is there a way to self reference within a list generation in python-CodePudding

I want to create the following list of tuples of strings:

pairs = [(s1,s2) for s1 in S for s2 in S if s1 != s2 and (s2,s1) not in self]

Obviously not in self doesn't work. Is there a way to achieve this with a generator expression? Thanks

CodePudding user response：

You don't want a list, you want a set:

pairs = {(s1,s2) for s1 in S for s2 in S if s1 != s2}

Sets can only have a single element of a given (hashable) value. Your self-referential filter clause (not in self) is essentially asking to ensure that there is only one such element, which is what a set is for. You can easily list(pairs) after this if you need a list instead of a set for what you're doing.

A downside of this is that the values must be hashable (immutable values, or mutable values that can be hashed).

CodePudding user response：

No, there's no way to reference the object being constructed by a comprehension.

Do it in two steps: First create the list, then add to the list in a loop.

pairs = []
for s1 in S:
    for s2 in S:
        if (s1, s2) not in pairs:
            pairs.append((s1, s2))