This Python __contains__ lesson is for a beginner. Read the short notes, run each example, then change one name or number and run it again.
__contains__ answers the in operator. Return True or False.
Without it, Python tries to iterate the object and compare each item.
Example 1: Start here
not in uses the same method and flips the answer.
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 __contains__(self, name): # define the function; it runs when you call it
return name in self.names # send this value back to the caller
print("Meera" in Batch(["Meera", "Arun"])) # in is True when the value is found
Example 2: in on your object
__contains__ answers the in operator.
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 __contains__(self, name): # define the function; it runs when you call it
return name in self.names # send this value back to the caller
print("Asha" in Roster(["Asha", "Arun"])) # in is True when the value is found
Example 3: A case-insensitive test
The method can normalize the text before it compares.
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 __contains__(self, name): # define the function; it runs when you call it
return name.lower() in [item.lower() for item in self.names] # send this value back to the caller
print("asha" in Roster(["Asha"])) # in is True when the value is found
Example 4: Without the method
Lists already support in. Write __contains__ only for your own class.
names = ["Asha", "Arun"] # a list; the values stay in this order
print("Asha" in names) # in is True when the value is found
not in uses the same method and flips the answer.
