python __main__ 里的变量是全局变量因此在函数里面可以访问到 - Python

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main and scoping in python

from:https://stackoverflow.com/questions/4775579/main-and-scoping-in-python

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I was somehow surprised by the following behavior:

def main():

    print "%s" % foo

if __name__ == "__main__":

    foo = "bar"

    main()

i.e. a module function has access to enclosing variables in the __main__ . What's the explanation for it?

python scope

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edited Jan 23 '11 at 18:41

SilentGhost

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asked Jan 23 '11 at 18:30

David Cournapeau

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4

There's no scope __main__ . There's an if that happens to compare a variable that happens to be called __name__ to something that happens to be the string literal "__main__" . – user395760 Jan 23 '11 at 18:53
@delnan: __main__ is a module, and each module has an associated scope. Try import __main__; type(__main__) in the interpreter (not in IPython). – Sven Marnach Jan 23 '11 at 21:48
@Sven: I know. OP should know at some point. But it seemed to me OP instead confuses if statements with parts of the condition and think if introduces a new scope - which would be a more pressing issue. (Admittedly, "There is no scope __main__ is misleading, strictly speaking) – user395760 Jan 23 '11 at 21:50
1

@delnan: you're right. In retrospect, I don't know why I was confused. Maybe debugging code at 3 a.m is not such a good idea after all :) – David Cournapeau Jan 24 '11 at 1:30
I just stumbled over this exact same behaviour...thought I was going mad. Glad to see someone else has already asked this question! The behaviour is obvious when the answer is explained... – ccbunney Jun 28 '13 at 8:58

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4 Answers

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Variables in the current modules global scope are visible everywhere in the module -- this rule also holds for the __main__ module.

From Guido's tutorial:

At any time during execution, there are at least three nested scopes whose namespaces are directly accessible:

the innermost scope, which is searched first, contains the local names

the scopes of any enclosing functions, which are searched starting with the nearest enclosing scope, contains non-local, but also non-global names

the next-to-last scope contains the current module’s global names

the outermost scope (searched last) is the namespace containing built-in names

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edited Jan 23 '11 at 18:45

answered Jan 23 '11 at 18:32

Sven Marnach

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__main__ module ? – user225312 Jan 23 '11 at 18:58
@AA: The main script is treated by Python as a module with the name __main__ . You even can do import __main__ (but usually that's a Bad Idea). – Sven Marnach Jan 23 '11 at 19:16

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The thing here is that:

if __name__ == "__main__":

    foo = "bar"

defines a global variable named foo in that script. so any function of that module will have access to it.

The piece of code listed above is global to the module and not inside any function.

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answered Jan 23 '11 at 18:34

Santiago Alessandri

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foo is a module global variable (it's not in any function). All scopes within the module can access it.

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answered Jan 23 '11 at 18:32

sinelaw

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In python there's the global scope, and functions have their own scopes. So it you define foo under the name==main, it's in the global scope. Also, it's not a mistake to use a variable which hasn't been declared yet, in a function, if it will be declared by the time the function will be called.

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edited Jan 23 '11 at 18:39

answered Jan 23 '11 at 18:34