RF: Pull compression detection logic into a central private module

Author: effigies

nibabel/_compression.py

@@ -0,0 +1,49 @@
+# emacs: -*- mode: python-mode; py-indent-offset: 4; indent-tabs-mode: nil -*-
+# vi: set ft=python sts=4 ts=4 sw=4 et:
+### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ##
+#
+#   See COPYING file distributed along with the NiBabel package for the
+#   copyright and license terms.
+#
+### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ##
+"""Constants and types for dealing transparently with compression"""
+from __future__ import annotations
+
+import bz2
+import gzip
+import io
+import typing as ty
+
+from .optpkg import optional_package
+
+if ty.TYPE_CHECKING:  # pragma: no cover
+    import indexed_gzip  # type: ignore
+    import pyzstd
+
+    HAVE_INDEXED_GZIP = True
+    HAVE_ZSTD = True
+else:
+    indexed_gzip, HAVE_INDEXED_GZIP, _ = optional_package('indexed_gzip')
+    pyzstd, HAVE_ZSTD, _ = optional_package('pyzstd')
+
+
+# Collections of types for isinstance or exception matching
+COMPRESSED_FILE_LIKES: tuple[type[io.IOBase], ...] = (
+    bz2.BZ2File,
+    gzip.GzipFile,
+)
+COMPRESSION_ERRORS: tuple[type[BaseException], ...] = (
+    OSError,  # BZ2File
+    gzip.BadGzipFile,
+)
+
+if HAVE_INDEXED_GZIP:
+    COMPRESSED_FILE_LIKES += (indexed_gzip.IndexedGzipFile,)
+    COMPRESSION_ERRORS += (indexed_gzip.ZranError,)
+    from indexed_gzip import IndexedGzipFile  # type: ignore
+else:
+    IndexedGzipFile = gzip.GzipFile
+
+if HAVE_ZSTD:
+    COMPRESSED_FILE_LIKES += (pyzstd.ZstdFile,)
+    COMPRESSION_ERRORS += (pyzstd.ZstdError,)

nibabel/filebasedimages.py

@@ -15,6 +15,7 @@
 from typing import Type
 from urllib import request
 
+from ._compression import COMPRESSION_ERRORS
 from .fileholders import FileHolder, FileMap
 from .filename_parser import TypesFilenamesError, _stringify_path, splitext_addext, types_filenames
 from .openers import ImageOpener
@@ -421,7 +422,7 @@ def _sniff_meta_for(
         try:
             with ImageOpener(meta_fname, 'rb') as fobj:
                 binaryblock = fobj.read(sniff_nbytes)
-        except (OSError, EOFError):
+        except COMPRESSION_ERRORS + (OSError, EOFError):
             return None
         return (binaryblock, meta_fname)
 

nibabel/openers.py

@@ -15,12 +15,11 @@
 from bz2 import BZ2File
 from os.path import splitext
 
-from nibabel.optpkg import optional_package
+from ._compression import HAVE_INDEXED_GZIP, IndexedGzipFile, pyzstd
 
 if ty.TYPE_CHECKING:  # pragma: no cover
     from types import TracebackType
 
-    import pyzstd
     from _typeshed import WriteableBuffer
 
     ModeRT = ty.Literal['r', 'rt']
@@ -32,8 +31,6 @@
     Mode = ty.Union[ModeR, ModeW]
 
     OpenerDef = tuple[ty.Callable[..., io.IOBase], tuple[str, ...]]
-else:
-    pyzstd = optional_package('pyzstd')[0]
 
 
 @ty.runtime_checkable
@@ -45,17 +42,6 @@ def write(self, b: bytes, /) -> int | None:
         ...  # pragma: no cover
 
 
-try:
-    from indexed_gzip import IndexedGzipFile  # type: ignore
-
-    HAVE_INDEXED_GZIP = True
-except ImportError:
-    # nibabel.openers.IndexedGzipFile is imported by nibabel.volumeutils
-    # to detect compressed file types, so we give a fallback value here.
-    IndexedGzipFile = gzip.GzipFile
-    HAVE_INDEXED_GZIP = False
-
-
 class DeterministicGzipFile(gzip.GzipFile):
     """Deterministic variant of GzipFile
 

nibabel/volumeutils.py

@@ -9,36 +9,28 @@
 """Utility functions for analyze-like formats"""
 from __future__ import annotations
 
-import gzip
 import io
 import sys
 import typing as ty
 import warnings
-from bz2 import BZ2File
 from functools import reduce
 from operator import getitem, mul
 from os.path import exists, splitext
 
 import numpy as np
 
+from ._compression import COMPRESSED_FILE_LIKES
 from .casting import OK_FLOATS, shared_range
 from .externals.oset import OrderedSet
-from .openers import IndexedGzipFile
-from .optpkg import optional_package
 
 if ty.TYPE_CHECKING:  # pragma: no cover
     import numpy.typing as npt
-    import pyzstd
-
-    HAVE_ZSTD = True
 
     Scalar = np.number | float
 
     K = ty.TypeVar('K')
     V = ty.TypeVar('V')
     DT = ty.TypeVar('DT', bound=np.generic)
-else:
-    pyzstd, HAVE_ZSTD, _ = optional_package('pyzstd')
 
 sys_is_le = sys.byteorder == 'little'
 native_code = sys_is_le and '<' or '>'
@@ -55,13 +47,6 @@
 #: default compression level when writing gz and bz2 files
 default_compresslevel = 1
 
-#: file-like classes known to hold compressed data
-COMPRESSED_FILE_LIKES: tuple[type[io.IOBase], ...] = (gzip.GzipFile, BZ2File, IndexedGzipFile)
-
-# Enable .zst support if pyzstd installed.
-if HAVE_ZSTD:
-    COMPRESSED_FILE_LIKES = (*COMPRESSED_FILE_LIKES, pyzstd.ZstdFile)
-
 
 class Recoder:
     """class to return canonical code(s) from code or aliases