This Python __len__ lesson is for a beginner. Read the short notes, run each example, then change one name or number and run it again.
__len__ lets len work on your object. Return a non-negative integer.
An empty collection should return 0, not None.
Example 1: Start here
bool uses __len__ when __bool__ is missing. A length of 0 is treated as false.
class Batch: # a template for objects
def __init__(self, names): # runs when you create the object
self.names = names # store this on the object
def __len__(self): # define the function; it runs when you call it
return len(self.names) # send this value back to the caller
print(len(Batch(["Meera", "Arun"]))) # len counts the items
Example 2: len on a class
__len__ must return a non-negative integer.
class Roster: # a template for objects
def __init__(self, names): # runs when you create the object
self.names = names # store this on the object
def __len__(self): # define the function; it runs when you call it
return len(self.names) # send this value back to the caller
print(len(Roster(["Asha", "Arun"]))) # len counts the items
Example 3: An empty roster
Zero is a valid length.
class Roster: # a template for objects
def __init__(self): # runs when you create the object
self.names = [] # store this on the object
def __len__(self): # define the function; it runs when you call it
return len(self.names) # send this value back to the caller
print(len(Roster())) # len counts the items
Example 4: Use it in an if
A container is true in an if when its length is not zero.
class Roster: # a template for objects
def __init__(self, names): # runs when you create the object
self.names = names # store this on the object
def __len__(self): # define the function; it runs when you call it
return len(self.names) # send this value back to the caller
if Roster(["Asha"]): # run the indented lines only when this is true
print("has students")
bool uses __len__ when __bool__ is missing. A length of 0 is treated as false.
