Question

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument

Using python 2.7 I came across strange behaviour and I’m not sure how to explain it or if it even exists in any python docs. Using the code

class MyClass:
@staticmethod
def func(objects=[], a=None, b=None):
    objects.append(a)
    print 'objects: %s'%objects
    print 'b: %s'%b


MyClass.func(a='one')
MyClass.func(a='two', b='foo')
MyClass.func(a='three')

I get the get output

objects: ['one']
b: None
objects: ['one', 'two']
b: foo
objects: ['one', 'two', 'three']
b: None

As you can see, the first list parameter (objects) of the method retains it’s values across calls.. new values being appended to the last list even though in it’s header declaration it has a default value of []. But the last parameter (b) does not retain it’s value, it is reset to the default value between calls.

The expected (for me anyway) is that the objects parameter should be reset to it’s default on any call to the method (like the b parameter is), but this doesn’t seem to happen and only seems to occur on the first call.

Can anyone explain this behaviour? Is it a bug in this version of python or is it intended behaviour? Possibly something to do with the list reference being retained across calls but the string variable (b) is not? I’m very confused by this behaviour.

Thanks

Answer 1

It has nothing to do with being an staticmethod. This is a very common mistake in Python.

Functions in Python are first class objects, not just a block of code, so the empty list you assign to objects in the parameters is a real list attached to the function object. Everytime you call it and append something to that list, you are using the same list. It’s easy to see it happening this way:

>>> def func(x, objects=[]):
...     objects.append(x)
... 
>>> func(1)
>>> func.func_defaults
([1],)
>>> func(2)
>>> func.func_defaults
([1, 2],)
>>> func(3)
>>> func.func_defaults
([1, 2, 3],)

func_defaults is the function object attribute that constains the default keyword parameters you set. See how the list is there and get changed?

The proper way to do what you want is:

class MyClass:
@staticmethod
def func(objects=None, a=None, b=None):
    if objects is None:
        objects = []
    objects.append(a)
    print 'objects: %s'%objects
    print 'b: %s'%b