Why Does `counter = 1` Inside a Closure Cause an `UnboundLocalError` in Python?

Understanding the Unbound Local Variable Error in Closures

When writing code using closures, it is crucial to consider the scope of variables. In this particular case, the question arises as to why an UnboundLocalError occurs in the following code:

counter = 0

def increment():
  counter += 1

increment()

To answer this question, we must first understand the concept of a closure in Python. A closure is a function that retains access to the variables defined in the scope in which it was defined, even after the scope has been exited. In this case, the counter variable is defined in the global scope and is referenced within the increment() function, which is considered a closure.

However, in Python, variables within a function are automatically treated as local variables unless explicitly declared otherwise. When the line counter = 1 is executed within increment(), the interpreter attempts to increment the local counter variable. However, since no value has been assigned to counter within the increment() function, it remains unbound. This results in the UnboundLocalError being raised.

To resolve this issue, there are two main approaches:

1. Using the global Keyword: If the intent is for counter to be a global variable, the global keyword can be used within the increment() function to explicitly declare that counter should reference the global variable rather than a local variable with the same name.

counter = 0

def increment():
  global counter
  counter += 1

increment()

2. Using nonlocal (Python 3.x only): In Python 3.x, the nonlocal keyword can be used within a nested function to declare that a variable should be treated as non-local, meaning it can access a variable with the same name in the enclosing scope.

counter = 0

def increment():
  nonlocal counter
  counter += 1

increment()

The above is the detailed content of Why Does `counter = 1` Inside a Closure Cause an `UnboundLocalError` in Python?. For more information, please follow other related articles on the PHP Chinese website!