Getting started

This chapter introduces some core concepts of mypy, including function annotations, the typing module and library stubs. Read it carefully, as the rest of documentation may not make much sense otherwise.

Installing mypy

Mypy requires Python 3.4 or later to run. Once you’ve installed Python 3, you can install mypy using pip:

python3 -m pip install mypy

Note that even though you need Python 3 to run mypy, type checking Python 2 code is fully supported, as discussed later in Type checking Python 2 code.

Running mypy

You can type check a program by using the mypy tool, which is basically a linter – it checks your program for errors without actually running it:

$ mypy

All errors reported by mypy are essentially warnings that you are free to ignore, if you so wish.


Depending on how mypy is configured, you may have to run mypy like this:

$ python3 -m mypy

If you haven’t added any type annotations to your program yet, you should add some first, as mypy won’t report many errors in unannotated functions. Don’t worry if you aren’t familiar with type annotations – we’ll discuss them in detail in much of the rest of this guide.

Mypy supports many command line options that you can use to tweak how mypy behaves. They are documented in The mypy command line.

Function signatures

A function without a type annotation is considered dynamically typed by mypy:

def greeting(name):
    return 'Hello, {}'.format(name)

You can declare the signature of a function using the Python 3 annotation syntax (Python 2 is discussed later in Type checking Python 2 code). This makes the function statically typed, which causes mypy to report type errors within the function.

Here’s a version of the above function that is statically typed and will be type checked:

def greeting(name: str) -> str:
    return 'Hello, {}'.format(name)

Now mypy will reject the following call, since the argument has an incompatible type:

def greeting(name: str) -> str:
    return 'Hello, {}'.format(name)

greeting(b'Alice')  # Argument 1 to "greeting" has incompatible type "bytes"; expected "str"

If a function does not explicitly return a value we give the return type as None. Using a None result in a statically typed context results in a type check error:

def p() -> None:

a = p()  # Error: "p" does not return a value

Arguments with default values can be annotated as follows:

def greeting(name: str, excited: bool = False) -> str:
    message = 'Hello, {}'.format(name)
    if excited:
        message += '!!!'
    return message

Mixing dynamic and static typing within a single file is often useful. For example, if you are migrating existing Python code to static typing, it may be easiest to do this incrementally, such as by migrating a few functions at a time. Also, when prototyping a new feature, you may decide to first implement the relevant code using dynamic typing and only add type signatures later, when the code is more stable.

def f():
    1 + 'x'  # No static type error (dynamically typed)

def g() -> None:
    1 + 'x'  # Type check error (statically typed)


The earlier stages of mypy, known as the semantic analysis, may report errors even for dynamically typed functions. However, you should not rely on this, as this may change in the future.

The typing module

The typing module contains many definitions that are useful in statically typed code. You typically use from ... import to import them (we’ll explain Iterable later in this document):

from typing import Iterable

def greet_all(names: Iterable[str]) -> None:
    for name in names:
        print('Hello, {}'.format(name))

For brevity, we often omit the typing import in code examples, but mypy will give an error if you use definitions such as Iterable without first importing them.

Library stubs and typeshed

Mypy uses library stubs to type check code interacting with library modules, including the Python standard library. A library stub defines a skeleton of the public interface of the library, including classes, variables and functions, and their types. Mypy ships with stubs from the typeshed project, which contains library stubs for the Python builtins, the standard library, and selected third-party packages.

For example, consider this code:

x = chr(4)

Without a library stub, mypy would have no way of inferring the type of x and checking that the argument to chr has a valid type.

Mypy complains if it can’t find a stub (or a real module) for a library module that you import. Some modules ship with stubs that mypy can automatically find, or you can install a 3rd party module with additional stubs (see Using installed packages for details). You can also create stubs easily. We discuss ways of silencing complaints about missing stubs in Using mypy with an existing codebase.

Next steps

If you are in a hurry and don’t want to read lots of documentation before getting started, here are some pointers to quick learning resources:

You can also continue reading this document and skip sections that aren’t relevant for you. You don’t need to read sections in order.