When a class or function is defined in a parent function or method, it is only visible in this parent function or method's scope. If the defined class or function is not used within this scope it is dead code, i.e. unnecessary, inoperative code that should be removed. Cleaning out dead code decreases the size of the maintained codebase, making it easier to understand the program and preventing bugs from being introduced.
def noncompliant():
def nested_function(): # Noncompliant
print("nested_function")
class NestedClass: # Noncompliant
def __init__(self):
print("NestedClass")
def compliant():
def nested_function():
print("nested_function")
class NestedClass:
def __init__(self):
print("NestedClass")
nested_function()
NestedClass()