How can I type hint an attribute in Python 3.5?
Question:
I have a class where I want the initial value of an attribute to be None
:
class SomeClass:
def __init__(self):
self.some_attribute = None
How can I add type hinting, so that the IDE understands that some_attribute
is usually of the type AnotherClass
?
Answers:
In Python 3.5, you have to write
self.some_attribute = None # type: AnotherClass
Since Python 3.6, new type hinting syntax was added for variables (PEP 526):
self.some_attribute: AnotherClass = None
This will probably make every type-checking system complain, because None is in fact not an instance of AnotherClass. Instead, you can use typing.Union[None, AnotherClass]
, or the shorthand:
from typing import Optional
...
self.some_attribute: Optional[AnotherClass] = None
Python 3.10+ uses pipe sign |
which many languages use as the or operator (PEP 604) to denote Typing.Union
.
So…
self.some_attribute: AnotherClass|None = None
More concise, no Typing
import needed.
FWIW, generally 3.10 also does away with many built containers needing Typing imports: list[int]
, not List[int]
(which still works).
Note: if using forward declarations, Optional["AnotherClass"]
is still needed, pipe doesn’t work.
Also pipe handles more than None, since it’s a shorthand for Typing.Union
:
some_attribute: AnotherClass|YetAnotherClass
I have a class where I want the initial value of an attribute to be None
:
class SomeClass:
def __init__(self):
self.some_attribute = None
How can I add type hinting, so that the IDE understands that some_attribute
is usually of the type AnotherClass
?
In Python 3.5, you have to write
self.some_attribute = None # type: AnotherClass
Since Python 3.6, new type hinting syntax was added for variables (PEP 526):
self.some_attribute: AnotherClass = None
This will probably make every type-checking system complain, because None is in fact not an instance of AnotherClass. Instead, you can use typing.Union[None, AnotherClass]
, or the shorthand:
from typing import Optional
...
self.some_attribute: Optional[AnotherClass] = None
Python 3.10+ uses pipe sign |
which many languages use as the or operator (PEP 604) to denote Typing.Union
.
So…
self.some_attribute: AnotherClass|None = None
More concise, no Typing
import needed.
FWIW, generally 3.10 also does away with many built containers needing Typing imports: list[int]
, not List[int]
(which still works).
Note: if using forward declarations, Optional["AnotherClass"]
is still needed, pipe doesn’t work.
Also pipe handles more than None, since it’s a shorthand for Typing.Union
:
some_attribute: AnotherClass|YetAnotherClass