Add cli command to generate openapi schema from app without running it

Author: Michael "M3" Lasevich

README.md

@@ -89,6 +89,12 @@ It will listen on the IP address `0.0.0.0`, which means all the available IP add
 
 In most cases you would (and should) have a "termination proxy" handling HTTPS for you on top, this will depend on how you deploy your application, your provider might do this for you, or you might need to set it up yourself. You can learn more about it in the <a href="https://fastapi.tiangolo.com/deployment/" class="external-link" target="_blank">FastAPI Deployment documentation</a>.
 
+## `fastapi schema`
+
+When you run `fastapi schema`, it will generate a swagger/openapi document.
+
+This document will be output to stderr by default, however `--output <filename>` option can be used to write output into file. You can control the format of the JSON file by specifying indent level with `--indent #`. If set to 0, JSON will be in the minimal/compress form. Default is 2 spaces.
+
 ## License
 
 This project is licensed under the terms of the MIT license.

src/fastapi_cli/cli.py

@@ -1,4 +1,6 @@
+import json
 import logging
+import sys
 from pathlib import Path
 from typing import Any, List, Union
 
@@ -7,7 +9,7 @@
 from rich.tree import Tree
 from typing_extensions import Annotated
 
-from fastapi_cli.discover import get_import_data
+from fastapi_cli.discover import get_app, get_import_data
 from fastapi_cli.exceptions import FastAPICLIException
 
 from . import __version__
@@ -370,5 +372,42 @@ def run(
     )
 
 
+@app.command()
+def schema(
+    path: Annotated[
+        Union[Path, None],
+        typer.Argument(
+            help="A path to a Python file or package directory (with [blue]__init__.py[/blue] files) containing a [bold]FastAPI[/bold] app. If not provided, a default set of paths will be tried."
+        ),
+    ] = None,
+    *,
+    app: Annotated[
+        Union[str, None],
+        typer.Option(
+            help="The name of the variable that contains the [bold]FastAPI[/bold] app in the imported module or package. If not provided, it is detected automatically."
+        ),
+    ] = None,
+    output: Annotated[
+        Union[str, None],
+        typer.Option(
+            help="The filename to write schema to. If not provided, write to stderr."
+        ),
+    ] = None,
+    indent: Annotated[
+        int,
+        typer.Option(help="JSON format indent. If 0, disable pretty printing"),
+    ] = 2,
+) -> Any:
+    """Generate schema"""
+    fastapi_app = get_app(path=path, app_name=app)
+    schema = fastapi_app.openapi()
+
+    stream = open(output, "w") if output else sys.stderr
+    json.dump(schema, stream, indent=indent if indent > 0 else None)
+    if output:
+        stream.close()
+    return 0
+
+
 def main() -> None:
     app()

src/fastapi_cli/discover.py

@@ -1,9 +1,10 @@
 import importlib
 import sys
+from contextlib import contextmanager
 from dataclasses import dataclass
 from logging import getLogger
 from pathlib import Path
-from typing import List, Union
+from typing import Iterator, List, Union
 
 from fastapi_cli.exceptions import FastAPICLIException
 
@@ -41,6 +42,18 @@ class ModuleData:
     extra_sys_path: Path
     module_paths: List[Path]
 
+    @contextmanager
+    def sys_path(self) -> Iterator[str]:
+        """Context manager to temporarily alter sys.path"""
+        extra_sys_path = str(self.extra_sys_path) if self.extra_sys_path else ""
+        if extra_sys_path:
+            logger.debug("Adding %s to sys.path...", extra_sys_path)
+            sys.path.insert(0, extra_sys_path)
+            yield extra_sys_path
+        if extra_sys_path and sys.path and sys.path[0] == extra_sys_path:
+            logger.debug("Removing %s from sys.path...", extra_sys_path)
+            sys.path.pop(0)
+
 
 def get_module_data_from_path(path: Path) -> ModuleData:
     use_path = path.resolve()
@@ -130,3 +143,16 @@ def get_import_data(
     return ImportData(
         app_name=use_app_name, module_data=mod_data, import_string=import_string
     )
+
+
+def get_app(
+    *, path: Union[Path, None] = None, app_name: Union[str, None] = None
+) -> FastAPI:
+    """Get the FastAPI app instance from the given path and app name."""
+    import_data: ImportData = get_import_data(path=path, app_name=app_name)
+    mod_data, use_app_name = import_data.module_data, import_data.app_name
+    with mod_data.sys_path():
+        mod = importlib.import_module(mod_data.module_import_str)
+    app = getattr(mod, use_app_name)
+    ## get_import_string_parts guarantees app is FastAPI object
+    return app  # type: ignore[no-any-return]

tests/assets/openapi.json

@@ -0,0 +1,25 @@
+{
+  "openapi": "3.1.0",
+  "info": {
+    "title": "FastAPI",
+    "version": "0.1.0"
+  },
+  "paths": {
+    "/": {
+      "get": {
+        "summary": "App Root",
+        "operationId": "app_root__get",
+        "responses": {
+          "200": {
+            "description": "Successful Response",
+            "content": {
+              "application/json": {
+                "schema": {}
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}

tests/test_cli.py

@@ -1,3 +1,4 @@
+import os
 import subprocess
 import sys
 from pathlib import Path
@@ -7,6 +8,7 @@
 from typer.testing import CliRunner
 
 from fastapi_cli.cli import app
+from fastapi_cli.exceptions import FastAPICLIException
 from fastapi_cli.utils.cli import get_uvicorn_log_config
 from tests.utils import changing_dir
 
@@ -15,6 +17,13 @@
 assets_path = Path(__file__).parent / "assets"
 
 
+def read_file(filename: str, strip: bool = True) -> str:
+    """Read file and return content as string"""
+    with open("openapi.json") as stream:
+        data = stream.read()
+    return data.strip() if data and strip else data
+
+
 def test_dev() -> None:
     with changing_dir(assets_path):
         with patch.object(uvicorn, "run") as mock_run:
@@ -251,6 +260,51 @@ def test_dev_help() -> None:
     assert "Use multiple worker processes." not in result.output
 
 
+def test_schema() -> None:
+    with changing_dir(assets_path):
+        with open("openapi.json") as stream:
+            expected = stream.read().strip()
+        assert expected != "", "Failed to read expected result"
+        result = runner.invoke(app, ["schema", "single_file_app.py"])
+        assert result.exit_code == 0, result.output
+        assert expected in result.output, result.output
+
+
+def test_schema_file() -> None:
+    with changing_dir(assets_path):
+        filename = "unit-test.json"
+        expected = read_file("openapi.json", strip=True)
+        assert expected != "", "Failed to read expected result"
+        result = runner.invoke(
+            app, ["schema", "single_file_app.py", "--output", filename]
+        )
+        assert os.path.isfile(filename)
+        actual = read_file(filename, strip=True)
+        os.remove(filename)
+        assert result.exit_code == 0, result.output
+        assert expected == actual
+
+
+def test_schema_invalid_path() -> None:
+    with changing_dir(assets_path):
+        result = runner.invoke(app, ["schema", "invalid/single_file_app.py"])
+        assert result.exit_code == 1, result.output
+        assert isinstance(result.exception, FastAPICLIException)
+        assert "Path does not exist invalid/single_file_app.py" in str(result.exception)
+
+
+#
+#
+# def test_schema_invalid_package() -> None:
+#     with changing_dir(assets_path):
+#         result = runner.invoke(
+#             app, ["schema", "broken_package/mod/app.py"]
+#         )
+#         assert result.exit_code == 1, result.output
+#         assert isinstance(result.exception, ImportError)
+#         assert "attempted relative import beyond top-level package" in str(result.exception)
+
+
 def test_run_help() -> None:
     result = runner.invoke(app, ["run", "--help"])
     assert result.exit_code == 0, result.output

tests/test_utils_package.py

@@ -3,7 +3,7 @@
 import pytest
 from pytest import CaptureFixture
 
-from fastapi_cli.discover import get_import_data
+from fastapi_cli.discover import get_app, get_import_data
 from fastapi_cli.exceptions import FastAPICLIException
 from tests.utils import changing_dir
 
@@ -169,6 +169,17 @@ def test_broken_package_dir(capsys: CaptureFixture[str]) -> None:
     assert "Ensure all the package directories have an __init__.py file" in captured.out
 
 
+def test_get_app_broken_package_dir(capsys: CaptureFixture[str]) -> None:
+    with changing_dir(assets_path):
+        # TODO (when deprecating Python 3.8): remove ValueError
+        with pytest.raises((ImportError, ValueError)):
+            get_app(path=Path("broken_package/mod/app.py"))
+
+    captured = capsys.readouterr()
+    assert "Import error:" in captured.out
+    assert "Ensure all the package directories have an __init__.py file" in captured.out
+
+
 def test_package_dir_no_app() -> None:
     with changing_dir(assets_path):
         with pytest.raises(FastAPICLIException) as e: