Detailed explanation of variables and scope in Python

Scope introduction

The scope in python is divided into 4 situations: L: local, local scope, that is, the variables defined in the function;

E: enclosing, the local scope of the nested parent function, that is, the local scope of the superior function that contains this function, but not global;

G: global variables, which are variables defined at the module level; B: built-in, variables in system fixed modules, such as int, bytearray, etc. The priority order of searching variables is: scope local > outer scope > global in the current module > python built-in scope, which is LEGB.

x = int(2.9) # int built-in
g_count = 0 # global
def outer():
o_count = 1 # enclosing
def inner():
i_count = 2 # local

#定义变量a
>>> a = 0
>>> print a
0
#定义函数p()
>>> def p():
... print a
... 
>>> p()
0
#定义函数p2()
>>> def p2():
... print a
... a = 3
... print a
... 
>>> p2() # 运行出错，外部变量a先被引用，不能重新赋值
Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
File "<interactive input>", line 2, in p2
UnboundLocalError: local variable 'a' referenced before assignment
#定义函数p3()
>>> def p3():
... a = 3 # 不引用直接赋值
... print a
... 
>>> p3()
3
>>> print a
0 # 外部变量a并未改变