Handling UnboundLocalError

UnboundLocalError is an exception that occurs when you try to reference a local variable that has not yet been assigned.

In Python, a variable declared inside a function is considered a local variable. Even if a variable with the same name is declared outside the function, it is considered a local variable within the function.

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counter = 0  # Global variable

def increase_counter():
    # UnboundLocalError occurs
    counter += 1  
    return counter

print(increase_counter())

In the example above, the counter variable declared outside the function is global, but inside the increase_counter function, it is treated as a separate local variable.

Therefore, when attempting to reference the counter variable inside the function, an UnboundLocalError is raised because the local variable is used before it has been assigned a value.

To use the counter variable as a global variable in this example, you should use the global keyword.

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counter = 0  # Global variable

def increase_counter():
    # Use as a global variable
    global counter
    counter += 1
    return counter

print(increase_counter())

In the code above, the global keyword is used to declare the counter variable inside the increase_counter function as a global variable.

One of the most common ways to handle an UnboundLocalError exception is to use a try-except block.

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counter = 0  # Global variable

def increase_counter():
    try:
        counter += 1
    except UnboundLocalError as e:
        print(f'UnboundLocalError occurred: {e}')
    return counter

print(increase_counter())

In the code above, when an UnboundLocalError exception occurs while referencing the counter variable inside the increase_counter function, the except block handles the exception and prints a message.

The UnboundLocalError often results from confusion between local and global variables, so it is important to reference and modify variables carefully within functions.