.
- For units finer than microseconds, returns an integer.

For conversion of timedelta to Python Object

- Not-a-time is returned as None.
- For microseconds or coarser, returns a datetime.timedelta.
- For Y/M/B (nonlinear units), generic units and units finer than microseconds, returns an integer.
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It might be easier to read and digest this as a table. I'd also like to see some code examples to explicitly show what happens. The nice part about code examples is they get run as part of our test suite.

Instead of "Not-a-time" I think we usually refer to it as NaT.

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@ngoldbaum ngoldbaum Aug 20, 2025 •
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I think also adding some text this page explaining what NaT is would be helpful. We use it in a few places above, but we never explicitly explain it's the datetime equivalent of NaN and that NaT is short for not-a-time.

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but we never explicitly explain it's the datetime equivalent of NaN and that NaT is short for not-a-time.

I think we should explain NaT explicitly under title "Datetime units", as we already had given an overview of it under "Basic datetimes".

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Also I have noticed a unknown non-linear timedelta type "B" mentioned in C code comments, is that a thing in NumPy? As I found no reference of it in docs.

For conversion of timedelta to Python Object

- Not-a-time is returned as None.
- For microseconds or coarser, returns a datetime.timedelta.
- For Y/M/B (nonlinear units), generic units and units finer than microseconds, returns an integer.

All I am aware of is that "B" is one of the Period aliases in Pandas. Is this a mistake?



Business day functionality
==========================

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5 changes: 3 additions & 2 deletions numpy/_core/src/multiarray/_datetime.h
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Expand Up @@ -242,6 +242,7 @@ convert_pyobject_to_timedelta(PyArray_DatetimeMetaData *meta, PyObject *obj,
/*
* Converts a datetime into a PyObject *.
*
* Not-a-time is returned as None.
* For days or coarser, returns a datetime.date.
* For microseconds or coarser, returns a datetime.datetime.
* For units finer than microseconds, returns an integer.
Expand All @@ -252,9 +253,9 @@ convert_datetime_to_pyobject(npy_datetime dt, PyArray_DatetimeMetaData *meta);
/*
* Converts a timedelta into a PyObject *.
*
* Not-a-time is returned as the string "NaT".
* Not-a-time is returned as None.
* For microseconds or coarser, returns a datetime.timedelta.
* For units finer than microseconds, returns an integer.
* For Y/M/B (nonlinear units), generic units and units finer than microseconds, returns an integer.
*/
NPY_NO_EXPORT PyObject *
convert_timedelta_to_pyobject(npy_timedelta td, PyArray_DatetimeMetaData *meta);
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6 changes: 3 additions & 3 deletions numpy/_core/src/multiarray/datetime.c
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Expand Up @@ -2761,7 +2761,7 @@ convert_pyobject_to_timedelta(PyArray_DatetimeMetaData *meta, PyObject *obj,
/*
* Converts a datetime into a PyObject *.
*
* Not-a-time is returned as the string "NaT".
* Not-a-time is returned as None.
* For days or coarser, returns a datetime.date.
* For microseconds or coarser, returns a datetime.datetime.
* For units finer than microseconds, returns an integer.
Expand Down Expand Up @@ -2945,9 +2945,9 @@ convert_timedelta_to_timedeltastruct(PyArray_DatetimeMetaData *meta,
/*
* Converts a timedelta into a PyObject *.
*
* Not-a-time is returned as the string "NaT".
* Not-a-time is returned as None.
* For microseconds or coarser, returns a datetime.timedelta.
* For units finer than microseconds, returns an integer.
* For Y/M/B (nonlinear units), generic units and units finer than microseconds, returns an integer.
*/
NPY_NO_EXPORT PyObject *
convert_timedelta_to_pyobject(npy_timedelta td, PyArray_DatetimeMetaData *meta)
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