Python's Mutable Default Argument

Intro

If you've written a lot of Python code, you might have run into one of Python's quirks, the Mutable Default Argument.

"What is that," you ask?

Take a look at this code:

def add_to_basket(item, basket=[]):
    basket.append(item)
    return basket

A reasonable implementation to add arbitrary items to a basket. It uses keyword arguments to provide a default value for basket if it is not passed in.

However, you would quickly realize that this does not give you the intended effect:

>>> add_to_basket('banana')
['banana']
>>> add_to_basket('apple')
['banana', 'apple']

It should be creating a new basket every time since the basket keyword parameter is not passed in, but instead it seems like the default list paramter is only created once rather than on every invocation.

This is a "feature" of Python called Mutable Default Arguments.

There is a great thread on Stack Overflow that discusses this and the official Python documentation actually covers this as well:

(emphasis added)

Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use None as the default, and explicitly test for it in the body of the function...

So what can you do about this?

The easiest thing to do is to make the default parameter's value None. Like so:

def add_to_basket(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

I personally like a similar flavor to the above solution; something like:

def add_to_basket(item, basket=None):
    basket = basket or []
    basket.append(item)
    return basket

This works, but if something is passed into basket that has a False-y value (e.g. empty string ('') or empty dictionary ({})), it will still return the default list value. I usually don't have to worry about this, so I like using the or notation.

Conclusion

Hopefully this gives you a better idea of the gotcha in Python. It's one of those things where if you've never seen it before, could be very disorientating.