python - 嵌套模块重新导入时出现 UnboundLocalError

Question

当我在 python 2.7 中重新导入一个已经导入的模块时，我得到一个 UnboundLocalError。一个最小的例子是

#!/usr/bin/python

import sys

def foo():
    print sys
    import sys

foo()

Traceback (most recent call last):
  File "./ptest.py", line 9, in <module>
    foo()
  File "./ptest.py", line 6, in foo
    print sys
UnboundLocalError: local variable 'sys' referenced before assignment

然而，当嵌套导入作为函数定义中的第一条语句放置时，一切正常:

#!/usr/bin/python

import sys

def foo():
    import sys
    print sys

foo()

<module 'sys' (built-in)>

有人可以解释为什么第一个脚本失败吗？谢谢。

Answer 1

这与引用全局变量相同。在 Python FAQ 中有很好的解释

This is because when you make an assignment to a variable in a scope, that variable becomes local to that scope and shadows any similarly named variable in the outer scope. Since the last statement in foo assigns a new value to x, the compiler recognizes it as a local variable. Consequently when the earlier print(x) attempts to print the uninitialized local variable and an error results.