如何在没有全局变量的情况下解决“ UnboundLocalError：赋值之前引用了本地变量'foo'”错误？

Question

Here is a simplified version of my code:

class foo:
    def __init__(self, thing):
        self.thing = thing

def bar():
    foo = foo('test')

bar()

And here's what it produces:

Traceback (most recent call last):
  File "filepath", line 8, in <module>
    bar()
  File "filepath", line 6, in bar
    foo = foo('test')
UnboundLocalError: local variable 'foo' referenced before assignment

I have a two part question regarding this.

Firstly, how can I get around this ? After some Googling I found that adding a global foo before the foo = foo('test') line does the trick. But the general consensus seems to be that global variables are bad things to be avoided, so is there a way around this that doesn't use global variables?

And secondly, why is this actually happening? I understand about functions only being able to access variables created inside that function (and global variables of course). But I'm not trying to access a variable that was defined outside of the function, I'm just creating a new one. Logically speaking, why is it forbidden to create a new object inside a function (unless you declare it as global first)?

Answer 1

As soon as the parser sees foo = ... , it marks foo as a local variable. Once that happens, your attempt to call foo('test') fails, because the local variable foo doesn't refer to anything yet, let alone a callable object.

You're going to have to pick a different name for the object you are creating.

Answer 2

A variable in Python is always in one scope.

You try to assign to a local variable foo while reading a global variable foo .

When you assign to a variable that isn't declared with the global keyword, it is assumed to be a local variable. So when you write foo = foo() Python assumes that foo is local and then you get the error, that you a reading foo before you wrote to it.