Python全局变量与global语句

2019-11-10 19:43:15

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如果将全局变量的名字声明在一个函数体内的时候，全局变量的名字能被局部变量给覆盖掉。

code1:

bar = 100def foo(): PRint "/ncalling foo()..." bar = 200 print "in foo(), bar is", barprint "in __main__, bar is", barfoo()print "/nin __main__, bar is (still)", bar

输出：

in __main__, bar is 100calling foo()...in foo(), bar is 200in __main__, bar is (still) 100

在foo()函数中局部的bar将全局的bar”覆盖掉了”。

code2:

bar = 100def foo(): print "/ncalling foo()..." #bar = 200 print "in foo(), bar is", barif __name__ == '__main__' : print "in __main__, bar is", bar foo() print "/nin __main__, bar is (still)", bar

输出：

in __main__, bar is 100calling foo()...in foo(), bar is 100in __main__, bar is (still) 100

这里将foo()函数内对bar赋值语句注释掉，发现函数内部可以访问全局的bar。

code 3:

bar = 100def foo(): print "/ncalling foo()..." bar += 1 print "in foo(), bar is", barif __name__ == '__main__' : print "in __main__, bar is", bar foo() print "/nin __main__, bar is (still)", bar

输出：

in __main__, bar is 100calling foo()...Traceback (most recent call last): File "/home/zhangjun/workspace/try/src/try.py", line 10, in <module> foo() File "/home/zhangjun/workspace/try/src/try.py", line 5, in foo bar += 1UnboundLocalError: local variable 'bar' referenced before assignment

这里在foo()函数内部，对bar变量值进行修改，发现修改的其实被视为一个局部变量。

code 4:

bar = 100def foo(): print "/ncalling foo()..." global bar bar += 1 print "in foo(), bar is", barif __name__ == '__main__' : print "in __main__, bar is", bar foo() print "/nin __main__, bar is (still)", bar

输出：

in __main__, bar is 100calling foo()...in foo(), bar is 101in __main__, bar is (still) 101

此时，在foo()函数内，我们首先对变量bar进行global声明，这样就可以顺利修改这个全局变量了。

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