Programming Fundamentals › Programming Basics

Null / None / nil

A value meaning "nothing here", and the source of countless crashes.

Also known as: None, nil, NULL, null pointer

Null (called None in Python, nil in Ruby and Go, null in JavaScript and SQL) is a value that means “there is no value here”. A user with no middle name, a search with no result, a field not filled in yet.

The problem is that it looks like any other value until you use it:

user = find_user(42)        # returns None when there's no such user
print(user.name)            # AttributeError: 'NoneType' object has no attribute 'name'

const user = findUser(42);  // null
console.log(user.name);     // TypeError: Cannot read properties of null

That crash, in one form or another, is one of the most common bugs there is.

How to handle it

• Check before you use it, at the point where nothing is a normal outcome:

user = find_user(42)
if user is None:
    return not_found()

• Use is None, not == False or if not user, when 0, "" or an empty list are valid values you don’t want to treat as “missing”.
• Decide what a function returns when there’s no answer, and document it. Returning an empty list is often safer than returning null.
• In JavaScript, optional chaining (user?.name) avoids the crash, but don’t use it to hide values that should never be missing. Also see how null differs from undefined.
• In SQL, NULL never equals anything, not even itself. Write WHERE x IS NULL, not WHERE x = NULL.

Null is not zero

0, "" and [] are real values. Null means no value at all. Mixing them up leads to bugs like treating a missing price as free.

Some languages make absence explicit in the type system (see the option type), so the compiler makes you handle it.