Understanding error messages

Let’s look at a (fictional) part of our pet shelter program:

def add_pet(pets, name, age, species):
    pet = {"name": name, "age": age, "species": species, "fed": False}
    pets.append(pet)
add_pet(my_pets, "Zia", 2, "cat")

Oops. We get an error:

Traceback (most recent call last):
  File "shelter.py", line 5, in <module>
    add_pet(my_pets, "Zia", 2, "cat")
NameError: name 'my_pets' is not defined

Let’s decode this traceback:

• File "shelter.py" tells us which file has the error.
• line 5 is the line where the error occurred.
• NameError is the type of error.
• name 'my_pets' is not defined tells us where we have the actual problem.

Now we know: we forgot to define my_pets before using it. This means that the my_pets variable doesn’t exist. In order to be able...