API: Treat 1D vectors as columns in some cases for arith/cmp ops
Definitely needs discussion.

There are a bunch of GH issues (will track down the list in a bit) that boil down to users expecting DataFrame+(Series|ndarray|listlike) ops to operate column-by-column.  This PR implements that behavior for a subset of cases.  In particular:

- `other` is a `Series`: indexes match exactly (and columns do not)
- `other` is a list/tuple/np.ndarray: length matches df.index (and not df.columns)

This means that things like `df == df['A']` will now work.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jbrockmendel! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/jbrockmendel/pandas/blob/9100010585fce40a97d823c7b506e064260ebdb0/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/core/ops.py`](https://github.com/jbrockmendel/pandas/blob/9100010585fce40a97d823c7b506e064260ebdb0/pandas/core/ops.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/conftest.py`](https://github.com/jbrockmendel/pandas/blob/9100010585fce40a97d823c7b506e064260ebdb0/pandas/tests/arithmetic/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_datetime64.py`](https://github.com/jbrockmendel/pandas/blob/9100010585fce40a97d823c7b506e064260ebdb0/pandas/tests/arithmetic/test_datetime64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_period.py`](https://github.com/jbrockmendel/pandas/blob/9100010585fce40a97d823c7b506e064260ebdb0/pandas/tests/arithmetic/test_period.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_timedelta64.py`](https://github.com/jbrockmendel/pandas/blob/9100010585fce40a97d823c7b506e064260ebdb0/pandas/tests/arithmetic/test_timedelta64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_operators.py`](https://github.com/jbrockmendel/pandas/blob/9100010585fce40a97d823c7b506e064260ebdb0/pandas/tests/frame/test_operators.py) !



##### Comment last updated on October 24, 2018 at 02:56 Hours UTC this is really confusing

we have .eq for this exact reason

pls justify why this should change > pls justify why this should change

I'm not 100% sure that it should.  But it is only now feasible following #23132 and it does come up often enough to merit discussion.  I'll try to fill out the list of Issues over the next couple days.
 Can you first open an issue for this to propose a specific behaviour and discuss it?