Type hints cheat sheet (Python 2)

This document is a quick cheat sheet showing how the PEP 484 type language represents various common types in Python 2.

Note

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

# 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: unicode

# 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
# ("Text" means  "unicode" in Python 2 and "str" in Python 3)
x = [u"one", u"two"]  # type: List[Text]

# Use Optional[] for values that could be None
x = some_function()  # type: Optional[str]
if x is not None:
   print x

Functions

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

# This is how you annotate a function definition
def stringify(num):
    # type: (int) -> str
    """Your function docstring goes here after the type definition."""
    return str(num)

# This function has no parameters and also returns nothing. Annotations
# can also be placed on the same line as their function headers.
def greet_world(): # type: () -> None
    print "Hello, world!"

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

# Add type annotations for arguments with default values as though they
# had no defaults
def f(num1, my_float=3.5):
    # type: (int, float) -> float
    return num1 + my_float

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

quux(3)  # Fine
quux(__x=3)  # Error

# This is how you annotate a callable (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):
    # type: (int) -> Iterable[int]
    i = 0
    while i < n:
        yield i
        i += 1

# There's an alternative syntax for functions with many arguments
def send_email(address,     # type: Union[str, List[str]]
               sender,      # type: str
               cc,          # type: Optional[List[str]]
               bcc,         # type: Optional[List[str]]
               subject='',
               body=None    # type: List[str]
               ):
    # type: (...) -> bool
    <code>

When you’re puzzled or when things are complicated

from typing import Union, Any, List, Optional, 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) # -> Revealed type is 'builtins.int'

# 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

# If you initialize a variable with an empty container or "None"
# you may have to help mypy a bit by providing a type annotation
x = []  # type: List[str]
x = None  # type: Optional[str]

# This makes each positional arg and each keyword arg a "str"
def call(self, *args, **kwargs):
    # type: (*str, **str) -> str
    request = make_request(*args, **kwargs)
    return self.do_api_query(request)

# Use a "type: ignore" comment to suppress errors 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 # https://github.com/python/mypy/issues/1167

# "cast" is a helper function that lets you override the inferred
# type of an expression. It's only for mypy -- there's no runtime check.
a = [4]
b = cast(List[int], a)  # Passes fine
c = cast(List[str], a)  # Passes fine (no runtime check)
reveal_type(c)  # -> Revealed type is 'builtins.list[builtins.str]'
print c  # -> [4]; the object is not cast

# If you want dynamic attributes on your class, have it override "__setattr__"
# or "__getattr__" in a stub or in your source code.
#
# "__setattr__" allows for dynamic assignment to names
# "__getattr__" allows for dynamic access to names
class A:
    # This will allow assignment to any A.x, if x is the same type as "value"
    # (use "value: Any" to allow arbitrary types)
    def __setattr__(self, name, value):
        # type: (str, int) -> None
        ...

a.foo = 42  # Works
a.bar = 'Ex-parrot'  # Fails type checking

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

# 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):
    # type: (Iterable[int]) -> List[str]
    return [str(x) for x in iterator_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):
    # type: (Mapping[int, str]) -> List[int]
    return list(my_dict.keys())

f({3: 'yes', 4: 'no'})

def f(my_mapping):
    # type: (MutableMapping[int, str]) -> Set[str]
    my_mapping[5] = 'maybe'
    return set(my_mapping.values())

f({3: 'yes', 4: 'no'})

Classes

class MyClass(object):
    # For instance methods, omit type for "self"
    def my_method(self, num, str1):
        # type: (int, str) -> str
        return num * str1

    # 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):
        # type: () -> None
        pass

# User-defined classes are valid as types in annotations
x = MyClass()  # type: MyClass

Miscellaneous

import sys
import re
from typing import Match, AnyStr, IO

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

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