Add --stdin option to CLI for reading Markdown from standard input

- Add --stdin flag to parse_args for reading from stdin
- Add convert_stdin() function to handle stdin parsing
- Update main() to call convert_stdin() when --stdin flag is used
- Add comprehensive tests for stdin functionality and CLI behavior

Author: mcepl

markdown_it/cli/parse.py

@@ -21,6 +21,8 @@ def main(args: Sequence[str] | None = None) -> int:
     namespace = parse_args(args)
     if namespace.filenames:
         convert(namespace.filenames)
+    elif namespace.stdin:
+        convert_stdin()
     else:
         interactive()
     return 0
@@ -31,6 +33,18 @@ def convert(filenames: Iterable[str]) -> None:
         convert_file(filename)
 
 
+def convert_stdin() -> None:
+    """
+    Parse a Markdown file and dump the output to stdout.
+    """
+    try:
+        rendered = MarkdownIt().render(sys.stdin.read())
+        print(rendered, end="")
+    except OSError:
+        sys.stderr.write("Cannot parse Markdown from the standard input.\n")
+        sys.exit(1)
+
+
 def convert_file(filename: str) -> None:
     """
     Parse a Markdown file and dump the output to stdout.
@@ -94,6 +108,9 @@ def parse_args(args: Sequence[str] | None) -> argparse.Namespace:
         formatter_class=argparse.RawDescriptionHelpFormatter,
     )
     parser.add_argument("-v", "--version", action="version", version=version_str)
+    parser.add_argument(
+        "--stdin", action="store_true", help="read Markdown from standard input"
+    )
     parser.add_argument(
         "filenames", nargs="*", help="specify an optional list of files to convert"
     )

tests/test_cli.py

@@ -1,5 +1,7 @@
+import io
 import pathlib
 import tempfile
+from contextlib import redirect_stdout
 from unittest.mock import patch
 
 import pytest
@@ -40,3 +42,56 @@ def mock_input(prompt):
     with patch("builtins.print") as patched, patch("builtins.input", mock_input):
         parse.interactive()
     patched.assert_called()
+
+
+def test_main_no_args_is_interactive():
+    with patch("markdown_it.cli.parse.interactive") as mock_interactive:
+        assert parse.main([]) == 0
+    mock_interactive.assert_called_once()
+
+
+def test_parse_output():
+    with tempfile.TemporaryDirectory() as tempdir:
+        path = pathlib.Path(tempdir).joinpath("test.md")
+        path.write_text("# a b c")
+        string_io = io.StringIO()
+        with redirect_stdout(string_io):
+            assert parse.main([str(path)]) == 0
+        assert string_io.getvalue() == "<h1>a b c</h1>\n"
+
+
+def test_stdin():
+    with patch("sys.stdin", io.StringIO("# a b c")):
+        string_io = io.StringIO()
+        with redirect_stdout(string_io):
+            assert parse.main(["--stdin"]) == 0
+        assert string_io.getvalue() == "<h1>a b c</h1>\n"
+
+
+def test_multiple_files():
+    with tempfile.TemporaryDirectory() as tempdir:
+        path1 = pathlib.Path(tempdir).joinpath("test1.md")
+        path1.write_text("# file 1")
+        path2 = pathlib.Path(tempdir).joinpath("test2.md")
+        path2.write_text("* file 2")
+        string_io = io.StringIO()
+        with redirect_stdout(string_io):
+            assert parse.main([str(path1), str(path2)]) == 0
+        assert string_io.getvalue() == "<h1>file 1</h1>\n<ul>\n<li>file 2</li>\n</ul>\n"
+
+
+def test_interactive_render():
+    # Simulate user typing '# hello', pressing Ctrl-D (renders), then Ctrl-C (exits)
+    # This is needed to break the infinite loop in interactive mode on EOF.
+    mock_input = patch(
+        "builtins.input", side_effect=["# hello", EOFError, KeyboardInterrupt]
+    )
+    string_io = io.StringIO()
+    with redirect_stdout(string_io), mock_input:
+        parse.interactive()
+
+    output = string_io.getvalue()
+    assert "markdown-it-py" in output  # from print_heading
+    # The rendered output is prefixed by a newline
+    assert "\n<h1>hello</h1>\n" in output
+    assert "Exiting" in output