Introduction to the declaration and parsing of keywords nonlocal and global in Python

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1. Global and nonlocal declarations in Python

The following code

a = 10 
 
def foo(): 
 a = 100

The result a of executing foo() is still the assignment to the
variable

function, the variable is always bound to The local namespace of this function, this behavior can be changed using the global statement.

>>> a 
10 
>>> def foo(): 
...  global a 
...  a = 100 
... 
>>> a 
10 
>>> foo() 
>>> a 
100

When parsing a name, first check the local scope, and then check the scope defined by the external nested function layer by layer from the inside out. If it cannot be found, Search the global command space and built-in namespace.

Although you can look up variables layer by layer, but! ..python2 only supports the innermost scope (local variables) and the global command space (gloabl), which means internal functions Local variables defined in external functions cannot be reassigned. For example, the following code does not work.

def countdown(start): 
 n = start 
 def decrement(): 
  n -= 1

In python2, the solution can be to modify The value is placed in a list or dictionary. In python3, you can use the nonlocal statement to complete the modification

def countdown(start): 
 n = start 
 def decrement(): 
  nonlocal n 
  n -= 1

2. Python nonlocal and global keywords Parsing

nonlocal

First of all, it is necessary to make it clear that the nonlocal keyword is defined in the closure. Please look at the following code:

x = 0
def outer():
 x = 1
 def inner():
  x = 2
  print("inner:", x)

 inner()
 print("outer:", x)

outer()
print("global:", x)

Result

# inner: 2
# outer: 1
# global: 0

Now, add the nonlocal keyword to the closure to declare it :

x = 0
def outer():
 x = 1
 def inner():
  nonlocal x
  x = 2
  print("inner:", x)

 inner()
 print("outer:", x)

outer()
print("global:", x)

Result

##

# inner: 2
# outer: 2
# global: 0

Still the same, look at an example:

x = 0
def outer():
 x = 1
 def inner():
  global x
  x = 2
  print("inner:", x)

 inner()
 print("outer:", x)

outer()
print("global:", x)

# inner: 2
# outer: 1
# global: 2

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