Python Variables Explained

Variables Inside Functions

Every function has its own local namespace. Names created inside a function exist only for the duration of that function call. If a function needs to read a global variable, Python finds it via LEGB without any declaration. If a function needs to rebind a global variable, it must declare it with global. Nested functions that rebind an enclosing variable use nonlocal.

copycount = 0   # global

def increment_bad():
    # This READS count fine...
    # But assigning to count here creates a LOCAL name, shadowing the global
    # Uncommenting the next line would raise UnboundLocalError:
    # count += 1   # Python sees the assignment and treats count as local
    pass

def increment_good():
    global count   # declare: 'count' here refers to the global
    count += 1

print(f"count before: {count}")
increment_good()
increment_good()
print(f"count after:  {count}")

# nonlocal — rebind a name in the nearest enclosing (non-global) scope
def make_counter():
    value = 0

    def step():
        nonlocal value   # refers to 'value' in make_counter's scope
        value += 1
        return value

    return step

counter = make_counter()
print(f"\ncounter() = {counter()}")
print(f"counter() = {counter()}")
print(f"counter() = {counter()}")

# UnboundLocalError — the classic scope surprise
total = 100

def broken():
    # Because there is an assignment to 'total' later in this function,
    # Python marks 'total' as local for the ENTIRE function body.
    # Reading it before the assignment therefore raises UnboundLocalError.
    try:
        print(total)     # ERROR — Python already decided 'total' is local
    except UnboundLocalError as e:
        print(f"UnboundLocalError: {e}")
    total = 200          # this assignment is what caused the problem

broken()
print(f"global total unchanged: {total}")

copy# del removes a NAME, not necessarily the object

x = [1, 2, 3]
y = x            # y points at the same object

del x            # removes the name 'x'

try:
    print(x)     # NameError — 'x' no longer exists
except NameError as e:
    print(f"NameError: {e}")

print(f"y still exists: {y}")   # object is alive via 'y'

# del on a list element or slice
nums = [10, 20, 30, 40, 50]
del nums[1]         # removes element at index 1
print(f"\nAfter del nums[1] : {nums}")

del nums[1:3]       # removes elements at indices 1 and 2
print(f"After del nums[1:3]: {nums}")

# del on a dict key
config = {"host": "localhost", "port": 8080, "debug": True}
del config["debug"]
print(f"\nAfter del config['debug']: {config}")

# Checking existence before access
data = {"name": "Alice"}
key  = "age"
if key in data:
    print(data[key])
else:
    print(f"Key {key!r} not present — use .get() for safe access")

print(f"Safe access: {data.get('age', 'unknown')}")