chore(tests): move gzip mtime-comparison helper out of library code (#4326)

* chore(tests): move gzip mtime-comparison helper out of library code

`_gzip_streams_equal_except_mtime` was only used by tests but lived in
`zarr.codecs.gzip`. Move it to `tests/conftest.py` alongside the other
shared test helpers (`nan_equal`, `deep_nan_equal`), rename it to
`gzip_streams_equal_except_mtime`, and update both importers.

Assisted-by: ClaudeCode:claude-fable-5-1

* chore: rename changelog fragment to the PR number

Assisted-by: ClaudeCode:claude-fable-5-1

Author: d-v-b

changes/4326.misc.md

@@ -0,0 +1 @@
+Removed the private test helper `_gzip_streams_equal_except_mtime` from `zarr.codecs.gzip`; it now lives in the test suite.

src/zarr/codecs/gzip.py

@@ -28,20 +28,6 @@ def parse_gzip_level(data: JSON) -> int:
     return parsed
 
 
-def _gzip_streams_equal_except_mtime(a: bytes, b: bytes) -> bool:
-    """Compare two gzip streams, ignoring the MTIME field of the header.
-
-    Per RFC 1952 the gzip header is [magic(2)][CM(1)][FLG(1)][MTIME(4)][XFL(1)][OS(1)],
-    so bytes 4-8 are MTIME. The fixed offsets assume the standard 10-byte header
-    with no FNAME/FEXTRA/FCOMMENT flags set, which holds here because numcodecs'
-    ``GZip.encode`` wraps ``gzip.GzipFile`` without a filename.
-    """
-    if len(a) != len(b):
-        return False
-
-    return a[:4] == b[:4] and a[8:] == b[8:]
-
-
 @dataclass(frozen=True)
 class GzipCodec(BytesBytesCodec):
     """gzip codec"""

tests/conftest.py

@@ -544,6 +544,20 @@ def deep_nan_equal(a: object, b: object) -> bool:
     return nan_equal(a, b)
 
 
+def gzip_streams_equal_except_mtime(a: bytes, b: bytes) -> bool:
+    """Compare two gzip streams, ignoring the MTIME field of the header.
+
+    Per RFC 1952 the gzip header is [magic(2)][CM(1)][FLG(1)][MTIME(4)][XFL(1)][OS(1)],
+    so bytes 4-8 are MTIME. The fixed offsets assume the standard 10-byte header
+    with no FNAME/FEXTRA/FCOMMENT flags set, which holds here because numcodecs'
+    ``GZip.encode`` wraps ``gzip.GzipFile`` without a filename.
+    """
+    if len(a) != len(b):
+        return False
+
+    return a[:4] == b[:4] and a[8:] == b[8:]
+
+
 # Shared mock-S3 (moto) backend. A single server is reused across the whole test session by
 # every test that needs S3 -- both the fsspec store tests and the documentation examples --
 # instead of each module standing up its own. Consumers create their own buckets and choose

tests/test_codecs/test_gzip.py

@@ -2,10 +2,10 @@
 import pytest
 
 import zarr
+from tests.conftest import gzip_streams_equal_except_mtime
 from zarr.abc.codec import SupportsSyncCodec
 from zarr.abc.store import Store
 from zarr.codecs import GzipCodec
-from zarr.codecs.gzip import _gzip_streams_equal_except_mtime
 from zarr.core.array_spec import ArrayConfig, ArraySpec
 from zarr.core.buffer import default_buffer_prototype
 from zarr.core.dtype import get_data_type_from_native_dtype
@@ -59,31 +59,31 @@ def test_gzip_streams_equal_except_mtime() -> None:
     suffix = b"\x00\xff\x10\x20"
 
     # Identical streams are equal.
-    assert _gzip_streams_equal_except_mtime(
+    assert gzip_streams_equal_except_mtime(
         prefix + mtime + suffix,
         prefix + mtime + suffix,
     )
 
     # Streams with different MTIME values are still equal.
-    assert _gzip_streams_equal_except_mtime(
+    assert gzip_streams_equal_except_mtime(
         prefix + b"\x01\x02\x03\x04" + suffix,
         prefix + b"\x05\x06\x07\x08" + suffix,
     )
 
     # Differences after MTIME are detected.
-    assert not _gzip_streams_equal_except_mtime(
+    assert not gzip_streams_equal_except_mtime(
         prefix + mtime + suffix,
         prefix + mtime + b"\x00\xff\x10\x21",
     )
 
     # Differences before MTIME are detected.
-    assert not _gzip_streams_equal_except_mtime(
+    assert not gzip_streams_equal_except_mtime(
         prefix + mtime + suffix,
         b"\x1f\x8b\x09\x00" + mtime + suffix,
     )
 
     # Different lengths are detected.
-    assert not _gzip_streams_equal_except_mtime(
+    assert not gzip_streams_equal_except_mtime(
         prefix + mtime + suffix,
         prefix + mtime + suffix + b"\x00",
     )

tests/test_fused_pipeline.py

@@ -11,7 +11,7 @@
 import pytest
 
 import zarr
-from tests.conftest import Expect
+from tests.conftest import Expect, gzip_streams_equal_except_mtime
 from zarr.abc.codec import (
     ArrayBytesCodec,
     ArrayBytesCodecPartialDecodeMixin,
@@ -20,7 +20,7 @@
 )
 from zarr.abc.store import Store, _store_supports_sync_io
 from zarr.codecs.bytes import BytesCodec
-from zarr.codecs.gzip import GzipCodec, _gzip_streams_equal_except_mtime
+from zarr.codecs.gzip import GzipCodec
 from zarr.codecs.transpose import TransposeCodec
 from zarr.codecs.zstd import ZstdCodec
 from zarr.core.array_spec import ArrayConfig, ArraySpec
@@ -785,7 +785,7 @@ def _bytes_identical(a: bytes, b: bytes) -> bool:
     ),
     Expect(
         input=(BytesCodec(), GzipCodec(level=1)),
-        output=_gzip_streams_equal_except_mtime,
+        output=gzip_streams_equal_except_mtime,
         id="bb",
     ),
     Expect(