Make SRC or code mandatory and mutually exclusive (#2360) (#2804)

Closes #2360: I'd like to make passing SRC or `--code` mandatory and the arguments mutually exclusive. This will change our (partially already broken) promises of CLI behavior, but I'll comment below.

CHANGES.md

@@ -38,6 +38,7 @@
   `--preview` (#2789)
 - Enable Python 3.10+ by default, without any extra need to specify
   `--target-version=py310`. (#2758)
+- Make passing `SRC` or `--code` mandatory and mutually exclusive (#2804)
 
 ### Packaging
 

docs/usage_and_configuration/the_basics.md

@@ -4,11 +4,11 @@ Foundational knowledge on using and configuring Black.
 
 _Black_ is a well-behaved Unix-style command-line tool:
 
-- it does nothing if no sources are passed to it;
+- it does nothing if it finds no sources to format;
 - it will read from standard input and write to standard output if `-` is used as the
   filename;
 - it only outputs messages to users on standard error;
-- exits with code 0 unless an internal error occurred (or `--check` was used).
+- exits with code 0 unless an internal error occurred or a CLI option prompted it.
 
 ## Usage
 

src/black/__init__.py

@@ -431,6 +431,17 @@ def main(
 ) -> None:
     """The uncompromising code formatter."""
     ctx.ensure_object(dict)
+
+    if src and code is not None:
+        out(
+            main.get_usage(ctx)
+            + "\n\n'SRC' and 'code' cannot be passed simultaneously."
+        )
+        ctx.exit(1)
+    if not src and code is None:
+        out(main.get_usage(ctx) + "\n\nOne of 'SRC' or 'code' is required.")
+        ctx.exit(1)
+
     root, method = find_project_root(src) if code is None else (None, None)
     ctx.obj["root"] = root
 
@@ -569,7 +580,6 @@ def get_sources(
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""
     sources: Set[Path] = set()
-    path_empty(src, "No Path provided. Nothing to do 😴", quiet, verbose, ctx)
 
     if exclude is None:
         exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES)

tests/test_black.py

@@ -972,10 +972,13 @@ def test_check_diff_use_together(self) -> None:
             # Multi file command.
             self.invokeBlack([str(src1), str(src2), "--diff", "--check"], exit_code=1)
 
-    def test_no_files(self) -> None:
+    def test_no_src_fails(self) -> None:
         with cache_dir():
-            # Without an argument, black exits with error code 0.
+            self.invokeBlack([], exit_code=1)
-            self.invokeBlack([])
+
+    def test_src_and_code_fails(self) -> None:
+        with cache_dir():
+            self.invokeBlack([".", "-c", "0"], exit_code=1)
 
     def test_broken_symlink(self) -> None:
         with cache_dir() as workspace:
@@ -1229,13 +1232,18 @@ def test_invalid_cli_regex(self) -> None:
 
     def test_required_version_matches_version(self) -> None:
         self.invokeBlack(
-            ["--required-version", black.__version__], exit_code=0, ignore_config=True
+            ["--required-version", black.__version__, "-c", "0"],
+            exit_code=0,
+            ignore_config=True,
         )
 
     def test_required_version_does_not_match_version(self) -> None:
-        self.invokeBlack(
+        result = BlackRunner().invoke(
-            ["--required-version", "20.99b"], exit_code=1, ignore_config=True
+            black.main,
+            ["--required-version", "20.99b", "-c", "0"],
         )
+        self.assertEqual(result.exit_code, 1)
+        self.assertIn("required version", result.stderr)
 
     def test_preserves_line_endings(self) -> None:
         with TemporaryDirectory() as workspace: