WIP/REF: BlockManager.setitem_blockwise

[jbrockmendel]

Posting largely for discussion with @phofl.

Motivations:

1. Try to end up with fewer code paths for setitem so that we can ensure consistent inplace/view/copy behavior (xref API: setitem copy/view behavior ndarray vs Categorical vs other EA #38896)
2. Simplify (ATM this does not simplify things)
3. Perf: iterating over blocks instead of columns may save some copies (though doing the Index.intersection calls may go the other way) (havent done any profiling yet)

Non-trivial overlap with #39044.
Some of this would be simplified by #39163.
No logical overlap with #39266, but will require rebase.

[pep8speaks]

Hello @jbrockmendel! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file pandas/core/arrays/datetimelike.py:

Line 619:89: E501 line too long (91 > 88 characters)

• In the file pandas/core/frame.py:

Line 3222:89: E501 line too long (104 > 88 characters)

[phofl]

Is there a specific place where I can be of help?

[jbrockmendel]

Mostly just want to keep you in the loop on what i have in mind since you're working on similar parts of the code.

[jbrockmendel]

@phofl this fails the test added in #39280. Thoughts on an appropriate fix?

(a branch that only has the edits to DataFrame._setitem_array has the same failure)

[phofl]

Not per se a fix, but I had similar difficulties in #39341

We have to account for duplicate column names in these checks. Maybe using get_indexer_for or something like that if there is a faster alternative. Simply comparing the lenght does not work with duplicates

[phofl]

You could use

if self.columns.is_unique:
    len_key = len(key)
else:
    len_key = len(self.columns.get_indexer_non_unique(key))
if len(value) != len_key:
    raise ValueError("Columns must be same length as key")

as a helper functions for both len(value) != len(key) checks. Test will keep failing because the column is now cast to integer, but this is good since this is consistent with other setitem cases. If you like I could implement this in #39341. I am dispatching to the else block there right now, since this does the trick currently.

[jbrockmendel]

mothballing