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bpo-46086: Add ratio_min(self,m) function by gi-ba-bu · Pull Request #30125 · python/cpython

Here I propose a new function, namely .ratio_min(self,m).

.ratio_min(self,m) is an extension of the difflib's function .ratio(self). Equivalently to .ratio(self), .ratio_min(self,m) returns a measure of two sequences' similarity (float in [0,1]). In addition to .ratio(), it can ignore matched substrings if these substrings have length less than a given threshold m. m is the second variable of the function.

It is very useful to avoid spurious high similarity scores.

def ratio_min(self,m):
    """Return a measure of the sequences' similarity (float in [0,1]).
    Where T is the total number of elements in both sequences, and
    M_min is the number of matches with every single match has length at least m, this is 2.0*M_min / T.
    Note that this is 1 if the sequences are identical, and 0 if
    they have no substring of length m or more in common.
    .ratio_min() is similar to .ratio(). 
    .ratio_min(1) is equivalent to .ratio().
    
    >>> s = SequenceMatcher(None, "abcd", "bcde")
    >>> s.ratio_min(1)
    0.75
    >>> s.ratio_min(2)
    0.75
    >>> s.ratio_min(3)
    0.75
    >>> s.ratio_min(4)
    0.0
    """

    matches = sum(triple[-1] for triple in self.get_matching_blocks() if triple[-1] >=m)
    return _calculate_ratio(matches, len(self.a) + len(self.b))