Using Python 3's function annotations, it is possible to specify the type of items contained within a homogeneous list (or other collection) for the purpose of type hinting in PyCharm and other IDEs?
A pseudo-python code example for a list of int:
def my_func(l:list<int>):
pass
I know it's possible using Docstring...
def my_func(l):
"""
:type l: list[int]
"""
pass
... but I prefer the annotation style if it's possible.
As of May 2015, PEP0484 (Type Hints) has been formally accepted. The draft implementation is also available at github under ambv/typehinting.
In September 2015, Python 3.5 was released with support for Type Hints and includes a new typing module. This allows for the specification of types contained within collections. As of November 2015, JetBrains PyCharm 5.0 fully supports Python 3.5 to include Type Hints as illustrated below.
Original Answer
As of Aug 2014, I have confirmed that it is not possible to use Python 3 type annotations to specify types within collections (ex: a list of strings).
The use of formatted docstrings such as reStructuredText or Sphinx are viable alternatives and supported by various IDEs.
It also appears that Guido is mulling over the idea of extending type annotations in the spirit of mypy: http://mail.python.org/pipermail/python-ideas/2014-August/028618.html
l: List[str]. Upvoted alecxe's answer, as that shows it in plain text.
Commented
Dec 20, 2021 at 0:58
list is favored instead of List since Python3.9
Commented
Mar 22, 2023 at 15:05
As of Python 3.9, builtin types are generic with respect to type annotations (see PEP 585). This allows to directly specify the type of elements:
def my_func(l: list[int]):
pass
This also extends to most other container types of the standard library, for example collections.deque or collections.abc.Mapping.
Various tools may support this syntax earlier than Python 3.9. When annotations are not inspected at runtime, the syntax is valid using quoting or __future__.annotations.
# quoted
def my_func(l: 'list[int]'):
pass
# postponed evaluation of annotation
from __future__ import annotations
def my_func(l: list[int]):
pass
As a consequence of PEP 585, most helpers in typing corresponding to standard library types are deprecated, such as typing.List, typing.Deque or typing.Mapping. They should only be used if compatibility with Python versions prior to 3.9 is required.
Since Python 3.5 has been officially out, there is the Type Hints supporting module - typing and the relevant List "type" for the generic containers.
In other words, now you could do:
from typing import List
def my_func(l: List[int]):
pass
This is deprecated since Python 3.9, now you can directly use the built-in list instead of typing.List.
Type comments have been added since PEP 484
from . import Monitor
from typing import List, Set, Tuple, Dict
active_monitors = [] # type: List[Monitor]
# or
active_monitors: List[Monitor] = []
# bonus
active_monitors: Set[Monitor] = set()
monitor_pair: Tuple[Monitor, Monitor] = (Monitor(), Monitor())
monitor_dict: Dict[str, Monitor] = {'codename': Monitor()}
# nested
monitor_pair_list: List[Dict[str, Monitor]] = [{'codename': Monitor()}]
This is currently working for me on PyCharm with Python 3.6.4
With support from the BDFL, it's almost certain now that python (probably 3.5) will provide a standardized syntax for type hints via function annotations.
https://www.python.org/dev/peps/pep-0484/
As referenced in the PEP, there is an experimental type-checker (kind of like pylint, but for types) called mypy that already uses this standard, and doesn't require any new syntax.
type object is not subscriptablewhen defining the function. Obviously you can use a string:def my_func(L: 'list[int]')but I don't know whether PyCharm will parse it as it parses the docstrings...'list[int]', apologies if that wasn't clear.