Closures
Writing Functions in Python
Shayne Miel
Software Architect @ Duo Security
Attaching nonlocal variables to nested functions
def foo():
a = 5
def bar():
print(a)
return bar
func = foo()
func()
5
Closures!
type(func.__closure__)
<class 'tuple'>
len(func.__closure__)
1
func.__closure__[0].cell_contents
5
Closures and deletion
x = 25
def foo(value):
def bar():
print(value)
return bar
my_func = foo(x)
my_func()
25
del(x)
my_func()
25
len(my_func.__closure__)
1
my_func.__closure__[0].cell_contents
25
Closures and overwriting
x = 25
def foo(value):
def bar():
print(value)
return bar
x = foo(x)
x()
25
len(x.__closure__)
1
x.__closure__[0].cell_contents
25
Definitions - nested function
Nested function: A function defined inside another function.
# outer function
def parent():
# nested function
def child():
pass
return child
Definitions - nonlocal variables
Nonlocal variables: Variables defined in the parent function that are used by the child function.
def parent(arg_1, arg_2):
# From child()'s point of view,
# `value` and `my_dict` are nonlocal variables,
# as are `arg_1` and `arg_2`.
value = 22
my_dict = {'chocolate': 'yummy'}
def child():
print(2 * value)
print(my_dict['chocolate'])
print(arg_1 + arg_2)
return child
Closure: Nonlocal variables attached to a returned function.
def parent(arg_1, arg_2):
value = 22
my_dict = {'chocolate': 'yummy'}
def child():
print(2 * value)
print(my_dict['chocolate'])
print(arg_1 + arg_2)
return child
new_function = parent(3, 4)
print([cell.cell_contents for cell in new_function.__closure__])
[3, 4, 22, {'chocolate': 'yummy'}]
Why does all of this matter?
Decorators use:
- Functions as objects
- Nested functions
- Nonlocal scope
- Closures
Let's practice!
Writing Functions in Python
