BUG: loc.setitem raising when expanding empty frame with array value

[phofl]

• closes BUG: Cannot use .loc to set a ndarray as the value of an empty dataframe #49972 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

So expected.iloc[0] will return a list and not necessarily the np.array? i.e. the ndarray is coerced to a list and then stored?

[jbrockmendel]

i think expected.iloc[0, 0] here is a scalar, which is probably 1) what the suer in 49972 wanted and 2) technically incorrect. i'd expect 1-D ndarray containing a zero

[phofl]

Hm in principal I agree, but this is consistent now with

df = DataFrame({0: [0]})
df.loc[1, 0] = np.asarray([0])

and

df = DataFrame({0: [0]})
df.loc[0, 1] = np.asarray([0])

which are similar expanding cases. This points to the bug being somewhere else regarding scalar vs array

[jbrockmendel]

thats reasonable. maybe let's see what it would take to fix the two cases you mention here and whether those are isolate-ish sketchy cases before adding another one?

if moving forward with this, should add a comment in test/code about living with technically-wrong behavior

[phofl]

There is a general inconsistency that might be expected? Not Sure.

If you have a column with a numeric dtype

df = DataFrame({0: [0]})

then

df.loc[1, 0] = np.asarray([0])

unpacks and sets a scalar. If the column has object dtype,

df = DataFrame({0: ["a"]})

we set the array. Is this expected?

[jbrockmendel]

Is this expected?

uhh maybe? with object-dtype it is more plausible that the user is intentionally trying to set an array as a value.

[jbrockmendel]

@phofl worth talking about in today's meeting?

[github-actions]

This pull request is stale because it has been open for thirty days with no activity. Please update and respond to this comment if you're still interested in working on this.

[simonjayhawkins]

Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.