✨ Support typing.Literal in Python 3.8+

[blackary]

Copied PR #174 from someone who deleted their account because I think it's worth reopening this PR and adding this functionality to typer.

Original PR message:

Support for typing.Literal for creating choices, without actually having to create Enum types. Requires Python 3.8, referenced in #76. I ran tests again 3.6 without problems, so I think the version check if working OK.

[github-actions]

📝 Docs preview for commit d83f792 at: https://62d04b8528f3e0240fbc7885--typertiangolo.netlify.app

[codecov]

Codecov Report

Attention: Patch coverage is 43.75000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 99.69%. Comparing base (1e43c6b) to head (d83f792).
Report is 237 commits behind head on master.

❗ Current head d83f792 differs from pull request most recent head a771189

Please upload reports for the commit a771189 to get more accurate results.

Files	Patch %	Lines
...test_parameter_types/test_enum/test_tutorial003.py	44.82%	16 Missing ⚠️
typer/main.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
+ Coverage   96.24%   99.69%   +3.44%     
==========================================
  Files         280      286       +6     
  Lines        5942     6456     +514     
==========================================
+ Hits         5719     6436     +717     
+ Misses        223       20     -203     

☔ View full report in Codecov by Sentry.
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[github-actions]

📝 Docs preview for commit 7a3466d at: https://639cea33a12b8e0af09de2a5--typertiangolo.netlify.app

[github-actions]

📝 Docs preview for commit 1e261e6 at: https://63a3728fbc88656dcfeedf1c--typertiangolo.netlify.app

[prompteus]

Any updates on merging this? @tiangolo

[hoishing]

Upvote for this PR, as using Literal is more succinct then using enum.

[atmartinez]

There is the possibility to import Literal from typing_extensions to allow this to work for 3.6 (since Typer still says it support it despite being EOL) and 3.7 as well. This may be a moot consideration soon though since 3.7 goes EOL in June 2023 as well if there are any plans to drop unsupported Python versions at some point.

This may require adding a specific dependency for typing_extensions, but it's currently being installed by another dep regardless from some of the CI runs I checked for 3.6+ 4.1-4.4 depending on the Python version.

Example implementation for using typing_extensions

[github-actions]

📝 Docs preview for commit d4cd3c3 at: https://6e1d3ac2.typertiangolo.pages.dev

[github-actions]

📝 Docs preview for commit c7dd353 at: https://89793d8d.typertiangolo.pages.dev

[github-actions]

📝 Docs preview for commit 01d8013 at: https://1d46deb9.typertiangolo.pages.dev

[github-actions]

📝 Docs preview for commit 6065329 at: https://85835217.typertiangolo.pages.dev

[github-actions]

📝 Docs preview for commit 3b9034b at: https://a0405a66.typertiangolo.pages.dev

[svlandeg]

Preview of new docs section: https://a0405a66.typertiangolo.pages.dev/tutorial/parameter-types/enum/#literal-choices

[svlandeg]

Thanks for this PR @blackary! I've taken the liberty to update this PR with the latest from master. I also added some improvements using new functionality from _typing.py, taking inspiration from the similar PR #669. This PR and that one now both cover largely the same functionality. I would personally merge this one and close the other, but will leave the final decision with Tiangolo as usual 😇

[blackary]

Thanks @svlandeg!

[github-actions]

📝 Docs preview for commit a771189 at: https://4a31fc21.typertiangolo.pages.dev

[k4black]

Gently ping @tiangolo

[svlandeg]

Hi all,

There's no need for pinging individual maintainers - this PR is on our internal board and will be reviewed further when Tiangolo has time. All the additional pings make our Github notifications difficult to use and only costs more time 😉 Thanks for your patience!

[github-actions]

📝 Docs preview for commit 74a6116 at: https://ffe72ce6.typertiangolo.pages.dev

Modified Pages

• https://ffe72ce6.typertiangolo.pages.dev/tutorial/parameter-types/enum/

[github-actions]

📝 Docs preview for commit c29c5d2 at: https://c23a0dec.typertiangolo.pages.dev

Modified Pages

• https://c23a0dec.typertiangolo.pages.dev/tutorial/parameter-types/enum/

[github-actions]

📝 Docs preview for commit cb93aa4 at: https://2f6cbaf9.typertiangolo.pages.dev

Modified Pages

• https://2f6cbaf9.typertiangolo.pages.dev/tutorial/parameter-types/enum/

[github-actions]

📝 Docs preview for commit 1834fb8 at: https://9dfe14ad.typertiangolo.pages.dev

Modified Pages

• https://9dfe14ad.typertiangolo.pages.dev/tutorial/parameter-types/enum/

[github-actions]

📝 Docs preview for commit f06dcf5 at: https://1c296473.typertiangolo.pages.dev

Modified Pages

• https://1c296473.typertiangolo.pages.dev/tutorial/parameter-types/enum/

[svlandeg]

Currently, the CI is failing with

Run ls -la coverage
ls: cannot access 'coverage': No such file or directory
Error: Process completed with exit code 2.

I'm pretty confused why this happens here though 🤔

Update: this looks to be an issue separate from this PR - we're looking into it!

Update 2: this should be fixed by #967

[github-actions]

📝 Docs preview for commit 20b55f6 at: https://43e3b3f1.typertiangolo.pages.dev

Modified Pages

• https://43e3b3f1.typertiangolo.pages.dev/tutorial/parameter-types/enum/