What is polymorphism in Python?

Polymorphism is the ability of objects from different classes to respond differently to the same method call.

Polymorphism through method overriding:

class Cat:
    def speak(self):
        return "Meow!"

class Dog:
    def speak(self):
        return "Woof!"

class Duck:
    def speak(self):
        return "Quack!"

# Same interface, different behavior
animals = [Cat(), Dog(), Duck()]
for animal in animals:
    print(animal.speak())

Duck Typing:

"If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck."

Python does not check the object's type — what matters is the presence of the required method:

class File:
    def read(self):
        return "data from a file"

class Database:
    def read(self):
        return "data from a DB"

def load_data(source):
    # The type doesn't matter, only that it has a 'read' method
    return source.read()

print(load_data(File()))      # data from a file
print(load_data(Database()))  # data from a DB

Polymorphism of built-in functions:

# len() works with different types
print(len("Python"))    # 6
print(len([1, 2, 3]))   # 3
print(len({"a": 1}))    # 1

# + behaves differently
print(1 + 2)           # 3 (addition)
print("Hello, " + "world!")  # Hello, world! (concatenation)