Only use Python-isal for levels 1 and 2

src/xopen/__init__.py

@@ -1021,17 +1021,17 @@ def _open_gz(  # noqa: C901
         compresslevel = XOPEN_DEFAULT_GZIP_COMPRESSION
 
     if threads != 0:
-        if igzip_threaded:
-            try:
-                return igzip_threaded.open(  # type: ignore
-                    filename,
-                    mode,
-                    compresslevel,
-                    **text_mode_kwargs,
-                    threads=1,
-                )
-            except ValueError:  # Wrong compression level
-                pass
+        # igzip level 0 compresses, while zlib outputs data in uncompressed
+        # deflate format. igzip level 3 is similar in size to 1 and 2 and slower.
+        # Other libraries do a better job at these levels.
+        if igzip_threaded and (compresslevel in (1, 2) or "r" in mode):
+            return igzip_threaded.open(  # type: ignore
+                filename,
+                mode,
+                compresslevel,
+                **text_mode_kwargs,
+                threads=1,
+            )
         if gzip_ng_threaded and zlib_ng:
             try:
                 return gzip_ng_threaded.open(
@@ -1099,12 +1099,8 @@ def _open_reproducible_gzip(filename, mode: str, compresslevel: int):
         mtime=0,
     )
     gzip_file = None
-    if igzip is not None:
-        try:
-            gzip_file = igzip.IGzipFile(**kwargs, compresslevel=compresslevel)
-        except ValueError:
-            # Compression level not supported, move to built-in gzip.
-            pass
+    if igzip is not None and (compresslevel in (1, 2) or "r" in mode):
+        gzip_file = igzip.IGzipFile(**kwargs, compresslevel=compresslevel)
     elif gzip_ng is not None:
         # Compression level should be at least 2 for zlib-ng to prevent very big files.
         gzip_file = gzip_ng.GzipNGFile(**kwargs, compresslevel=max(compresslevel, 2))