Lifetime of default function arguments in python [duplicate]

Question

I just started learning python, just got struck by the default argument concept.

It is mentioned in python doc that the default argument value of a function is computed only once when the def statement is encountered. This makes a large difference between values of immutable and mutable default function arguments.

>>> def func(a,L=[]):
      L.append(a)
      return L
>>> print(func(1))
[1]
>>> print(func(2))
[1, 2]

here the mutable function argument L retains the last assigned value (since the default value is calculated not during function call as like in C)

is the lifetime of the default argument's value in python is lifetime of the program (like static variables in C) ?

Edit :

>>> Lt = ['a','b']
>>> print(func(3,Lt))
['a', 'b', 3]
>>> print(func(4))
[1, 2, 4]

here during the function call func(3,Lt) the default value of L is preserved, it is not overwritten by Lt.

so is default argument have two memory? one for actual default value (with program scope) and other for when an object is passed to it (with scope of function call)?

Answer 1

As the argument is an attribute of the function object, it normally has the same lifetime as the function. Usually, functions exist from the moment their module is loaded, until the interpreter exits.

However, Python functions are first-class objects and you can delete all references (dynamically) to the function early. The garbage collector may then reap the function and subsequently the default argument:

>>> def foo(bar, spam=[]):
...     spam.append(bar)
...     print(spam)
... 
>>> foo
<function foo at 0x1088b9d70>
>>> foo('Monty')
['Monty']
>>> foo('Python')
['Monty', 'Python']
>>> foo.func_defaults
(['Monty', 'Python'],)
>>> del foo
>>> foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined

Note that you can directly reach the func_defaults attribute (__defaults__ in python 3), which is writable, so you can clear the default by reassigning to that attribute.