python python-typing mutable default-arguments

How to type mutable default arguments

The way to deal with mutable default arguments in Python is to set them to None.

For example:

def foo(bar=None):
    bar = [] if bar is None else bar
    return sorted(bar)

If I type in the function definition, then the only type for bar says that bar is Optional when, clearly, it is not Optional by the time I expect to run that sorted function on it:

def foo(bar: Optional[List[int]]=None):
    bar = [] if bar is None else bar
    return sorted(bar) # bar cannot be `None` here

So then should I cast?

def foo(bar: Optional[List[int]]=None):
    bar = [] if bar is None else bar
    bar = cast(List[int], bar) # make it explicit that `bar` cannot be `None`
    return sorted(bar)

Should I just hope that whoever reads through the function sees the standard pattern of dealing with default mutable arguments and understands that for the rest of the function, the argument should not be Optional?

What's the best way to handle this?

EDIT: To clarify, the user of this function should be able to call foo as foo() and foo(None) and foo(bar=None). (I don't think it makes sense to have it any other way.)

EDIT #2: Mypy will run with no errors if you never type bar as Optional and instead only type it as List[int], despite the default value being None. However, this is highly not recommended because this behavior may change in the future, and it also implicitly types the parameter as Optional. (See this for details.)

Solution

None is not the only sentinel available. You can choose your own list value to use as a sentinel, replacing it (rather than None) with a new empty list at run time.

_sentinel = []

def foo(bar: List[int]=_sentinel):
    bar = [] if bar is _sentinel else bar
    return sorted(bar)

As long as no one calls foo using _sentinel as an explicit argument, bar will always get a fresh empty list. In a call like foo([]), bar is _sentinel will be false: the two empty lists are not the same object, as the mutability of lists means that you cannot have a single empty list that always gets referenced by [].