Python Arena
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Everything is an object

This module starts the second half of the course, which the MOOC calls the Advanced Course in Programming. Its big idea is object-oriented programming: building programs out of objects that keep data together with the operations on that data. You have been using objects all along without calling them that.

Data that belongs together

When several values describe one thing, you have kept them together in a dictionary (or a tuple): a pet's name, species and age, for example. Here are two pets:

pet1 = {"name": "Biscuit", "species": "cat", "age": 4}
pet2 = {"name": "Pepper", "species": "dog", "age": 2}

pet1["age"] += 1

print(pet1["name"], pet1["age"])
print(pet2["name"], pet2["age"])
Output
Biscuit 5
Pepper 2

Each dictionary is an object: a whole of its own in the computer's memory, holding data that belongs together. The two dictionaries have the same keys, but they are separate objects, so changing one doesn't change the other. This is what programmers mean when they say that objects are independent.

Every value is an object

In Python, every value is an object: numbers, strings, lists, dictionaries, even True and None. A variable doesn't hold the object itself but a reference to it, as you saw in the References module, where two variables referred to the same list.

Every object has a type, and the function type tells you which:

In the Python interpreter
>>> type(42)
<class 'int'>
>>> type("Biscuit")
<class 'str'>
>>> type([4, 2])
<class 'list'>
>>> type({"name": "Pepper"})
<class 'dict'>

Python calls the type of an object its class. Classes are what this part of the course is about, and soon you'll write your own.

You met two more words in the References module. Lists and dictionaries are mutable: they can be changed after they are made. Numbers, strings and tuples are immutable: they can't. When a program seems to change a string, Python really makes a new string, and the variable then refers to the new one:

word = "cat"
other = word
word = word + "s"
print(word)
print(other)
Output
cats
cat

The string "cat" stays as it was, and other still refers to it.

Objects in and out of functions

A function can take objects as its arguments and return one of them: the whole object, with all its data.

def older(pet1, pet2):
    if pet1["age"] >= pet2["age"]:
        return pet1
    return pet2


cat = {"name": "Biscuit", "species": "cat", "age": 4}
dog = {"name": "Pepper", "species": "dog", "age": 2}

winner = older(cat, dog)
print(winner["name"])
print(winner)
Output
Biscuit
{'name': 'Biscuit', 'species': 'cat', 'age': 4}

The caller gets the whole dictionary back and can use any of its data, not just the name.

Your task

Challenge 1 of 3

What does this program print? Type each line it prints.

Challenge 2

Write a function oldest(pets) that takes a list of pet dictionaries, each with the keys "name", "species" and "age", and returns the whole dictionary of the oldest pet. When two pets are equally old, return the one that comes first in the list. The list always has at least one pet.

For example, with the pets in the starter code, oldest(pets) returns {'name': 'Rex', 'species': 'dog', 'age': 9}.

Challenge 3

Write two functions:

  • new_pet(name, species) creates and returns a new pet dictionary with the keys "name", "species" and "age". A new pet's age is 0.
  • birthday(pet) adds one to the age of the pet it is given. It returns nothing: it changes the dictionary itself.

This code:

cat = new_pet("Biscuit", "cat")
dog = new_pet("Rex", "dog")
birthday(cat)
birthday(cat)
print(cat)
print(dog)

prints

{'name': 'Biscuit', 'species': 'cat', 'age': 2}
{'name': 'Rex', 'species': 'dog', 'age': 0}
Next: Methods belong to objects