[ENH] fill_direction improve

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• use existing functions to check type and presence of columns

This PR improves #704 .

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Relevant Reviewers

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• @ericmjl

[codecov]

Codecov Report

Merging #778 (22dc841) into dev (9e96d67) will increase coverage by 0.70%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #778      +/-   ##
==========================================
+ Coverage   94.49%   95.20%   +0.70%     
==========================================
  Files          17       17              
  Lines         745      875     +130     
==========================================
+ Hits          704      833     +129     
- Misses         41       42       +1     

[samukweku]

@ericmjl @hectormz @sallyhong and others. Need your advice on the API. Currently different values for different columns cannot be applied for the limit parameter; if there are multiple columns, the same limit will be applied to all the columns. I am of the opinion that the API should be changed to allow different limit for different columns. It looks likely to break current API. Or should I just leave as is; probably users don’t care much for limit?

[ericmjl]

The only change I'm requesting, @samukweku, is about docstring triple quotes, nothing else!

[ericmjl]

🎉

[ericmjl]

This is a classic "do we apply this function to one column, or multiple columns" situation!

As the library matures, I've grown to realize that having singular and plural functions is not necessarily the best, so allowing a function to be applied to multiple columns simultaneously is probably the better way. There must be some way we can "compose" them together, I think, such that we get the desired behaviour. I'm kind of inspired by what I see in JAX, where we have vmap to map a function over an array axis. This makes composing functions together more natural.

Maybe a better solution to this problem is to provide a .map_columns() function, which allows us to map any pyjanitor function across columns. Let's talk about this in another setting. For now, I like what you've done here, @samukweku, in that you've left this API piece as a TODO to think about later on.

[ericmjl]

I approve, @samukweku! Let's get this one merged in. You've done a massive job with recent PRs, please take the honour of merging this one!