Don't understand why UnboundLocalError occurs (closure)

python scope closures global-variables
Randomblue picture Randomblue · Feb 13, 2012 · Viewed 203.9k times · Source

What am I doing wrong here?

counter = 0

def increment():
  counter += 1

increment()

The above code throws an UnboundLocalError.

Answer

Sven Marnach picture Sven Marnach · Feb 13, 2012

Python doesn't have variable declarations, so it has to figure out the scope of variables itself. It does so by a simple rule: If there is an assignment to a variable inside a function, that variable is considered local.[1] Thus, the line

counter += 1

implicitly makes counter local to increment(). Trying to execute this line, though, will try to read the value of the local variable counter before it is assigned, resulting in an UnboundLocalError.[2]

If counter is a global variable, the global keyword will help. If increment() is a local function and counter a local variable, you can use nonlocal in Python 3.x.

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