Python全局变量

Question

def say_boo_twice():
  global boo
  boo = 'Boo!'
  print boo, boo

boo = 'boo boo'
say_boo_twice()

输出是

Boo! Boo!

不是我想的那样。既然我将boo声明为全局，为什么输出不：

boo boo boo boo

Answer 1

在将boo声明为global之后，您将重新分配它，因此该值是最后一个分配给它的值。如果删除第三行，将得到预期的输出。

Answer 2

因为你要提前重新分配。注释掉boo = 'Boo!'，您将得到所描述的内容。

def say_boo_twice():
   global boo
   #boo = 'Boo!'
   print boo, boo  

boo = 'boo boo' 
say_boo_twice() 

而且global boo是不必要的，boo已经在全局范围内。 这就是global起作用的地方

def say_boo_twice():   
   global boo
   boo = 'Boo!'
   print boo, boo  

say_boo_twice() 
print "outside the function: " + boo #works

鉴于：

def say_boo_twice():   
   #global boo
   boo = 'Boo!'
   print boo, boo  

say_boo_twice() 
print "outside the function: " + boo # ERROR.  boo is only known inside function, not to this scope

Answer 3

你已经改变了函数内部的boo，为什么它不改变呢？而且，全局变量也不好。