Why default arguments value is mutable if param initiated by call?

September 5, 2022

I am using PyCharm (Python 3) to write a Python function which accepts an empty dictionary or list as an argument.

def my_function(param1: list = list(), param2: dict = dict()):
    ...

But PyCharm yellowed this.
How dict() or list() can be mutable? Is this PyCharm bug?

>Solution :

No, it’s not a PyCharm bug, it’s PyCharm trying to prevent you from writing bugs.

The list() and dict() functions are called at the time you declare that function, and the single (reusable, mutable) list or dict returned by that call becomes the parameter default.

From there on it’s just "Least Astonishment" and the Mutable Default Argument all again.

To wit:

>>> def my_function(param1: list = list(), param2: dict = dict()):
...     param1.append("hi")
...     param2[len(param2)] = "hello"
...
>>> my_function.__defaults__
([], {})
>>> my_function()
>>> my_function.__defaults__
(['hi'], {0: 'hello'})
>>> my_function()
>>> my_function.__defaults__
(['hi', 'hi'], {0: 'hello', 1: 'hello'})
>>> my_function()
>>> my_function.__defaults__
(['hi', 'hi', 'hi'], {0: 'hello', 1: 'hello', 2: 'hello'})