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

Question

For example, I have a basic method that will return a list of permutations.

import itertools def perms(elements, set_length=elements):     data=[]     for x in range(elements):         data.append(x+1)     return list(itertools.permutations(data, set_length)) 

Now I understand, that in its current state this code won't run because the second elements isn't defined, but is there and elegant way to accomplish what I'm trying to do here? If that's still not clear, I want to make the default setLength value equal to the first argument passed in. Thanks.

Answer 1

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.

Answer 2

Because of the way Python handles bindings and default parameters...

The standard way is:

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

And another option is:

def perms(elements, **kwargs):     setLength = kwargs.pop('setLength', elements) 

Although this requires you to explicitly use perms(elements, setLength='something else') if you don't want a default...