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global The sentence is Python The only statement in that looks something like a declaration statement . however , It is not a declaration of type or size , It is a namespace declaration . It tells Python The function is intended to generate one or more global variable names . in other words , Exists within the scope of the entire module （ Namespace ） Variable name ：
X = 88 # Global X
def func（）:
global X
X = 99 # Global X: outside def
func（）
print（X） # Prints 99
In this example, we added a global Statement , In order to be in def Within X Can be referenced in def In addition to the X.
Let's take another example
y,z = 1,2 # Global variables in module
def all_global（）:
global x # Declare globals assigned
x = y + z # No need to declare y,z: LEGB rule
here ,x、y and z All are all_global Global variables in functions .y and z Global variable , Because they are not assigned in a function ;x Global variable , Because it passes through global Statement explicitly maps itself to the scope of the module . If not used global In words ,x Will be considered a local variable due to the assignment .
y and z It didn't go on global Statement .Python Of LEGB The lookup rule will automatically find them from the module . Besides , Be careful x It may not exist before the function runs . If so , The assignment statement in the function will be automatically created in the module x This variable .
