Improvements to str_cat

[hack-c]

closes #11435
closes #11334.

Modified behavior of str_cat to ignore NaN values in the Series by default.

Added some doc examples to make the behavior with nan more clear, and catch an error condition when a sep is passed instead of an others (array) as an argument and print a more intuitive error message.

This is a retry of PR #12000.

Before, if NaN values were present in the Series, calling cat() on it would return NaN, unlike the rest of the Series.str. API. This behavior has been amended to ignore NaN values by default.
(#11435)

A new, friendlier ValueError was also added to protect against the mistake of supplying the sep as an arg, rather than a kwarg.
(#11334)

    >>> Series(['a','b',np.nan,'c']).str.cat(sep=' ')
    'a b c'

    >>> Series(['a','b',np.nan,'c']).str.cat(sep=' ', na_rep='?')
    'a b ? c'

    >>> Series(['a','b',np.nan,'c']).str.cat(' ')
    ---------------------------------------------------------------------------
    ValueError                                Traceback (most recent call last)
    <ipython-input-3-338c379049e9> in <module>()
    ----> 1 Series(['a','b',np.nan,'c']).str.cat(' ')

    [...]

    ValueError: Did you mean to supply a `sep` keyword?

[jreback]

make the first 2 an ipython block so they run, the 3rd can be a code-block (just so you can show the error and not the entire traceback)

[jreback]

looks good.

some small comments and you have linting errors:

linting -> pandas/core
pandas/core/strings.py:4:80: E501 line too long (84 > 79 characters)
linting -> pandas/io
linting -> pandas/stats
linting -> pandas/compat
linting -> pandas/sparse
linting -> pandas/tools
linting -> pandas/tseries
linting -> pandas/tests
pandas/tests/test_strings.py:2463:24: E231 missing whitespace after ','
pandas/tests/test_strings.py:2463:28: E231 missing whitespace after ','
pandas/tests/test_strings.py:2463:32: E231 missing whitespace after ','
pandas/tests/test_strings.py:2471:5: E303 too many blank lines (2)
linting -> pandas/computation
linting -> pandas/util

[jreback]

ping when green.

[jreback]

@hack-c thanks, I make the flake fixes.

[hack-c]

Hey @jreback so is this all done?

[jreback]

yep all done. thanks!

just trying to get the 0.18.0RC1 out the door :)

[hack-c]

Ok, great @jreback!

How can I help? I want to do more :)

[jreback]

@hack-c great!

just troll issues, you can select the difficulty labels as you want!