[Python-ideas] fnmatch.filter_false

Steven D'Aprano steve at pearwood.info
Wed May 17 21:00:40 EDT 2017

On Wed, May 17, 2017 at 12:14:05PM -0400, Alex Walters wrote:
> Fnmath.filter works great.  To remind people what it does, it takes an
> iterable of strings and a pattern and returns a list of the strings that
> match the pattern.  And that is wonderful
> However, I often need to filter *out* the items that match the pattern (to
> ignore them).  In every project that I need this I end up copying the
> function out of the fnmatch library and adding 'not' to the test clause.

At the cost of a slight inefficiency, you could use the pure Python 
equivalent given in the docs:


fnmatch.filter(names, pattern)

    Return the subset of the list of names that match pattern. It is 
    the same as [n for n in names if fnmatch(n, pattern)], but implemented
    more efficiently.

So your filter_false is:

    [n for n in names if not fnmatch(n, pattern)]

which avoids the need for the copy-and-paste anti-pattern.

Otherwise, I would support:

- filter_false

- or a glob symbol to reverse the sense of the test, e.g. ~ or ! 
  as the first character;

but I dislike functions that take boolean arguments to change their 
behaviour. It's not a 100% hard and fast rule, but in general I prefer 
to avoid functions that take a constant bool argument:

# rather than this:
function(important_args, True)
function(important_args, False)

# use this:

(although there may be exceptions). 


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