Prevent calling a function more than once if the parameters have been used before

I would like a way to limit the calling of a function to once per values of parameters.

For example

def unique_func(x):
    return x

>>> unique_func([1])
[1]
>>> unique_func([1])
*** wont return anything ***
>>> unique_func([2])
[2]

Any suggestions? I’ve looked into using memoization but not established a solution just yet.

This is not solved by the suggested Prevent a function from being called twice in a row since that only solves when the previous func call had them parameters.

Answer

Memoization uses a mapping of arguments to return values. Here, you just want a mapping of arguments to None, which can be handled with a simple set.

def idempotize(f):
    cache = set()
    def _(x):
        if x in cache:
            return
        cache.add(x)
        return f(x)
    return _


@idempotize
def unique_fun(x):
    ...

With some care, this can be generalized to handle functions with multiple arguments, as long as they are hashable.

def idempotize(f):
    cache = set()
    def _(*args, **kwargs):
        k = (args, frozenset(kwargs.items()))
        if k in cache:
            return
        return f(*args, **kwargs)
    return _