I wonder how to make this getter more type-safe:
VALUES = {
'1': 'One',
'2': 'Two',
'3': 'Three'
}
def get(key : str) -> str:
return VALUES[key]
Instead of the type str
I would love to have a keyof VALUES
and type(VALUES[key])
types.
get('4')
should throw a invalid type warning then as this key does not exist. Not sure if this is possible with Python as I properly live in a TypeScript wonderland... :-)
TypeScript would properly look like this:
get<K extends keyof VALUES>(key : K): typeof K
{
return VALUES[key];
}
You cannot do this in general . However, you can accomplish what you want in this particular case using typing.Literal
:
import typing
def get(key: typing.Literal['1','2','3']) -> str:
return VALUES[key]
As was suggested in the comments, the enum
module provides a nice solution here. By mixing in str
with enum.Enum
, we can create an Enum
that is fully backwards-compatible with str
(ie, can be used wherever a str
type is expected.
from enum import Enum
# The mixin type has to come first if you're combining
# enum.Enum with other types
class Values(str, Enum):
N1 = 'One'
N2 = 'Two'
N3 = 'Three'
If we enter this definition into the interactive console, we'll see that this class has the following behaviour:
>>> Values['N1']
<Values.N1: 'One'>
>>> Values('One')
<Values.N1: 'One'>
>>> Values.N1
<Values.N1: 'One'>
>>> Values('One') is Values.N1 is Values['N1']
True
>>> Values.N1.name
'N1'
>>> Values.N1.value
'One'
>>> Values.N1 == 'One'
True
>>> Values.N1 is 'One'
False
>>> Values.N1.startswith('On')
True
>>> type(Values.N1)
<enum 'Values'>
>>> for key in Values:
... print(key)
...
Values.N1
Values.N2
Values.N3
>>> list(Values)
[Values.N1, Values.N2, Values.N3]
As you can see, we have defined a new type which:
str
— it can freely be compared with str
objects, and str
methods can be used on its members.typing.Literal
. If I have a function like this:def do_something(some_string):
if some_string not in ('One', 'Two', 'Three'):
raise Exception('NO.')
then I can either annotate it like this:
from typing import Literal
def do_something(some_string: Literal['One', 'Two', 'Three']) -> None:
...
or like this (in which case you'll have to pass in a member of the Values
enum rather than a vanilla string, or the type-checker will raise an error):
# Values enum as defined above
def do_something(some_string: Values) -> None:
...
There's an even more detailed guide to the intricacies of python Enum
s here .
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.