Why Do I Get an UnboundLocalError in Python?

UnboundLocalError: Understanding Unbound Variables in Python

In Python, an UnboundLocalError occurs when a local variable is referenced before it has been assigned a value. Unlike other programming languages, Python doesn't require explicit variable declarations. Instead, variables are bound to values when assigned.

One way to trigger an UnboundLocalError is by accessing an unassigned variable:

>>> foobar
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foobar' is not defined

Another way is when an assignment operation fails to execute, such as within a conditional block:

def foo():
    if False:
        spam = 'eggs'
    print(spam)

>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in foo
UnboundLocalError: local variable 'spam' referenced before assignment

In Python, names are bound to values through various operations: assignment, function parameters, import statements, exception handlers, and context managers. When a name is bound in a function scope, it becomes a local variable. To access a global variable within a function, a global or nonlocal statement must be used (in Python 3).

For example, the following function attempts to access a global variable foo but fails because it's bound within the function scope:

foo = None
def bar():
    if False:
        foo = 'spam'
    print(foo)

>>> bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in bar
UnboundLocalError: local variable 'foo' referenced before assignment

However, using global foo fixes the issue:

foo = None
def bar():
    global foo
    if False:
        foo = 'spam'
    print(foo)

>>> bar()
None

Understanding the concept of variable binding is crucial to avoid UnboundLocalErrors in Python.

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