Specialized Powerset Requirement

Question

Let's say I have compiled five regular expression patterns and then created five Boolean variables:

a =  re.search(first, mystr)
b =  re.search(second, mystr)
c =  re.search(third, mystr)
d = re.search(fourth, mystr)
e = re.search(fifth, mystr)

I want to use the Powerset of (a, b, c, d, e) in a function so it finds more specific matches first then falls through. As you can see, the Powerset (well, its list representation) should be sorted by # of elements descending.

Desired behavior:

 if a and b and c and d and e:
     return 'abcde' 
 if a and b and c and d:
     return 'abcd'
 [... and all the other 4-matches ]
 [now the three-matches]
 [now the two-matches]
 [now the single matches]
 return 'No Match'  # did not match anything

Is there a way to utilize the Powerset programmatically and ideally, tersely, to get this function's behavior?

Answer 1

You could use the powerset() generator function recipe in the itertools documentation like this:

from itertools import chain, combinations
from pprint import pprint
import re

def powerset(iterable):
    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

mystr   = "abcdefghijklmnopqrstuvwxyz"
first   = "a"
second  = "B"  # won't match, should be omitted from result
third   = "c"
fourth  = "d"
fifth   = "e"

a = 'a' if re.search(first, mystr) else ''
b = 'b' if re.search(second, mystr) else ''
c = 'c' if re.search(third, mystr) else ''
d = 'd' if re.search(fourth, mystr) else ''
e = 'e' if re.search(fifth, mystr) else ''

elements = (elem for elem in [a, b, c, d, e] if elem is not '')
spec_ps = [''.join(item for item in group)
              for group in sorted(powerset(elements), key=len, reverse=True)
                  if any(item for item in group)]

pprint(spec_ps)

Output:

['acde',
 'acd',
 'ace',
 'ade',
 'cde',
 'ac',
 'ad',
 'ae',
 'cd',
 'ce',
 'de',
 'a',
 'c',
 'd',
 'e']