Python全局变量与global语句

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 = 100 def foo(): print "\ncalling foo()..." bar = 200 print "in foo(), bar is", bar print "in __main__, bar is", bar foo() print "\nin __main__, bar is (still)", bar

输出：

in __main__, bar is 100 calling foo()... in foo(), bar is 200 in __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 = 100 def foo(): print "\ncalling foo()..." #bar = 200 print "in foo(), bar is", bar if __name__ == '__main__' : print "in __main__, bar is", bar foo() print "\nin __main__, bar is (still)", bar

输出：

in __main__, bar is 100 calling foo()... in foo(), bar is 100 in __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 = 100 def foo(): print "\ncalling foo()..." bar += 1 print "in foo(), bar is", bar if __name__ == '__main__' : print "in __main__, bar is", bar foo() print "\nin __main__, bar is (still)", bar

输出：

in __main__, bar is 100 calling 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 += 1 UnboundLocalError: 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 = 100 def foo(): print "\ncalling foo()..." global bar bar += 1 print "in foo(), bar is", bar if __name__ == '__main__' : print "in __main__, bar is", bar foo() print "\nin __main__, bar is (still)", bar

输出：

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