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

Question

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?

this is something that every python newbie stubs their toe on once or twice, it's pretty cool that pylint has stopped you from writing a horrible bug ! — wim, Mar 02 '12 at 00:59
Another really simple trick is to use an empty tuple: `def func(items=())`. A tuple is not mutable so pylint shuts up, but the question asks for a list so this won't always be relevant or possible. — Ant, Sep 04 '19 at 10:28

score 107 · Accepted Answer · answered Mar 02 '12 at 00:43

107

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.

answered Mar 02 '12 at 00:43

Sven Marnach

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Ah wow, I'm surprised that people know about this. =) I hated this feature of python so much that I wrote my own decorator library to allow for `func(items=new([]))` syntax. – ninjagecko Mar 02 '12 at 00:51
1

Thanks! It seems like Python thinks None is a completely different type than list, so I was wondering is it generally acceptable in Python to set the default value of a parameter to a different type (such as None) (I know Python is a type-less language, but I come from C++... lol)? – Jack Z Mar 02 '12 at 00:54
It is quite different to a strongly typed language like C++. Just think of variables as names which reference objects, and everything is passed by reference. – wim Mar 02 '12 at 00:57
3

@wim: I think you mean 'statically', not strongly; python is strongly but dynamically typed. – DSM Mar 02 '12 at 00:59
Thanks, wim! Then when you use a variable, how can you decide if it's an actual list or still keeping its default value None? Or do you always have to check before using it? – Jack Z Mar 02 '12 at 01:00
1

You don't have to check it every time, only when you expect that it won't be what you think it should be. It's like checking if a pointer/reference is null in C++ .. you don't check it every time, but in many cases it does make sense to guard against it. – cwa Mar 02 '12 at 02:10
7

I usually write `items = items or []` if I expect `items` to be an iterable type. – Neil G Mar 04 '12 at 21:41

score 10 · Answer 2 · answered Sep 17 '13 at 17:20

10

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.

answered Sep 17 '13 at 17:20

sfink

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And if inside the function you need to make a copy, use `my_copy = list(items)`. You came up with a simple and very clever solution to a common problem. – Mark Ransom Sep 17 '13 at 18:12

score 3 · Answer 3 · answered Mar 02 '12 at 02:10

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.

Odomontois · Answer 4 · 2012-03-02T02:44:08.450

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]

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

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