STY: Remove an unnecessary else (#3179)

---------

Co-authored-by: Stefan <96178532+stefan6419846@users.noreply.github.com>

Author: j-t-1

pypdf/_reader.py

@@ -876,8 +876,7 @@ def _read_xref_tables_and_trailers(
                             f"Previous trailer cannot be read: {e.args}", __name__
                         )
                         break
-                    else:
-                        raise PdfReadError(f"Trailer cannot be read: {e.args}")
+                    raise PdfReadError(f"Trailer cannot be read: {e!s}")
                 self._process_xref_stream(xrefstream)
                 if "/Prev" in xrefstream:
                     startxref = cast(int, xrefstream["/Prev"])
@@ -972,7 +971,8 @@ def _read_pdf15_xref_stream(
         stream.seek(-1, 1)
         idnum, generation = self.read_object_header(stream)
         xrefstream = cast(ContentStream, read_object(stream, self))
-        assert cast(str, xrefstream["/Type"]) == "/XRef"
+        if cast(str, xrefstream["/Type"]) != "/XRef":
+            raise PdfReadError(f"Unexpected type {xrefstream['/Type']!r}")
         self.cache_indirect_object(generation, idnum, xrefstream)
         stream_data = BytesIO(xrefstream.get_data())
         # Index pairs specify the subsections in the dictionary.

tests/test_reader.py

@@ -1835,3 +1835,11 @@ def test_infinite_loop_for_length_value():
     writer = PdfWriter()
     with pytest.raises(PdfReadError, match=r"^Detected loop with self reference for IndirectObject\(165, 0, \d+\)\.$"):
         writer.add_page(reader.pages[0])
+
+
+def test_trailer_cannot_be_read():
+    path = RESOURCE_ROOT / "crazyones.pdf"
+    data = path.read_bytes().replace(b"/Type/XRef", b"/Type/Invalid")
+    with pytest.raises(PdfReadError, match=r"^Trailer cannot be read: Unexpected type '/Invalid'$"):
+        reader = PdfReader(BytesIO(data))
+        list(reader.pages)