Default arguments in Python are evaluated once when the function is defined, not each time the function is called. This means that if you have a mutable default argument and mutate it, you will have mutated that object for all future calls to the function as well.
For example, the following append_one function appends 1 to a list
and returns it. foo is set to an empty list by default.
>>> def append_one(foo=[]):
... foo.append(1)
... return foo
...>>> append_one()
[1]
>>> append_one()
[1, 1]
>>> append_one()
[1, 1, 1]1 to our list foo. It does not
receive a new empty list on each call, it is the same list everytime.
To avoid this problem, you have to create a new object every time the function is called:
>>> def append_one(foo=None):
... if foo is None:
... foo = []
... foo.append(1)
... return foo
...
>>> append_one()
[1]
>>> append_one()
[1]Note:
- This behavior can be used intentionally to maintain state between calls of a function (eg. when writing a caching function).
- This behavior is not unique to mutable objects, all default arguments are evaulated only once when the function is defined.