What is the scope of a defaulted parameter in Python?

by

The scope is as you would expect.

The perhaps surprising thing is that the default value is only calculated once and reused, so each time you call the function you get the same list, not a new list initialized to [].

The list is stored in f.__defaults__ (or f.func_defaults in Python 2.)

def f(a, L=[]):
    L.append(a)
    return L

print f(1)
print f(2)
print f(3)
print f.__defaults__
f.__defaults__ = (['foo'],) # Don't do this!
print f(4)

Result:

[1]
[1, 2]
[1, 2, 3]
([1, 2, 3],)
['foo', 4]

More Related Contents:

  1. Short description of the scoping rules?
  2. Local variables in nested functions
  3. Accessing class variables from a list comprehension in the class definition
  4. Python 3: UnboundLocalError: local variable referenced before assignment [duplicate]
  5. Passing a dictionary to a function as keyword parameters
  6. Getting list of parameter names inside python function [duplicate]
  7. Why does this UnboundLocalError occur (closure)? [duplicate]
  8. Why does assigning to my global variables not work in Python?
  9. Python scope: “UnboundLocalError: local variable ‘c’ referenced before assignment” [duplicate]
  10. How do Python functions handle the types of parameters that you pass in?
  11. Python: How can I run eval() in the local scope of a function
  12. How to create module-wide variables in Python? [duplicate]
  13. How to inject variable into scope with a decorator?
  14. Python: Sharing global variables between modules and classes therein
  15. Tensorflow: How to get a tensor by name?
  16. Why is one class variable not defined in list comprehension but another is?
  17. Python and lmfit: How to fit multiple datasets with shared parameters?
  18. Named tuple and default values for optional keyword arguments
  19. Getting “global name ‘foo’ is not defined” with Python’s timeit
  20. How references to variables are resolved in Python [duplicate]
  21. Getting a default value on index out of range in Python [duplicate]
  22. Global dictionaries don’t need keyword global to modify them? [duplicate]
  23. Why do I get a `NameError` (or `UnboundLocalError`) from using a named exception after the `except` block?
  24. UnboundLocalError trying to use a variable (supposed to be global) that is (re)assigned (even after first use)
  25. What does the slash mean when help() is listing method signatures?
  26. python – returning a default value
  27. Python global variable scoping
  28. Why can functions in Python print variables in enclosing scope but cannot use them in assignment? [duplicate]
  29. Defining a default argument as a global variable
  30. Scope of variable within “with” statement?

Leave a Comment