Mypy syntax cheat sheet (Python 3)

This document is a quick cheat sheet showing how the PEP 484 type language represents various common types in Python 3. Unless otherwise noted, the syntax is valid on all versions of Python 3.


Technically many of the type annotations shown below are redundant, because mypy can derive them from the type of the expression. So many of the examples have a dual purpose: show how to write the annotation, and show the inferred types.

Built-in types

from typing import List, Set, Dict, Tuple, Text, Optional, AnyStr

# For simple built-in types, just use the name of the type.
x = 1  # type: int
x = 1.0  # type: float
x = True  # type: bool
x = "test"  # type: str
x = u"test"  # type: str
x = b"test"  # type: bytes

# For collections, the name of the type is capitalized, and the
# name of the type inside the collection is in brackets.
x = [1]  # type: List[int]
x = {6, 7}  # type: Set[int]

# For mappings, we need the types of both keys and values.
x = {'field': 2.0}  # type: Dict[str, float]

# For tuples, we specify the types of all the elements.
x = (3, "yes", 7.5)  # type: Tuple[int, str, float]

# For textual data, use Text.
# This is `unicode` in Python 2 and `str` in Python 3.
x = ["string", u"unicode"]  # type: List[Text]

# Use Optional for values that could be None.
input_str = f()  # type: Optional[str]
if input_str is not None:


Python 3 introduces an annotation syntax for function declarations in PEP 3107.

from typing import Callable, Iterable, Union, Optional, List

# This is how you annotate a function definition.
def stringify(num: int) -> str:
    return str(num)

# And here's how you specify multiple arguments.
def plus(num1: int, num2: int) -> int:
    return num1 + num2

# Add type annotations for kwargs as though they were positional args.
def f(num1: int, my_float: float = 3.5) -> float:
    return num1 + my_float

# An argument can be declared positional-only by giving it a name
# starting with two underscores:
def quux(__x: int) -> None:
quux(3)  # Fine
quux(__x=3)  # Error

# This is how you annotate a function value.
x = f # type: Callable[[int, float], float]

# A generator function that yields ints is secretly just a function that
# returns an iterable (see below) of ints, so that's how we annotate it.
def f(n: int) -> Iterable[int]:
    i = 0
    while i < n:
        yield i
        i += 1

# For a function with many arguments, you can of course split it over multiple lines
def send_email(address: Union[str, List[str]],
               sender: str,
               cc: Optional[List[str]],
               bcc: Optional[List[str]],
               body: List[str] = None
               ) -> bool:


When you’re puzzled or when things are complicated

from typing import Union, Any, List, cast

# To find out what type mypy infers for an expression anywhere in
# your program, wrap it in reveal_type.  Mypy will print an error
# message with the type; remove it again before running the code.
reveal_type(1)  # -> error: Revealed type is ''

# Use Union when something could be one of a few types.
x = [3, 5, "test", "fun"]  # type: List[Union[int, str]]

# Use Any if you don't know the type of something or it's too
# dynamic to write a type for.
x = mystery_function()  # type: Any

# Use `ignore` to suppress type-checking on a given line, when your
# code confuses mypy or runs into an outright bug in mypy.
# Good practice is to comment every `ignore` with a bug link
# (in mypy, typeshed, or your own code) or an explanation of the issue.
x = confusing_function()  # type: ignore #

# cast is a helper function for mypy that allows for guidance of how to convert types.
# it does not cast at runtime
a = [4]
b = cast(List[int], a)  # passes fine
c = cast(List[str], a)  # passes fine (no runtime check)
reveal_type(c)  # -> error: Revealed type is 'builtins.list[builtins.str]'
print(c)  # -> [4] the object is not cast

# TODO: explain "Need type annotation for variable" when
# initializing with None or an empty container

Standard duck types

In typical Python code, many functions that can take a list or a dict as an argument only need their argument to be somehow “list-like” or “dict-like”. A specific meaning of “list-like” or “dict-like” (or something-else-like) is called a “duck type”, and several duck types that are common in idiomatic Python are standardized.

from typing import Mapping, MutableMapping, Sequence, Iterable, List, Set

# Use Iterable for generic iterables (anything usable in `for`),
# and Sequence where a sequence (supporting `len` and `__getitem__`) is required.
def f(iterable_of_ints: Iterable[int]) -> List[str]:
    return [str(x) for x in iterable_of_ints]
f(range(1, 3))

# Mapping describes a dict-like object (with `__getitem__`) that we won't mutate,
# and MutableMapping one (with `__setitem__`) that we might.
def f(my_dict: Mapping[int, str])-> List[int]:
    return list(my_dict.keys())
f({3: 'yes', 4: 'no'})
def f(my_mapping: MutableMapping[int, str]) -> Set[str]:
    my_mapping[5] = 'maybe'
    return set(my_mapping.values())
f({3: 'yes', 4: 'no'})


class MyClass:
    # The __init__ method doesn't return anything, so it gets return
    # type None just like any other method that doesn't return anything.
    def __init__(self) -> None:
    # For instance methods, omit `self`.
    def my_method(self, num: int, str1: str) -> str:
        return num * str1

# User-defined classes are written with just their own names.
x = MyClass() # type: MyClass

Other stuff

import sys
import re
# typing.Match describes regex matches from the re module.
from typing import Match, AnyStr, IO
x = re.match(r'[0-9]+', "15")  # type: Match[str]

# You can use AnyStr to indicate that any string type will work
# but not to mix types
def full_name(first: AnyStr, last: AnyStr) -> AnyStr:
    return first+last
full_name('Jon','Doe')  # same str ok
full_name(b'Bill', b'Bit')  # same binary ok
full_name(b'Terry', 'Trouble')  # different str types, fails

# Use IO[] for functions that should accept or return any
# object that comes from an open() call. The IO[] does not
# distinguish between reading, writing or other modes.
def get_sys_IO(mode='w') -> IO[str]:
    if mode == 'w':
        return sys.stdout
    elif mode == 'r':
        return sys.stdin
        return sys.stdout

# forward references are useful if you want to referemce a class before it is designed

def f(foo: A) -> int:  # this will fail

class A:

# however, using the string 'A', it will pass as long as there is a class of that name later on
def f(foo: 'A') -> int:

# TODO: add TypeVar and a simple generic function

Variable Annotation in Python 3.6 with PEP 526

Python 3.6 brings new syntax for annotating variables with PEP 526. Mypy brings limited support for PEP 526 annotations.

# annotation is similar to arguments to functions
name: str = "Eric Idle"

# class instances can be annotated as follows
mc : MyClass = MyClass()

# tuple packing can be done as follows
tu: Tuple[str, ...] = ('a', 'b', 'c')

# annotations are not checked at runtime
year: int = '1972'  # error in type checking, but works at runtime

# these are all equivalent
hour = 24 # type: int
hour: int; hour = 24
hour: int = 24

# you do not (!) need to initialize a variable to annotate it
a: int # ok for type checking and runtime

# which is useful in conditional branches
child: bool
if age < 18:
    child = True
    child = False

# annotations for classes are for instance variables (those created in __init__ or __new__)
class Battery:
    charge_percent: int = 100  # this is an instance variable with a default value
    capacity: int  # an instance variable without a default

# you can use the ClassVar annotation to make the variable a class variable instead of an instance variable.
class Car:
    seats: ClassVar[int] = 4
    passengers: ClassVar[List[str]]

 # You can also declare the type of an attribute in __init__
 class Box:
     def __init__(self) -> None:
         self.items: List[str] = []

Please see New features in Python 3.6 for more on mypy’s compatability with Python 3.6’s new features.