Separate error listing from formatting in format_lines

This allows to add more formaters based on the same error list.

AUTHORS.rst

@@ -150,3 +150,4 @@ Contributors
 - rajivsunar07 <[email protected]>
 - Сергій <[email protected]>
 - Mersho <[email protected]>
+- Michal Čihař <[email protected]>

src/reuse/lint.py

@@ -2,6 +2,7 @@
 # SPDX-FileCopyrightText: 2022 Florian Snow <[email protected]>
 # SPDX-FileCopyrightText: 2023 DB Systel GmbH
 # SPDX-FileCopyrightText: 2024 Nico Rikken <[email protected]>
+# SPDX-FileCopyrightText: 2024 Michal Čihař <[email protected]>
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
@@ -16,7 +17,7 @@
 from io import StringIO
 from pathlib import Path
 from textwrap import TextWrapper
-from typing import IO, Any, Optional
+from typing import IO, Any, Iterable, Optional
 
 from . import __REUSE_version__
 from .project import Project
@@ -264,7 +265,9 @@ def custom_serializer(obj: Any) -> Any:
     )
 
 
-def format_lines(report: ProjectReport) -> str:
+def get_errors(
+    report: ProjectReport,
+) -> Iterable[tuple[Path | str | None, str]]:
     """Formats data dictionary as plaintext strings to be printed to sys.stdout
     Sorting of output is not guaranteed.
     Symbolic links can result in multiple entries per file.
@@ -275,7 +278,6 @@ def format_lines(report: ProjectReport) -> str:
     Returns:
         String (in plaintext) that can be output to sys.stdout
     """
-    output = StringIO()
 
     def license_path(lic: str) -> Optional[Path]:
         """Resolve a license identifier to a license path."""
@@ -285,55 +287,58 @@ def license_path(lic: str) -> Optional[Path]:
         # Bad licenses
         for lic, files in sorted(report.bad_licenses.items()):
             for path in sorted(files):
-                output.write(
-                    _("{path}: bad license {lic}\n").format(path=path, lic=lic)
-                )
+                yield (path, _("bad license {lic}").format(lic=lic))
 
         # Deprecated licenses
         for lic in sorted(report.deprecated_licenses):
             lic_path = license_path(lic)
-            output.write(
-                _("{lic_path}: deprecated license\n").format(lic_path=lic_path)
-            )
+            yield (lic_path, _("deprecated license"))
 
         # Licenses without extension
         for lic in sorted(report.licenses_without_extension):
             lic_path = license_path(lic)
-            output.write(
-                _("{lic_path}: license without file extension\n").format(
-                    lic_path=lic_path
-                )
-            )
+            yield (lic_path, _("license without file extension"))
 
         # Unused licenses
         for lic in sorted(report.unused_licenses):
             lic_path = license_path(lic)
-            output.write(
-                _("{lic_path}: unused license\n").format(lic_path=lic_path)
-            )
+            yield lic_path, _("unused license")
 
         # Missing licenses
         for lic, files in sorted(report.missing_licenses.items()):
             for path in sorted(files):
-                output.write(
-                    _("{path}: missing license {lic}\n").format(
-                        path=path, lic=lic
-                    )
-                )
+                yield (path, _("missing license {lic}").format(lic=lic))
 
         # Read errors
         for path in sorted(report.read_errors):
-            output.write(_("{path}: read error\n").format(path=path))
+            yield (path, _("read error"))
 
         # Without licenses
         for path in report.files_without_licenses:
-            output.write(_("{path}: no license identifier\n").format(path=path))
+            yield (path, _("no license identifier"))
 
         # Without copyright
         for path in report.files_without_copyright:
-            output.write(_("{path}: no copyright notice\n").format(path=path))
+            yield (path, _("no copyright notice"))
 
-    return output.getvalue()
+
+def format_lines(report: ProjectReport) -> str:
+    """Formats data dictionary as plaintext strings to be printed to sys.stdout
+    Sorting of output is not guaranteed.
+    Symbolic links can result in multiple entries per file.
+
+    Args:
+        report: ProjectReport data
+
+    Returns:
+        String (in plaintext) that can be output to sys.stdout
+    """
+    if not report.is_compliant:
+        return "".join(
+            f"{path}: {error}\n" for path, error in get_errors(report)
+        )
+
+    return ""
 
 
 def run(args: Namespace, project: Project, out: IO[str] = sys.stdout) -> int: