This Python Magic Methods lesson is for a beginner. Read the short notes, run each example, then change one name or number and run it again.
Methods with two underscores on each side are hooks Python calls for you. __str__ shapes what print shows. __eq__ shapes ==.
Add one when your objects should work with a built-in operation. Do not invent new double-underscore names.
Example 1: Start here
__str__ is for people reading output. __repr__ is for a developer reading a log.
class Score: # a template for objects
def __init__(self, value): # runs when you create the object
self.value = value # store this on the object
def __str__(self): # define the function; it runs when you call it
return str(self.value) + " marks" # send this value back to the caller
def __eq__(self, other): # define the function; it runs when you call it
return self.value == other.value # send this value back to the caller
print(Score(82))
print(Score(82) == Score(82)) # a comparison is True or False
Example 2: A readable object
__str__ decides what print shows.
class Course: # a template for objects
def __init__(self, title): # runs when you create the object
self.title = title # store this on the object
def __str__(self): # define the function; it runs when you call it
return "Course: " + self.title # send this value back to the caller
print(Course("Python"))
Example 3: Length
__len__ lets the built-in len work on your object.
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(["Asha", "Arun"]))) # len counts the items
Example 4: Equal values
__eq__ defines what == means for this class.
class Score: # a template for objects
def __init__(self, value): # runs when you create the object
self.value = value # store this on the object
def __eq__(self, other): # define the function; it runs when you call it
return self.value == other.value # send this value back to the caller
print(Score(82) == Score(82)) # a comparison is True or False
__str__ is for people reading output. __repr__ is for a developer reading a log.
