global.md

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。