Python之global

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