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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
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原文地址：http://www.cnblogs.com/mengdie/p/4567154.html
