Question

I would like to do something like the following:

class Foo(object):
    def __init__(self):
        self.member = 10
        pass

def factory(foo):
    foo = Foo()

aTestFoo = None
factory(aTestFoo)

print aTestFoo.member

However it crashes with AttributeError: 'NoneType' object has no attribute 'member':
the object aTestFoo has not been modified inside the call of the function factory.

What is the pythonic way of performing that ? Is it a pattern to avoid ? If it is a current mistake, how is it called ?

In C++, in the function prototype, I would have added a reference to the pointer to be created in the factory… but maybe this is not the kind of things I should think about in Python.

In C#, there’s the key word ref that allows to modify the reference itself, really close to the C++ way. I don’t know in Java… and I do wonder in Python.

Answer 1

Python does not have pass by reference. One of the few things it shares with Java, by the way. Some people describe argument passing in Python as call by value (and define the values as references, where reference means not what it means in C++), some people describe it as pass by reference with reasoning I find quite questionable (they re-define it to use to what Python calls “reference”, and end up with something which has nothing to do with what has been known as pass by reference for decades), others go for terms which are not as widely used and abused (popular examples are “{pass,call} by {object,sharing}”). See Call By Object on effbot.org for a rather extensive discussion on the defintions of the various terms, on history, and on the flaws in some of the arguments for the terms pass by reference and pass by value.

The short story, without naming it, goes like this:

• Every variable, object attribute, collection item, etc. refers to an object.
• Assignment, argument passing, etc. create another variable, object attribute, collection item, etc. which refers to the same object but has no knowledge which other variables, object attributes, collection items, etc. refer to that object.
• Any variable, object attribute, collection item, etc. can be used to modify an object, and any other variable, object attribute, collection item, etc. can be used to observe that modification.
• No variable, object attribute, collection item, etc. refers to another variable, object attribute, collection items, etc. and thus you can’t emulate pass by reference (in the C++ sense) except by treating a mutable object/collection as your “namespace”. This is excessively ugly, so don’t use it when there’s a much easier alternative (such as a return value, or exceptions, or multiple return values via iterable unpacking).

You may consider this like using pointers, but not pointers to pointers (but sometimes pointers to structures containing pointers) in C. And then passing those pointers by value. But don’t read too much into this simile. Python’s data model is significantly different from C’s.