从词汇范围界定的Google Style Guide个例子中:
嵌套的Python函数可以引用封闭函数中定义的变量
这两个问题一开始似乎都得到了验证:
# Reference
def toplevel():
a = 5
def nested():
print(a + 2)
nested()
return a
toplevel()
7
Out[]: 5
# Assignment
def toplevel():
a = 5
def nested():
a = 7 # a is still 5, can't modify enclosing scope variable
nested()
return a
toplevel()
Out[]: 5
那么,为什么嵌套函数中引用和赋值的组合会导致异常呢?
# Reference and assignment
def toplevel():
a = 5
def nested():
print(a + 2)
a = 7
nested()
return a
toplevel()
# UnboundLocalError: local variable 'a' referenced before assignment