cli: backports.zstd is replaced by zstandard for Python < 3.14

The zstandard package is widely available across distributions and
Python environments, while backports.zstd is not always packaged or
readily accessible. So backports.zstd is replaced by zstandard.

Make the zstandard dependency optional by importing it only when required.

The zstandard backend does not support text mode when opening compressed files
through zstd.open().
Switch to binary mode and let json.load() consume the resulting binary steam,
which is supported by both backends.

Signed-off-by: Christophe Guerreiro <christophe.guerreiro@non.se.com>

Author: Christophe Guerreiro

CHANGELOG.md

@@ -8,6 +8,8 @@ All notable changes to this project will be documented in this file.
 
 ### Changed
 
+- zstandard replaced backports.zstd for python <3.14 and considered as an optional dependency
+
 ### Fixed
 
 ### Removed
@@ -40,7 +42,7 @@ boolean options, all defaulting to `True`:
   - The JSON output file is no longer generated by default.
   - The `--output <PATH>` option have been renamed to `--json-output <PATH>`
   - The `--json-output <PATH>` option must now be explicitly specified to enable JSON file generation.
-  
+
 
 ### Fixed
 

pyproject.toml

@@ -20,8 +20,9 @@ classifiers = [
     "Topic :: Software Development :: Build Tools",
 ]
 
-dependencies = [
-    "backports.zstd; python_version < '3.14'"
+[project.optional-dependencies]
+zstandard = [
+    "zstandard;python_version<'3.14'",
 ]
 
 [project.scripts]

src/spdx_diff/cli.py

@@ -17,9 +17,12 @@
 from typing import Any
 
 if sys.version_info >= (3, 14):
-    from compression import zstd
+    from compression.zstd import open as open_zstd
 else:
-    from backports import zstd
+    try:
+        from zstandard import open as open_zstd
+    except ImportError:
+        open_zstd = None
 
 from . import __version__
 
@@ -61,7 +64,12 @@ def _parse(self, json_path: pathlib.Path) -> None:
 
         try:
             if json_path.suffix == ".zst":
-                with zstd.open(json_path, "rt") as f:
+                if open_zstd is None:
+                    raise RuntimeError(
+                        "Zstd support is not available."
+                        "Please install the 'zstandard' package."
+                    )
+                with open_zstd(json_path, "rb") as f:
                     data = json.load(f)
             else:
                 with json_path.open(encoding="utf-8") as f: