Double-underscore methods, or "dunder" methods, are special methods defined in a class that are invoked implicitly. Like the name suggests, they are prefixed and suffixed with dunders. You've probably already seen some, such as the __init__ dunder method, also known as the "constructor" of a class, which is implicitly invoked when you instantiate an instance of a class.
When you create a new class, there will be default dunder methods inherited from the object class. However, we can override them by redefining these methods within the new class. For example, the default __init__ method from object doesn't take any arguments, so we almost always override that to fit our needs.
Other common dunder methods to override are __str__ and __repr__. __repr__ is the developer-friendly string representation of an object - usually the syntax to recreate it - and is implicitly called on arguments passed into the repr function. __str__ is the user-friendly string representation of an object, and is called by the str function. Note here that, if not overriden, the default __str__ invokes __repr__ as a fallback.
class Foo:
def __init__(self, value): # constructor
self.value = value
def __str__(self):
return f"This is a Foo object, with a value of {self.value}!" # string representation
def __repr__(self):
return f"Foo({self.value!r})" # way to recreate this object
bar = Foo(5)
# print also implicitly calls __str__
print(bar) # Output: This is a Foo object, with a value of 5!
# dev-friendly representation
print(repr(bar)) # Output: Foo(5)Another example: did you know that when you use the <left operand> + <right operand> syntax, you're implicitly calling <left operand>.__add__(<right operand>)? The same applies to other operators, and you can look at the operator built-in module documentation for more information!