function argument dependent on another function argument?

[Paul Rubin]

Steven D'Aprano <steve at REMOVE-THIS-cybersource.com.au> writes:
> def foo(self, x, y=None):
>     if y is None:
>         y = self.a
> 
> I don't find that clumsy in the least. I find it perfectly readable and a 
> standard idiom.

That has the same problem as the earlier version.  If the person
passes None, they get self.a.  I prefer:

    sentinel = object()
    ...

    def foo(x, y=sentinel):
      if y is sentinel:
          y = self.a