What Causes UnboundLocalError in Python Functions?

Unbound Local Variables in Python

In Python, an unbound local variable refers to a situation where a variable is used within a function but has not been assigned a value. This can occur when the variable is declared but not assigned, or when the assignment statement is unreachable within the function.

When a variable is not bound, an UnboundLocalError exception is raised. This is a subclass of NameError, indicating that the variable name was not found. However, unlike NameError, UnboundLocalError specifically refers to local variables that have not been bound.

The Python interpreter checks for unbound local variables at the time of name resolution. This means that even if a variable is declared in a function, it will still raise an UnboundLocalError if it is used before it is bound.

For example:

def my_function():
    print(variable)  # Raises UnboundLocalError
    variable = "Hello"

In this example, the variable variable is declared but not assigned when it is used in the print statement. Thus, the interpreter raises an UnboundLocalError.

To resolve this issue, ensure that the variable is assigned a value before it is used. This can be done by moving the assignment statement to the beginning of the function, or by using a default value for the variable.

It's important to note that Python does not have declarations for variables. Instead, variables are created when they are first assigned a value. This means that the order of assignment and usage is crucial to avoid UnboundLocalError exceptions.

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