ENH: Add dict support for read_csv date_format

[phofl]

• closes ENH: let date_format take a mapping of columns to formats #51240 (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]

Thanks @phofl

[phofl]

thx @MarcoGorelli for helping out

[mroeschke]

So it appears the tests added here can raise a UserWarning

============================================================================= warnings summary =============================================================================
pandas/tests/io/parser/test_parse_dates.py::test_parse_dates_dict_format_two_columns[c_high-foo-parse_dates1]
pandas/tests/io/parser/test_parse_dates.py::test_parse_dates_dict_format_two_columns[c_low-foo-parse_dates1]
pandas/tests/io/parser/test_parse_dates.py::test_parse_dates_dict_format_two_columns[python-foo-parse_dates1]
  /.../opt/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/_pytest/unraisableexception.py:78: PytestUnraisableExceptionWarning: Exception ignored in: 'pandas._libs.tslibs.parsing._maybe_warn_about_dayfirst'

  Traceback (most recent call last):
    File ".../pandas/core/tools/datetimes.py", line 137, in _guess_datetime_format_for_array
      guessed_format = guess_datetime_format(
  UserWarning: Parsing dates in %d- %m-%Y format when dayfirst=False (the default) was specified. Pass `dayfirst=True` or specify a format to silence this warning.

    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

I would expect no warning since an explicit format is passed?

[MarcoGorelli]

That's a good question - so, parse_dates is {'foo': [0, 1]}, and date_format specifies column 'foo', but in

pandas/pandas/io/parsers/base_parser.py

Lines 1117 to 1119 in 7598f71

	date_fmt = (
	date_format.get(col) if isinstance(date_format, dict) else date_format
	)

, col is 'a_b', and so it doesn't find the date format in the dict and so date_fmt is None, so the format has to be guessed. It's reassuring that the correct format was guessed 😄

Thanks for spotting this

@phofl should the user have passed date_format={'a_b': '%d- %m-%Y'}, or does this need fixing in the code?

[phofl]

code I'd say, good catch btw.

[phofl]

#51479