[Python-Dev] can't assign to function call (original) (raw)

Chris Angelico rosuav at gmail.com
Mon Mar 18 15:23:07 CET 2013

On Tue, Mar 19, 2013 at 12:50 AM, Neal Becker <ndbecker2 at gmail.com> wrote:

def F(x): return x

x = 2 F(x) = 3 F(x) = 3 SyntaxError: can't assign to function call Do we really need this restriction? There do exist other languages without it.

The languages that permit you to assign to a function call all have some notion of a reference type. In C++, for instance, you can return a reference to an object, and assigning to the function call achieves the same thing as assigning to the referent. C has similar semantics with pointers; you can dereference a returned pointer:

int somevalue; int *F() {return &somevalue;}

F() = 5; / will assign to somevalue */

With Python, there are no pointers, there are no variables. But you can do something somewhat similar:

x = [0] def F(): return x

F()[0]=3; x[0] 3

If you think of x as a pointer to the value x[0], then Python lets you "dereference" the function return value. But this is fiddling with terminology; the concept of assigning to a function return value doesn't really make sense in Python.

Further discussion on exactly why this is the case can be found on python-list's or python-tutor's archives, such as this excellent post by Steven D'Aprano:

http://mail.python.org/pipermail/tutor/2010-December/080505.html

TLDR: Python != C. :)

ChrisA

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