Fix misleading error on opening empty file

src/eventio/file_types.py

@@ -1,34 +1,39 @@
 import gzip
 try:
     import zstandard as zstd
-    has_zstd = True
-except ImportError:
-    has_zstd = False
+except ModuleNotFoundError:
+    zstd = None
 
 from .constants import (
     SYNC_MARKER_SIZE,
     SYNC_MARKER_LITTLE_ENDIAN,
     SYNC_MARKER_BIG_ENDIAN,
 )
 
+ZSTD_MARKER = b'\x28\xb5\x2f\xfd'
+GZIP_MARKER = b'\x1f\x8b'
+
+
+def _check_marker(path, marker):
+    with open(path, 'rb') as f:
+        marker_bytes = f.read(len(marker))
+
+    if len(marker_bytes) < len(marker):
+        return False
+
+    return marker_bytes == marker
 
 def is_gzip(path):
     '''Test if a file is gzipped by reading its first two bytes and compare
     to the gzip marker bytes.
     '''
-    with open(path, 'rb') as f:
-        marker_bytes = f.read(2)
-
-    return marker_bytes[0] == 0x1f and marker_bytes[1] == 0x8b
+    return _check_marker(path, GZIP_MARKER)
 
 
 def is_zstd(path):
     '''Test if a file is compressed using zstd using its magic marker bytes
     '''
-    with open(path, 'rb') as f:
-        marker_bytes = f.read(4)
-
-    return marker_bytes == b'\x28\xb5\x2f\xfd'
+    return _check_marker(path, ZSTD_MARKER)
 
 
 def is_eventio(path):
@@ -39,7 +44,7 @@ def is_eventio(path):
         with gzip.open(path, 'rb') as f:
             marker_bytes = f.read(SYNC_MARKER_SIZE)
     elif is_zstd(path):
-        if not has_zstd:
+        if zstd is None:
             raise IOError('You need the `zstandard` module to read zstd files')
         with open(path, 'rb') as f:
             cctx = zstd.ZstdDecompressor()

tests/test_open_file.py

@@ -1,6 +1,7 @@
-import eventio
 from os import path
 from itertools import zip_longest
+import eventio
+import pytest
 
 
 def test_is_install_folder_a_directory():
@@ -19,6 +20,13 @@ def test_file_is_iterable():
     for event in f:
         pass
 
+def test_empty(tmp_path):
+    path = tmp_path / "empty.dat"
+    path.write_bytes(b"")
+
+    with pytest.raises(ValueError, match="^File .* is not an eventio file$"):
+        eventio.EventIOFile(path)
+
 
 def test_file_has_objects_at_expected_position():
     expected = [