Warning about mutable default argument in PyCharm

Question

I am using PyCharm (Python 3) to write a Python function which accepts a dictionary as an argument with attachment={}.

def put_object(self, parent_object, connection_name, **data):     ...  def put_wall_post(self, message, attachment={}, profile_id="me"):     return self.put_object(profile_id, "feed", message=message, **attachment) 

In the IDE, attachment={} is colored yellow. Moving the mouse over it shows a warning.

Default arguments value is mutable

This inspection detects when a mutable value as list or dictionary is detected in a default value for an argument.

Default argument values are evaluated only once at function definition time, which means that modifying the default value of the argument will affect all subsequent calls of the function.

What does this mean and how can I resolve it?

Answer 1

If you don't alter the "mutable default argument" or pass it anywhere where it could be altered just ignore the message, because there is nothing to be "fixed".

In your case you only unpack (which does an implicit copy) the "mutable default argument" - so you're safe.

If you want to "remove that warning message" you could use None as default and set it to {} when it's None:

def put_wall_post(self,message,attachment=None,profile_id="me"):     if attachment is None:         attachment = {}      return self.put_object(profile_id,"feed",message = message,**attachment) 

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Just to explain the "what it means": Some types in Python are immutable (int, str, ...) others are mutable (like dict, set, list, ...). If you want to change immutable objects another object is created - but if you change mutable objects the object remains the same but it's contents are changed.

The tricky part is that class variables and default arguments are created when the function is loaded (and only once), that means that any changes to a "mutable default argument" or "mutable class variable" are permanent:

def func(key, value, a={}):     a[key] = value     return a  >>> print(func('a', 10))  # that's expected {'a': 10} >>> print(func('b', 20))  # that could be unexpected {'b': 20, 'a': 10} 

PyCharm probably shows this Warning because it's easy to get it wrong by accident (see for example “Least Astonishment” and the Mutable Default Argument and all linked questions). However, if you did it on purpose (Good uses for mutable function argument default values?) the Warning could be annoying.

Answer 2

You can replace mutable default arguments with None. Then check inside the function and assign the default:

def put_wall_post(self, message, attachment=None, profile_id="me"):     attachment = attachment if attachment else {}      return self.put_object(profile_id, "feed", message=message, **attachment) 

This works because None evaluates to False so we then assign an empty dictionary.

In general you may want to explicitly check for None as other values could also evaluate to False, e.g. 0, '', set(), [], etc, are all False-y. If your default isn't 0 and is 5 for example, then you wouldn't want to stomp on 0 being passed as a valid parameter:

def function(param=None):     param = 5 if param is None else param