Filtering a String for a Set of Characters

Question

this is from the 'python cookbook' but isn't explained that well.

allchars = string.maketrans('','')
def makefilter(keep):
    delchars = allchars.translate(allchars, keep)
    def thefilter(s):
        return s.translate(allchars,delchars)
    return thefilter

if __name__ == '__main__':
    just_vowels = makefilter('aeiou')
    print just_vowels('four score and seven years ago')

    print just_vowels('tigere, igers, bigers')

my question is, how does thefilters 's' get passed in as a parameter?

Answer 1

makefilter returns a function.

In the example code:

just_vowels = makefilter('aeiou')

the variable just_vowels now refers to a function based on thefilter.

The code:

print just_vowels('tigere, igers, bigers')

is calling that function, and setting its s parameter to the string 'tigere, igers, bigers'.

Answer 2

It's possible to simplify that cookbook code using a list comprehension or generator expression:

def make_filter(keep):
    def the_filter(string):
        return ''.join(char for char in string if char in keep)
    return the_filter

This will work the same way as the provided makefilter.

>>> just_vowels = make_filter('aeiou')
>>> just_vowels('four score and seven years ago')
'ouoeaeeeaao'
>>> just_vowels('tigere, igers, bigers')
'ieeieie'

Like Richie explained, it will dynamically create a function, which you can later call on a string. The (char for char in string if char in keep) bit of code creates a generator which will iterate over the characters of the original string and perform the filtering. ''.join(...) then combines those characters back into a string.

Personally, I find that level of abstraction (writing a function to return a function) to be overkill for this sort of problem. It's a question of taste, but I think your code would be clearer if you just call the significant line directly:

>>> string = 'tigere, igers, bigers'
>>> keep = 'aeiou'
>>> ''.join(char for char in string if char in keep)
'ieeieie'