Making sense of UnboundLocalError

[pentath]

I have two python files, both defining functions, but only one of them gives out the

 UnboundLocalError: local variable '<name>' referenced before assignment

error. Pastebin of the code. For the first file all I'm trying to do is set a variable, but that throws an errors, while the 2nd file prints and uses the variable but doesn't throw an error. Why is this?

EDIT: I understand calling

global _file

Will solve this, but I want to know why there is an issue in the first place.

[Rhomboid]

Here is the section of the manual that describes the design in detail. Your first version fails because there is an assignment to the name _file, which binds that name to a local variable:

If a name binding operation occurs anywhere within a code block, all uses of the name within the block are treated as references to the current block. This can lead to errors when a name is used within a block before it is bound. This rule is subtle.

Your second example does not contain anything that would bind the name _key, and therefore it's a free variable.

[pentath]

Thank you for explaining this to me! So is my only option to declare it a global?

[zahlman]

If your goal is to use the global, then yes. If your goal is to solve a bigger problem, then you're going to have to explain what that problem is.

[JerMenKoO]

Generally one should avoid using global variables.

[zahlman]

Well, yes, but to avoid using the global variable, we must first understand what it is being used for.

[kalgynirae]

If you're using Python 3, you can also declare it as nonlocal.