Update docs for Literal types #8152
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This pull request is a long-overdue update of the Literal type docs. It: 1. Removes the "this is alpha" warning we have at the top. 2. Mentions Literal enums are a thing (and works in a brief example of one). 3. Adds a section about "intelligent indexing". 4. Adds a section about the "tagged union" pattern (see python#8151). I made the example focus on the TypedDict/JSON use case -- IMO that's really the only realistically useful use case for the pattern. 5. Cross-references the "tagged union" docs with the TypedDicts docs -- IMO, tagged unions are mostly useful when you're working with JSON I also thought about making the "Unions of TypedDict" section I added to the TypedDicts doc mention using [pydantic][0] as an alternative to the "tagged union" pattern. I personally prefer using libraries like this which handle validation and let me use regular classes (and `isinstance`) instead of dicts, but I wasn't sure whether we want to be recommending 3rd party libraries so held off for now. [0]: https://pydantic-docs.helpmanual.io
ilevkivskyi
approved these changes
Dec 20, 2019
docs/source/literal_types.rst
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| from typing import overload, Union | ||
| from typing_extensions import Literal | ||
| # Note: if you are using Python 3.7 or earlier, you will need to import | ||
| # Literal from the typing_extensions module. |
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I would make it a proper .. note:: (for example above at the start), we do this for Protocol for example.
docs/source/literal_types.rst
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| @@ -115,7 +106,9 @@ you can instead change the variable to be ``Final`` (see :ref:`final_attrs`): | |||
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| .. code-block:: python | |||
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| from typing_extensions import Final, Literal | |||
| # Note: if you are using Python 3.7 or earlier, you will need to import | |||
| # Literal and Final from the typing_extensions module. | |||
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If you add the above one, this one would be unnecessary.
| lit_index: Literal[1] = 1 | ||
| fin_index: Final = 1 | ||
| reveal_type(tup[lit_index]) # Revealed type is 'str' | ||
| reveal_type(tup[fin_index]) # Revealed type is 'str' |
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Maybe add an example with TypedDict? Maybe also an example with union of literals, something along these lines:
i: Literal[1, 2]
t: Tuple[str, int, int]
reveal_type(t[i]) # This is "int"
docs/source/literal_types.rst
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| .. code-block:: python | ||
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| # Note: if you are using Python 3.7 or earlier, you will need to import | ||
| # Literal and TypedDict from the typing_extensions module. |
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Again, it is not necessary to repeat everywhere if there is a dedicated note at the top.
| print(event["job_id"]) | ||
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| While this feature is mostly useful when working with TypedDicts, you can also | ||
| use the same technique wih regular objects, tuples, or namedtuples. |
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Mention that tags can also be proper enum values (using is) and classes (using isinstance()), not just strings.
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This pull request is a long-overdue update of the Literal type docs. It:
Removes the "this is alpha" warning we have at the top.
Mentions Literal enums are a thing (and works in a brief example of one).
Adds a section about "intelligent indexing".
Adds a section about the "tagged union" pattern (see Add support for narrowing Literals using equality #8151). I made the example focus on the TypedDict/JSON use case -- IMO that's really the only realistically useful use case for the pattern.
Cross-references the "tagged union" docs with the TypedDicts docs.
I also thought about making the "Unions of TypedDict" section I added to the TypedDicts doc mention using pydantic as an alternative to the "tagged union" pattern.
I personally prefer using libraries like this which handle validation and let me use regular classes (and
isinstance) instead of dicts, but I wasn't sure whether we want to be recommending 3rd party libraries so held off for now. LMK if you think it's worth including it, and I can update the PR.