Do not assign a lambda expression, use a def (E731)

Lambdas should not be assigned to a variable. Instead, they should be defined as functions.

The primary reason for this is debugging. Lambdas show as <lambda> in tracebacks, where functions will display the function’s name.

Anti-pattern

root = lambda folder_name: os.path.join(BASE_DIR, folder_name)

Best practice

def root(folder_name):
    return os.path.join(BASE_DIR, folder_name)

Additional links

• https://www.python.org/dev/peps/pep-0008/#programming-recommendations