[Numpy-discussion] Fixing the dtype of np.full's return value

[Antony Lee]

Hi all,

The docstring of np.full indicates that the result of the dtype is
`np.array(fill_value).dtype`, as long as the keyword argument `dtype`
itself is not set. This is actually not the case: the current
implementation always returns a float array when `dtype` is not set, see
e.g.

In [1]: np.full(1, 1)
Out[1]: array([ 1.])

In [2]: np.full(1, None)
Out[2]: array([ nan])

In [3]: np.full(1, None).dtype
Out[3]: dtype('float64')

In [4]: np.array(None)
Out[4]: array(None, dtype=object)

The note about return value of the dtype was actually explicitly discussed
in https://github.com/numpy/numpy/pull/2875 but the tests failed to cover
the case where the `dtype` argument is not passed.

We could either change the docstring to match the current behavior, or fix
the behavior to match what the docstring says (my preference). @njsmith
mentioned in https://github.com/numpy/numpy/issues/6366 that this may be
acceptable as a bug fix, as "it's a very new function so there probably
aren't many people relying on it" (it was introduced in 1.8).

I guess the options are:
- Fix the behavior outright and squeeze this in 1.10 as a bugfix (my
preference).
- Emit a warning in 1.10, fix in 1.11.
- Do nothing for 1.10, warn in 1.11, fix in 1.12 (at that point the
argument of `np.full` being a very new function starts becoming invalid...).
- Change the docstring.

Thoughts?

Antony

[Nathaniel Smith]

Post by Antony Lee
Hi all,
The docstring of np.full indicates that the result of the dtype is

`np.array(fill_value).dtype`, as long as the keyword argument `dtype`
itself is not set. This is actually not the case: the current
implementation always returns a float array when `dtype` is not set, see
e.g.

Post by Antony Lee
In [1]: np.full(1, 1)
Out[1]: array([ 1.])
In [2]: np.full(1, None)
Out[2]: array([ nan])
In [3]: np.full(1, None).dtype
Out[3]: dtype('float64')
In [4]: np.array(None)
Out[4]: array(None, dtype=object)
The note about return value of the dtype was actually explicitly

discussed in https://github.com/numpy/numpy/pull/2875 but the tests failed
to cover the case where the `dtype` argument is not passed.

Post by Antony Lee
We could either change the docstring to match the current behavior, or

fix the behavior to match what the docstring says (my preference).
@njsmith mentioned in https://github.com/numpy/numpy/issues/6366 that this
may be acceptable as a bug fix, as "it's a very new function so there
probably aren't many people relying on it" (it was introduced in 1.8).

Post by Antony Lee
- Fix the behavior outright and squeeze this in 1.10 as a bugfix (my

preference).

Post by Antony Lee
- Emit a warning in 1.10, fix in 1.11.
- Do nothing for 1.10, warn in 1.11, fix in 1.12 (at that point the

argument of `np.full` being a very new function starts becoming invalid...).

Post by Antony Lee
- Change the docstring.

If this had been caught last month, I'd be in favor of making the change in
the 1.10 prereleases and seeing if it caused any problems. At this point I
think it's too risky for the 1.10 cycle, because we're very near the final
release so there's little chance to notice if this does cause problems.

Given that, my vote is for a FutureWarning in 1.10 and change it in 1.11
(assuming we don't get a chorus of complaints before then).

-n