From 09d56ba971e68e22c5a752c6abe08033f6f401d2 Mon Sep 17 00:00:00 2001
From: Dan Walters <dan@walters.io>
Date: Thu, 8 Oct 2020 15:03:09 -0500
Subject: [PATCH] Support for typing.Literal as an alternative to Enum in
 Python 3.8+.

---
 docs/tutorial/parameter-types/enum.md         | 39 +++++++++++++
 docs_src/parameter_types/enum/tutorial003.py  | 11 ++++
 .../test_enum/test_tutorial003.py             | 56 +++++++++++++++++++
 typer/main.py                                 |  7 +++
 4 files changed, 113 insertions(+)
 create mode 100644 docs_src/parameter_types/enum/tutorial003.py
 create mode 100644 tests/test_tutorial/test_parameter_types/test_enum/test_tutorial003.py

diff --git a/docs/tutorial/parameter-types/enum.md b/docs/tutorial/parameter-types/enum.md
index 6af834bf54..561e889d0d 100644
--- a/docs/tutorial/parameter-types/enum.md
+++ b/docs/tutorial/parameter-types/enum.md
@@ -66,3 +66,42 @@ Training neural network of type: lstm
 ```
 
 </div>
+
+### Literal choices
+
+With Python 3.8+, you can also use `typing.Literal` to represent a set of possible predefined choices, without having to use an `Enum`:
+
+```Python hl_lines="13"
+{!../docs_src/parameter_types/enum/tutorial003.py!}
+```
+
+<div class="termy">
+
+```console
+$ python main.py --help
+
+// Notice the predefined values [simple|conv|lstm]
+Usage: main.py [OPTIONS]
+
+Options:
+  --network [simple|conv|lstm]  [default: simple]
+  --install-completion          Install completion for the current shell.
+  --show-completion             Show completion for the current shell, to copy it or customize the installation.
+  --help                        Show this message and exit.
+
+// Try it
+$ python main.py --network conv
+
+Training neural network of type: conv
+
+// Invalid value
+$ python main.py --network capsule
+
+Usage: main.py [OPTIONS]
+Try "main.py --help" for help.
+
+Error: Invalid value for '--network': invalid choice: capsule. (choose from simple, conv, lstm)
+```
+
+</div>
+
diff --git a/docs_src/parameter_types/enum/tutorial003.py b/docs_src/parameter_types/enum/tutorial003.py
new file mode 100644
index 0000000000..6271eac338
--- /dev/null
+++ b/docs_src/parameter_types/enum/tutorial003.py
@@ -0,0 +1,11 @@
+from typing import Literal
+
+import typer
+
+
+def main(network: Literal["simple", "conv", "lstm"] = "simple"):
+    typer.echo(f"Training neural network of type: {network}")
+
+
+if __name__ == "__main__":
+    typer.run(main)
diff --git a/tests/test_tutorial/test_parameter_types/test_enum/test_tutorial003.py b/tests/test_tutorial/test_parameter_types/test_enum/test_tutorial003.py
new file mode 100644
index 0000000000..700fb469c7
--- /dev/null
+++ b/tests/test_tutorial/test_parameter_types/test_enum/test_tutorial003.py
@@ -0,0 +1,56 @@
+import subprocess
+import sys
+
+import pytest
+import typer
+from typer.testing import CliRunner
+
+pytestmark = pytest.mark.skipif(sys.version_info < (3, 8), reason="requires python3.8")
+
+
+runner = CliRunner()
+
+
+@pytest.fixture(scope="module")
+def mod():
+    from docs_src.parameter_types.enum import tutorial003 as mod
+
+    return mod
+
+
+@pytest.fixture(scope="module")
+def app(mod):
+    app = typer.Typer()
+    app.command()(mod.main)
+    return app
+
+
+def test_help(app):
+    result = runner.invoke(app, ["--help"])
+    assert result.exit_code == 0
+    assert "--network [simple|conv|lstm]" in result.output
+
+
+def test_main(app):
+    result = runner.invoke(app, ["--network", "conv"])
+    assert result.exit_code == 0
+    assert "Training neural network of type: conv" in result.output
+
+
+def test_invalid(app):
+    result = runner.invoke(app, ["--network", "capsule"])
+    assert result.exit_code != 0
+    assert (
+        "Error: Invalid value for '--network': invalid choice: capsule. (choose from simple, conv, lstm)"
+        in result.output
+    )
+
+
+def test_script(mod):
+    result = subprocess.run(
+        ["coverage", "run", mod.__file__, "--help"],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        encoding="utf-8",
+    )
+    assert "Usage" in result.stdout
diff --git a/typer/main.py b/typer/main.py
index 02d9a5d7fe..e7f48d2a0d 100644
--- a/typer/main.py
+++ b/typer/main.py
@@ -1,4 +1,6 @@
 import inspect
+import sys
+import typing
 from datetime import datetime
 from enum import Enum
 from functools import update_wrapper
@@ -584,6 +586,11 @@ def get_click_type(
             [item.value for item in annotation],
             case_sensitive=parameter_info.case_sensitive,
         )
+    elif sys.version_info >= (3, 8) and typing.get_origin(annotation) == typing.Literal:
+        return click.Choice(
+            tuple(typing.get_args(annotation)),
+            case_sensitive=parameter_info.case_sensitive,
+        )
     raise RuntimeError(f"Type not yet supported: {annotation}")  # pragma no cover