Return bool if a string shares all characters with another string including doubles Python-3.x

Question

I'm sorry if a similar problem has been asked, I could not find it.

I need to check whether string_a contains all the characters from string_b including the non-unique ones.

Example 1:

... string_a = 'baba'
... string_b = 'baaa'
... <solution here>
False

Example 2 (returns True because now string_a has enough 'a's):

... string_a = 'ababa'
... string_b = 'baaa'
... <solution here>
True

I tried set() method but it only works if the characters of the strings are unique. So I have this:

... string_a = 'baba'
... string_b = 'baaa'
... return set(string_b) <= set(string_a)
True

I want it to be False because string_b has three 'a's and string_a only two.

Answer 1

What you're doing is interpreting the strings as multisets and checking if one is a subset of the other. Python's multiset representation is collections.Counter:

>>> from collections import Counter
>>> Counter('baba')
Counter({'b': 2, 'a': 2})
>>> Counter('baaa')
Counter({'a': 3, 'b': 1})

Unfortunately, Counter doesn't implement a is_subset method, so we have to write our own. Here are two ways to do it:

def is_sub_multiset(haystack, needle):
    haystack = Counter(haystack)
    needle = Counter(needle)

    return not needle - haystack
    # OR
    # return all(haystack[elem] >= count for elem, count in needle.items())

>>> is_sub_multiset('baba', 'baaa')
False
>>> is_sub_multiset('ababa', 'baaa')
True