(Updated from previous PR description.)

This PR deprecates the generic unit in np.timedelta64 and np.datetime64. Using this unit can lead to unexpected behavior in some cases (see #28287 for details). Using generic unit now raises a FutureWarning.

• Allowed behavior (no warning):

import numpy as np

np.timedelta64(10, 's')  # seconds
np.timedelta64("NaT") # NaT is allowed with generic

• Deprecated behavior (raises FutureWarning):

import numpy as np

np.timedelta64(10)  # generic unit
np.timedelta64(10, 's') + 5 # adding integer to timedelta with generic unit
np.datetime64('2020-01-01') + 2

Changes

Main

• Raise FutureWarning when np.timedelta64 or np.datetime64 is constructed with the generic unit.
• Add tests to ensure the warning is raised in relevant scenarios.
• Add Release notes

Test

Test updates fall into three categories.

• Suppressing the warning in tests where the generic unit seems to be used intentionally. (@pytest.mark.filterwarning("ignore::FutureWarning")

• Adding tests to check generic unit's behavior. Some tests uses generic timedelta in pytest.mark.parametrize. In such cases, the warnings cannot be suppressed with pytest.mark.filterwarnings. Instead, this PR add additional tests to check only generic unit's behavior.

• Modifying tests to use an explicit unit instead of generic.

Future Work

These works are not included in this PR and I am planning to do them in future PRs. But, if you think they should be included in this PR, please let me know. I am happy to do so.

• NaT is allowed with generic unit. We may want to disallow this in future.

  • This may requires to seek consensus.

• np.timedelta64() creates a generic timedelta with value 0. We may want to change this to create a timedelta with an explicit unit (e.g., np.timedelta64(0, 's')) instead.

  • This may requires to seek consensus.

• np.ones_like now raises FutureWarning when the input array is of timedelta type.

  • It seems that we should set 1 value with specific unit.

• np.all_close now raises FutureWarning when the input arrays are of datetime or timedelta type.

  • atol is not allowed to be timedelta64 because it is supposed to be printed with g format. ("{atol:g}" in the code.)