ImportError: cannot import name 'Literal' from 'typing'

Question

I have recently started using PEP 484 and PEP 586 to make my code clearer and more accessible. So far everything was ok, but when I wanted to use Literal from the package typing it appears it couldn't be imported. What is the most surprising is that PyCharm isn't complaining at all for importing it or using it.

The code I want to use in the end is looking like that :

SomeVar = TypeVar("SomeVar", Literal['choice1'], Literal['choice2'], someType)

It would be used in the cases where you can have a string to describe what you want or an already made solution e.g :

def someFunc(my_var: SomeVar = 'choice1'):
    result = []
    if my_var == 'choice1':
        result.append(...)
    else:
        result = my_var
    return result

I use an Anaconda environment with Python 3.7.7.

Factitive

Answer 1

Using Literal in Python 3.8 and later

from typing import Literal

Using Literal in all Python versions (1)

Literal was added to typing.py in 3.8, but you can use Literal in older versions anyway.

First install typing_extensions (pip install typing_extensions) and then

from typing_extensions import Literal

This approach is supposed to work also in Python 3.8 and later.

Using Literal in all Python versions (2)

For completeness, I'm also adding the try-except approach to import Literal:

try:
    from typing import Literal
except ImportError:
    from typing_extensions import Literal

This should also work for all Python versions, given that typing_extensions is installed if you're using Python 3.7 or older.

Syringomyelia

Answer 2

As stated in the docs, typing.Literal is only available from Python 3.8 and up.

Parthenogenesis

Thanks ! What alternative do you advise then ? – Rhoea

Alternatively (if you don't want to update your Python version), you can use the Literal type from the typing-extensions package. – Eryn

Answer 3

If the developers are using Python 3.8+ but you are want your code to be able to run on older Python versions (like LTS Ubuntu versions) without requiring typing-extensions, then you can just fallback to a string. The people who use your software won't benefit from strict type checking anyway.

import sys

if sys.version_info >= (3, 8):
    from typing import Literal
    MyType = Literal["A", "B", "C"]
else:
    MyType = str

You can of course combine this with first trying to fallback to typing_extensions (but then you will have to declare your literal type twice).

Egon