Returning None or a tuple and unpacking

python return-value
Stefano Borini picture Stefano Borini · Aug 14, 2009 · Viewed 18.2k times · Source

I am always annoyed by this fact:

$ cat foo.py
def foo(flag):
    if flag:
        return (1,2)
    else:
        return None

first, second = foo(True)
first, second = foo(False)

$ python foo.py
Traceback (most recent call last):
  File "foo.py", line 8, in <module>
    first, second = foo(False)
TypeError: 'NoneType' object is not iterable

The fact is that in order to correctly unpack without troubles I have either to catch the TypeError or to have something like

values = foo(False)
if values is not None:
    first, second = values

Which is kind of annoying. Is there a trick to improve this situation (e.g. to so set both first and second to None without having foo returning (None, None)) or a suggestion about the best design strategy for cases like the one I present ? *variables maybe ?

Answer

Amber picture Amber · Aug 14, 2009

Well, you could do...

first,second = foo(True) or (None,None)
first,second = foo(False) or (None,None)

but as far as I know there's no simpler way to expand None to fill in the entirety of a tuple.

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