No compression level errors should be thrown in read mode.

CHANGELOG.rst

@@ -9,6 +9,7 @@ Changelog
 
 version 1.6.0-dev
 -----------------
++ Fix a bug where compression levels for IGzipFile where checked in read mode.
 + Update statically linked ISA-L release to 2.31.0
 + Fix an error that occurred in the ``__close__`` function when a threaded
   writer was initialized with incorrect parameters.

src/isal/igzip.py

@@ -151,11 +151,12 @@ def __init__(self, filename=None, mode=None,
         If omitted or None, the current time is used.
         """
         if not (isal_zlib.ISAL_BEST_SPEED <= compresslevel
-                <= isal_zlib.ISAL_BEST_COMPRESSION):
+                <= isal_zlib.ISAL_BEST_COMPRESSION) and "r" not in mode:
             raise ValueError(
-                "Compression level should be between {0} and {1}.".format(
-                    isal_zlib.ISAL_BEST_SPEED, isal_zlib.ISAL_BEST_COMPRESSION
-                ))
+                f"Compression level should be between "
+                f"{isal_zlib.ISAL_BEST_SPEED} and "
+                f"{isal_zlib.ISAL_BEST_COMPRESSION}, got {compresslevel}."
+            )
         super().__init__(filename, mode, compresslevel, fileobj, mtime)
         if self.mode == WRITE:
             self.compress = isal_zlib.compressobj(compresslevel,

tests/test_gzip_compliance.py

@@ -456,6 +456,13 @@ def test_with_open(self):
         else:
             self.fail("1/0 didn't raise an exception")
 
+    def test_read_and_compresslevel(self):
+        with igzip.GzipFile(self.filename, "wb") as f:
+            f.write(b"xxx")
+        with igzip.GzipFile(self.filename, "rb", compresslevel=17) as f:
+            data = f.read()
+        assert data == b"xxx"
+
     def test_zero_padded_file(self):
         with igzip.GzipFile(self.filename, "wb") as f:
             f.write(data1 * 50)
@@ -785,6 +792,13 @@ def test_newline(self):
         with igzip.open(self.filename, "rt", newline="\r") as f:
             self.assertEqual(f.readlines(), [uncompressed])
 
+    def test_reading_and_compresslevel(self):
+        with igzip.open(self.filename, "wb") as f:
+            f.write(data1)
+        with igzip.open(self.filename, "rb", compresslevel=17) as f:
+            text = f.read()
+        assert text == data1
+
 
 def create_and_remove_directory(directory):
     def decorator(function):

tests/test_igzip_threaded.py

@@ -191,3 +191,11 @@ def test_igzip_threaded_append_text_mode(tmp_path):
     with gzip.open(test_file, "rt") as f:
         contents = f.read()
     assert contents == "ABCD"
+
+
+def test_igzip_threaded_open_compresslevel_and_reading(tmp_path):
+    test_file = tmp_path / "test.txt.gz"
+    test_file.write_bytes(gzip.compress(b"thisisatest"))
+    with igzip_threaded.open(test_file, compresslevel=5) as f:
+        text = f.read()
+    assert text == b"thisisatest"