Question

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument

consider the following python test.py module:

class Container:
    def __init__(self, str_list=[]):
        self.str_list = str_list

    def from_strings(self, st=""):
        self.str_list.append(st)
        return self

o1 = Container().from_strings(st="o1")
o2 = Container().from_strings(st="o2")
o3 = Container().from_strings(st="o3")

def prnt():
    print("o1's list:"+str(o1.str_list))
    print("o2's list:"+str(o2.str_list))
    print("o3's list:"+str(o3.str_list))

if __name__ == '__main__':
    prnt()

Why is the output of python test.py:

  o1's list:['o1', 'o2', 'o3']
  o2's list:['o1', 'o2', 'o3']
  o3's list:['o1', 'o2', 'o3']

instead of:

  o1's list:['o1']
  o2's list:['o2']
  o3's list:['o3']

(It seems like I am missing why a field (str_list) of different instances in the same module could mix up. A pointer to the python doc explaining this concept would be highly appreciated)

Answer 1

You created an empty list as default value in your constructor:

def __init__(self, str_list=[]):
    self.str_list = str_list

This list is only created once and then shared by all instances. If you wanted a separate list for each instance, you could do something like:

def __init__(self, str_list=None):
    self.str_list = [] if str_list is None else str_list

Now each instance creates it’s own list.

edit:

What is going on?

You have probably used default arguments like 1, or 'foo' and never had a problem.
These are immutable objects which can not be manipulated. Whenever you think you do, you get a new instance in stead. The collection of immutable object types includes int, str, tuple, among others.

Now [], a list, is a mutable object which can be manipulated. So whenever you change an attribute on it, it changes the actual object and every reference to it will reflect the changes.