MAINT: Let PdfMerger._create_stream raise NotImplemented (#1251)

... if arg is none of str/Path/stream/PdfReader

PyPDF2/_merger.py

@@ -250,7 +250,12 @@ def _create_stream(
             stream = BytesIO(filecontent)
             my_file = True
         else:
-            stream = fileobj
+            raise NotImplementedError(
+                "PdfMerger.merge requires an object that PdfReader can parse. "
+                "Typically, that is a Path or a string representing a Path, "
+                "a file object, or an object implementing .seek and .read. "
+                "Passing a PdfReader directly works as well."
+            )
         return stream, my_file, encryption_obj
 
     @deprecate_bookmark(bookmark="outline_item", import_bookmarks="import_outline")

tests/test_merger.py

@@ -30,6 +30,14 @@ def merger_operate(merger):
     merger.append(pdf_path, pages=PyPDF2.pagerange.PageRange(slice(0, 0)))
     merger.append(pdf_forms)
     merger.merge(0, pdf_path, import_outline=False)
+    with pytest.raises(NotImplementedError) as exc:
+        with open(pdf_path, "rb") as fp:
+            data = fp.read()
+        merger.append(data)
+    assert exc.value.args[0].startswith(
+        "PdfMerger.merge requires an object that PdfReader can parse. "
+        "Typically, that is a Path"
+    )
 
     # Merging an encrypted file
     reader = PyPDF2.PdfReader(pdf_pw)