为什么给我的全局变量赋值在 Python 中不起作用？

我在理解 Python 范围规则方面遇到了很大的麻烦。

剧本如下:

a = 7


def printA():
print "Value of a is %d" % (a)


def setA(value):
a = value
print "Inside setA, a is now %d" %(a)




print "Before setA"
printA()
setA(42)
print "After setA"
printA()

给出(对我来说)意外的输出:

Before setA
Value of a is 7
Inside setA, a is now 42
After setA
Value of a is 7

我希望最后一次打印出 a 的值是42，而不是7。关于 Python 的全局变量作用域规则，我漏掉了什么？

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最佳答案

Global variables are special. If you try to assign to a variable a = value inside of a function, it creates a new local variable inside the function, even if there is a global variable with the same name. To instead access the global variable, add a global statement inside the function:

a = 7
def setA(value):
global a   # declare a to be a global
a = value  # this sets the global value of a

See also Naming and binding for a detailed explanation of Python's naming and binding rules.

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The trick to understanding this is that when you assign to a variable, using =, you also declare it as a local variable. So instead of changing the value of the global variable a, setA(value) actually sets a local variable (which happens to be called a) to the value passed in.

This becomes more obvious if you try to print the value of a at the start of setA(value) like so:

def setA(value):
print "Before assignment, a is %d" % (a)
a = value
print "Inside setA, a is now %d" % (a)

If you try to run this Python will give you a helpful error:

Traceback (most recent call last):
File "scopeTest.py", line 14, in
setA(42)
File "scopeTest.py", line 7, in setA
print "Before assignment, a is %d" % (a)
UnboundLocalError: local variable 'a' referenced before assignment

This tells us that Python has decided that the setA(value) function has a local variable called a, which is what you alter when you assign to it in the function. If you don't assign to a in the function (as with printA()) then Python uses the global variable A.

To mark a variable as global you need to use the global keyword in Python, in the scope that you want to use the global variable. In this case that is within the setA(value) function. So the script becomes:

a = 7


def printA():
print "Value of a is %d" % (a)


def setA(value):
global a
a = value
print "Inside setA, a is now %d" %(a)




print "Before setA"
printA()
setA(42)
print "After setA"
printA()

This one line addition tells Python that when you use the variable a in the setA(value) function that you are talking about the global variable, not a local variable.

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Python does not have a concept of variables as other languages. You have objects which are "somewhere" and you have references to these objects. = is used to assign these objects to references in the current namespace.

You create a name a in the namespace of the setA function which refers to the object to which value refers.

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inside the function,a is treated as a local variable,you need to define

global a

inside the function before using it inside the method, even though you had defined it outside the method.

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The execution of a function introduces a new symbol table used for the local variables of the function. More precisely, all variable assignments in a function store the value in the local symbol table; whereas variable references first look in the local symbol table, then in the local symbol tables of enclosing functions, then in the global symbol table, and finally in the table of built-in names. Thus, global variables cannot be directly assigned a value within a function (unless named in a global statement), although they may be referenced.

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Accepted answer here should be helpful:

using global incorrectly. See Python reference. You should declare variable without global and then inside the function when you want to access global variable you declare it global yourvar


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