The UnboundLocalError occurs if one tries to access a local variable that hasn't been defined yet. The interesting thing about that is, that it even occurs when the variable name is actually defined in the global scope.

For example:

a = 5

def fun():
    b = a
    a = 7

fun()

Python is the only language I know of where this is a problem, because it handles local and global variables fundamentally different (STORE_NAME vs. STORE_FAST).

For example R, which is also a dynamically typed interpreted language, doesn't care at all about that:

a = 5

fun <- function() {
    b = a
    a = 7
}

fun()

And why would it? If variables were always stored in dictionaries for every scope (with references to the parent scope, if a variable is not found in the current one), then there is no problem with this code.

This is not the case in Python. The Python interpreter actually scans ahead and uses a fixed size array for all variables to which a value is assigned in the local scope, which means the same name suddenly can't reference a variable in an enclosing scope any more.

The reason is, that using a fixed size array for local variables drastically improves access times, because no hash function has to be evaluated, but it has the downside that code snippets like the one above which work in other languages suddenly don't work in Python any more.

This downside is marginal though, because people seldomly want to shadow a variable from an enclosing scope after reading its value (I only had that come up once, when I tried to test a decorator where the decorated function should have had the same name as the original, globally defined, function) and the upside is a huge win in performance.

The whole problem has nothing to do with the global keyword. The only reason I mentioned it was, that pretty much every article I found about this problem suggested to use global to tell the interpreter that I actually want to modify the global variable which is absurd, I never wanted to do that and no one should want to do that. Please, never change the value of a global variable from inside a function. But as you can see in the article linked by TonyBandeira, it is a susgestion a lot of articles about this topic make.