Python之global

mingche

　　
　　1　　Global
　　The global statement and its nonlocal cousin are the only things that are remotely like declaration statements in Python. They are not type or size declarations; they are namespace declarations. The global statement tells Python that a function plans to change one or more global names.
　　· Global names are variables assigned at the top level of the enclosing module file.
　　· Global names must be declared only if they are assigned within a function.
　　· Global names may be referenced within a function without being declared.
　　In other words, global allows us to change names that live outside a def at the top level of a module file.
　　
　　2　　Example
　　The global statement consists of the keyword global, followed by one or more names separated by commas.



X = 88                         # Global X
def func():
global X
X = 99                     # Global X: outside def

func()
print(X)                       # Prints 99
　　



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
　　　　x, y, and z are all globals inside the function all_global. y and z are global because they aren’t assigned in the function; x is global because it was listed in a global statement to map it to the module’s scope explicitly. Without the global here, x would be considered local by virtue of the assignment.
　　
　　3　　Access globals



# thismod.py
var = 99                              # Global variable == module attribute
def local():
var = 0                           # Change local var
def glob1():
global var                        # Declare global (normal)
var += 1                          # Change global var
def glob2():
var = 0                           # Change local var
import thismod                    # Import myself
thismod.var += 1                  # Change global var
def glob3():
var = 0                           # Change local var
import sys                        # Import system table
glob = sys.modules['thismod']     # Get module object (or use __name__)
glob.var += 1                     # Change global var
def test():
print(var)
local();
print(var)
glob1();
print(var)
glob2();
print(var)
glob3()
print(var)
　　run and get results



>>> import thismod
>>> thismod.test()
99
99
100
101
102