ENH: Print Option: Always show an array's dtype

Proposed new feature or change:

Motive

The default dtype of array is platform-dependant. ( #9464 )

When running tests in a continuous integration context, that are ran on multiple platforms (Windows, macOS, Linux), the fact that the default dtypes of arrays can vary must be taken into account.

The issue appears for tests relying on numpy's arrays representations. Indeed, the default dtype of the array is not displayed in the array representation. This means that an expected output representation is now dependant on the platform. Writing OS-specific tests is now unavoidable.

What I would like is being able to write platform independent repeatable outputs that can be used for automated testing.

Example

Actual

On my machine, the default dtype for integer arrays is int64. Here are some examples of array creations and their representations:

In [3]: import numpy as np

In [4]: np.array([1, 2, 3])
Out[4]: array([1, 2, 3])

In [5]: np.array([1, 2, 3], dtype=np.int64)
Out[5]: array([1, 2, 3])

In [6]: np.array([1, 2, 3], dtype=np.int32)
Out[6]: array([1, 2, 3], dtype=int32)

We can see that:

• When creating an array with no dtype kwarg, the default dtype is used. The array representation solely is not enough to know the actual dtype.
• When creating an array with a dtype kwarg matching the default integer dtype of the platform, the resulting array representation is the same, and dtype is also implicit.
• The last case is the most explicit: the user provides the expected dtype, and the representation reflects that. This only works for non-default dtypes.

Desired

In [3]: import numpy as np

In [4]: np.array([1, 2, 3])
Out[4]: array([1, 2, 3], dtype=int64)

In [5]: np.array([1, 2, 3], dtype=np.int64)
Out[5]: array([1, 2, 3], dtype=int64)

In [6]: np.array([1, 2, 3], dtype=np.int32)
Out[6]: array([1, 2, 3], dtype=int32)

The dtype is always printed out, and the default dtype does not influence the representation. So, since the default dtype depends on the platform, and the representation depends on the dtype, the chain is broken and the representation does not depend anymore on the platform. Writing platform independant tests relying on representation is now easier.

from
platform <- default dtype <- repr => platform <- repr
to
platform <- default dtype </- repr => platform </- repr

Existing solutions I looked for

np.set_printoptions

I first looked into https://numpy.org/doc/stable/reference/generated/numpy.set_printoptions.html
I experimented with kwarg legacy='1.13' and legacy='1.21, without success. Also, even if I were successful, I would have dislike relying on a kwarg named legacy, strongly implying it should not be used anymore in new code.

Proposed solution

Adding a new dtype printing option

import numpy


np.set_printoptions(dtype="default") # current behaviour
np.set_printoptions(dtype="always") # always print dtype
np.set_printoptions(dtype="never") # never print dtype

Technical Analysis

The function _array_repr_implementation implements the array representation logic. We can see the logic where it adds the suffix, and there is no way to force print the dtype, or force not printing it.

Allow to override this param could be helpful:

def _array_repr_implementation(
        arr, max_line_width=None, precision=None, suppress_small=None,
+       skipdtype: bool | None = None,       
        array2string=array2string):
        ...
- skipdtype = dtype_is_implied(arr.dtype) and arr.size > 0
+ if skipdtype is None:
+     skipdtype = dtype_is_implied(arr.dtype) and arr.size > 0

Role of the proposed new skipdtype three-valued kwarg:

• None: current behaviour, platform-dependant
• False: always print the , dtype=... suffix
• True: never print the , dtype=... suffix

Additional links

• https://github.com/pydata/xarray/pull/8702

Start in numpy/core/arrayprint.py at _array_repr_implementation and inspect how array print options reach the dtype suffix logic. Compare the proposed dtype modes—default, always, and never—with the current behavior of np.set_printoptions. Done means the option is exposed and array representations consistently follow the selected dtype-printing behavior across the examples described.