Best practice for setting the default value of a parameter that's supposed to be a list in Python?

I have a Python function that takes a list as a parameter. If I set the parameter's default value to an empty list like this:

def func(items=[]):
    print items

Pylint would tell me "Dangerous default value [] as argument". So I was wondering what is the best practice here?

Use None as a default value:

def func(items=None):
    if items is None:
        items = []
    print items

The problem with a mutable default argument is that it will be shared between all invocations of the function -- see the "important warning" in the relevant section of the Python tutorial.

I just encountered this for the first time, and my immediate thought is "well, I don't want to mutate the list anyway, so what I really want is to default to an immutable list so Python will give me an error if I accidentally mutate it." An immutable list is just a tuple. So:

def func(items=()):
print items

Sure, if you pass it to something that really does want a list (eg isinstance(items, list)), then this'll get you in trouble. But that's a code smell anyway.

For mutable object as a default parameter in function- and method-declarations the problem is, that the evaluation and creation takes place at exactly the same moment. The python-parser reads the function-head and evaluates it at the same moment.

Most beginers asume that a new object is created at every call, but that's not correct! ONE object (in your example a list) is created at the moment of DECLARATION and not on demand when you are calling the method.

For imutable objects that's not a problem, because even if all calls share the same object, it's imutable and therefore it's properties remain the same.

As a convention you use the None object for defaults to indicate the use of a default initialization, which now can take place in the function-body, which naturally is evaluated at call-time.

In addition and also to better understand what python is, here my little themed snippet:

from functools import wraps
def defaultFactories(func):
    'wraps function to use factories instead of values for defaults in call'
    defaults = func.func_defaults
    @wraps(func)
    def wrapped(*args,**kwargs):
        func.func_defaults = tuple(default() for default in defaults)
        return func(*args,**kwargs)
    return wrapped

def f1(n,b = []):
    b.append(n)
    if n == 1: return b
    else: return f1(n-1) + b

@defaultFactories
def f2(n,b = list):
    b.append(n)
    if n == 1: return b
    else: return f2(n-1) + b

>>> f1(6)
[6, 5, 4, 3, 2, 1, 6, 5, 4, 3, 2, 1, 6, 5, 4, 3, 2, 1, 6, 5, 4, 3, 2, 1, 6, 5, 4, 3, 2, 1, 6, 5, 4, 3, 2, 1]
>>> f2(6)
[1, 2, 3, 4, 5, 6]