What's the scope of a variable initialized in an if statement?

 

I'm new to Python, so this is probably a simple scoping question. The following code in a Python file (module) is confusing me slightly: 

if __name__ == '__main__':
    x = 1

print x

In other languages I've worked in, this code would throw an exception, as the x variable is local to the if statement and should not exist outside of it. But this code executes, and prints 1. Can anyone explain this behavior? Are all variables created in a module global/available to the entire module?

Python variables are scoped to the innermost function, class, or module in which they're assigned. Control blocks like if and while blocks don't count, so a variable assigned inside an if is still scoped to a function, class, or module.

(Implicit functions defined by a generator expression or list/set/dict comprehension do count, as do lambda expressions. You can't stuff an assignment statement into any of those, but lambda parameters and for clause targets are implicit assignment.)

Yes, they're in the same "local scope", and actually code like this is common in Python: 

if condition:
  x = 'something'
else:
  x = 'something else'

use(x)

Note that x isn't declared or initialized before the condition, like it would be in C or Java, for example.

In other words, Python does not have block-level scopes. Be careful, though, with examples such as 

if False:
    x = 3
print(x)

which would clearly raise a NameError exception.

Scope in python follows this order:

  • Search the local scope

  • Search the scope of any enclosing functions

  • Search the global scope

  • Search the built-ins

    • (source)

    Notice that if and other looping/branching constructs are not listed - only classes, functions, and modules provide scope in Python, so anything declared in an if block has the same scope as anything decleared outside the block. Variables aren't checked at compile time, which is why other languages throw an exception. In python, so long as the variable exists at the time you require it, no exception will be thrown.

    链接地址: http://www.djcxy.com/p/36330.html

    上一篇: 如何使用指针从不同的函数访问局部变量？

    下一篇: if语句中初始化变量的范围是什么？