Is there a way to set a default parameter equal to another parameter value?

by

No, function keyword parameter defaults are determined when the function is defined, not when the function is executed.

Set the default to None and detect that:

def perms(elements, setLength=None):
    if setLength is None:
        setLength = elements

If you need to be able to specify None as a argument, use a different sentinel value:

_sentinel = object()

def perms(elements, setLength=_sentinel):
    if setLength is _sentinel:
        setLength = elements

Now callers can set setLength to None and it won’t be seen as the default.

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