问题描述
我在试图理解 Python 范围规则时遇到了严重的麻烦.
使用以下脚本:
a = 7定义打印A():打印a 的值是 %d" % (a)定义集A(值):a = 值打印在 setA 中,a 现在是 %d" %(a)打印在setA之前"打印A()集A(42)打印在 setA 之后"打印A()
给出意想不到的(对我来说)输出:
<前>设置A之前a 的值为 7在 setA 中,a 现在是 42设置A后a 的值为 7我希望 a 的最后一次打印的值是 42,而不是 7.关于 Python 的全局变量范围规则,我遗漏了什么?
解决方案全局变量是特殊的.如果您尝试在函数内部为变量 a = value
赋值,它会在函数内部创建一个新的局部变量,即使存在同名的全局变量.要访问全局变量,请添加 global
函数内的语句:
a = 7定义集A(值):global a # 声明 a 为全局a = value # 设置 a 的全局值
另请参阅命名和绑定以获取详细说明Python 的命名和绑定规则.
I'm having terrible trouble trying to understand python scoping rules.
With the following script:
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()
Gives the unexpected (to me) output of:
Before setA Value of a is 7 Inside setA, a is now 42 After setA Value of a is 7
Where I would expect the last printing of the value of a to be 42, not 7. What am I missing about Python's scope rules for the scoping of global variables?
解决方案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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