TST: Add tests for dtype changes when assigning values via indexing

[phofl]

#37680 will fix the assigning problems reported in #26395.

We should add further tests for iloc/iat and different dtypes for all cases (loc, at, iloc, iat)

cc @jreback

[allenmac347]

take

[Rprabudeva]

I would like to contribute this one

[phofl]

Ping @allenmac347 are you still working on this?

[allenmac347]

Hi, yes I am still working on making tests. I have been busy with classes lately but with Thanksgiving break coming up I should be more free to work on this issue. My apologies for the delay.

[allenmac347]

@phofl hello, so after testing loc, iloc, at, and iat, it seems like at and iat does not seem to be able to upcast at all for different dtypes. This code for example throws an error:
// column D initially has an int64 dtype
df = pd.DataFrame(index=['A','B','C'])
df['D'] = 0
df.at['B', 'D'] = "hello"

ValueError: invalid literal for int() with base 10: 'hello'

This is the same case for iat. Both loc and iloc seem to work fine and promotes column D to an object dtype. It seems like both iat and at are forcing the assigned value to cast to the dtype of the column, hence why assigning a float to int via at/iat does not change the dtype of the column as found in the bug+fix you mentioned. Please let me know if there are further specific tests you wanted me to try before I submit a pull request.

[phofl]

cc @jbrockmendel is this done on purpose?