[python] "Least Astonishment" and the Mutable Default Argument

I am going to demonstrate an alternative structure to pass a default list value to a function (it works equally well with dictionaries).

As others have extensively commented, the list parameter is bound to the function when it is defined as opposed to when it is executed. Because lists and dictionaries are mutable, any alteration to this parameter will affect other calls to this function. As a result, subsequent calls to the function will receive this shared list which may have been altered by any other calls to the function. Worse yet, two parameters are using this function's shared parameter at the same time oblivious to the changes made by the other.

Wrong Method (probably...):

def foo(list_arg=[5]):
    return list_arg

a = foo()
a.append(6)
>>> a
[5, 6]

b = foo()
b.append(7)
# The value of 6 appended to variable 'a' is now part of the list held by 'b'.
>>> b
[5, 6, 7]  

# Although 'a' is expecting to receive 6 (the last element it appended to the list),
# it actually receives the last element appended to the shared list.
# It thus receives the value 7 previously appended by 'b'.
>>> a.pop()             
7

You can verify that they are one and the same object by using id:

>>> id(a)
5347866528

>>> id(b)
5347866528

Per Brett Slatkin's "Effective Python: 59 Specific Ways to Write Better Python", Item 20: Use None and Docstrings to specify dynamic default arguments (p. 48)

The convention for achieving the desired result in Python is to provide a default value of None and to document the actual behaviour in the docstring.

This implementation ensures that each call to the function either receives the default list or else the list passed to the function.

Preferred Method:

def foo(list_arg=None):
   """
   :param list_arg:  A list of input values. 
                     If none provided, used a list with a default value of 5.
   """
   if not list_arg:
       list_arg = [5]
   return list_arg

a = foo()
a.append(6)
>>> a
[5, 6]

b = foo()
b.append(7)
>>> b
[5, 7]

c = foo([10])
c.append(11)
>>> c
[10, 11]

There may be legitimate use cases for the 'Wrong Method' whereby the programmer intended the default list parameter to be shared, but this is more likely the exception than the rule.

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