UnboundLocalError local variable 'index' referenced before assignment

The Problem

The exception UnboundLocalError: local variable 'index' referenced before assignment happens in Python when you use a global variable in a function that also defines a local version of the same variable.

index = 0

def foo():
    if index == 0:
        print("ZERO")

    index = 1

foo()

Because index is defined globally as well as inside the foo() function, Python throws an exception if you try to use the variable inside foo before it’s declared.

The Solution

In this case either move the index declaration up higher in the foo function, or rename the variable to avoid the conflict.

index = 0

def foo():
    if findex == 0:
        print("ZERO")

    findex = 1

foo()

Further Reading

If you’re looking to get a deeper understanding of how Python application monitoring works, take a look at the following articles:

Expert

Evan Hicks

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