Add SourceType enum and use it for source_type in ReuseInfo

This commit adds a new enum `SourceType` with three possible values to indicate if the source type is a `.license file`, `file header` or `DEP5 file`. It then updates the usage of `source_type` by replacing the string type with the new `SourceType` enum type. This improves readability and makes the code more maintainable.

src/reuse/__init__.py

@@ -20,6 +20,7 @@
 import os
 import re
 from dataclasses import dataclass, field
+from enum import Enum, auto
 from typing import NamedTuple, Optional, Set
 
 try:
@@ -88,6 +89,21 @@
 _IGNORE_FILE_PATTERNS.extend(_IGNORE_SPDX_PATTERNS)
 
 
+class SourceType(Enum):
+    """
+    An enumeration representing the types of sources for license information.
+
+    Potential values:
+        DOT_LICENSE_FILE: A .license file containing license information.
+        FILE_HEADER: A file header containing license information.
+        DEP5_FILE: A .reuse/dep5 file containing license information.
+    """
+
+    DOT_LICENSE_FILE = ".license file"
+    FILE_HEADER = "file header"
+    DEP5_FILE = ".reuse/dep5 file"
+
+
 @dataclass(frozen=True)
 class ReuseInfo:
     """Simple dataclass holding licensing and copyright information"""
@@ -96,7 +112,7 @@ class ReuseInfo:
     copyright_lines: Set[str] = field(default_factory=set)
     contributor_lines: Set[str] = field(default_factory=set)
     source_path: Optional[str] = None
-    source_type: Optional[str] = None
+    source_type: Optional[SourceType] = None
 
     def contains_copyright_or_licensing(self) -> bool:
         """Either *spdx_expressions* or *copyright_lines* is non-empty."""

src/reuse/project.py

@@ -25,6 +25,7 @@
     _IGNORE_MESON_PARENT_DIR_PATTERNS,
     IdentifierNotFound,
     ReuseInfo,
+    SourceType,
 )
 from ._licenses import EXCEPTION_MAP, LICENSE_MAP
 from ._util import (
@@ -151,7 +152,7 @@ def reuse_info_of(self, path: PathLike) -> ReuseInfo:
         """
         path = _determine_license_path(path)
         source_path = ""
-        source_type = ""
+        source_type = None
 
         _LOGGER.debug(f"searching '{path}' for SPDX information")
 
@@ -189,9 +190,9 @@ def reuse_info_of(self, path: PathLike) -> ReuseInfo:
                 if file_result:
                     source_path = str(path)
                     if path.suffix == ".license":
-                        source_type = ".license file"
+                        source_type = SourceType.DOT_LICENSE_FILE
                     else:
-                        source_type = "file header"
+                        source_type = SourceType.FILE_HEADER
 
             except (ExpressionError, ParseError):
                 _LOGGER.error(
@@ -224,7 +225,7 @@ def reuse_info_of(self, path: PathLike) -> ReuseInfo:
                 spdx_expressions=dep5_result.spdx_expressions,
                 copyright_lines=dep5_result.copyright_lines,
                 source_path=source_path,
-                source_type="DEP5 file",
+                source_type=SourceType.DEP5_FILE,
             )
         # There is a file header or a .license file
         return ReuseInfo(