Improvements based on code review comments.

pypdf/_writer.py

@@ -235,9 +235,9 @@ def _get_clone_from(
                 or Path(str(fileobj)).stat().st_size == 0
             ):
                 cloning = False
-            if isinstance(fileobj, (IO, BytesIO)):
+            if isinstance(fileobj, (IOBase, BytesIO)):
                 t = fileobj.tell()
-                fileobj.seek(-1, 2)
+                fileobj.seek(0, 2)
                 if fileobj.tell() == 0:
                     cloning = False
                 fileobj.seek(t, 0)
@@ -249,6 +249,7 @@ def _get_clone_from(
         # to prevent overwriting
         self.temp_fileobj = fileobj
         self.fileobj = ""
+        self.cloned = False
         # The root of our page tree node.
         pages = DictionaryObject()
         pages.update(
@@ -266,6 +267,7 @@ def _get_clone_from(
             if not isinstance(clone_from, PdfReader):
                 clone_from = PdfReader(clone_from)
             self.clone_document_from_reader(clone_from)
+            self.cloned = True
         else:
             self._pages = self._add_object(pages)
             # root object
@@ -352,9 +354,11 @@ def xmp_metadata(self, value: Optional[XmpInformation]) -> None:
         return self.root_object.xmp_metadata  # type: ignore
 
     def __enter__(self) -> "PdfWriter":
-        """Store that writer is initialized by 'with'."""
+        """Store how writer is initialized by 'with'."""
+        c: bool = self.cloned
         t = self.temp_fileobj
         self.__init__()  # type: ignore
+        self.cloned = c
         self.fileobj = t  # type: ignore
         return self
 
@@ -365,11 +369,8 @@ def __exit__(
         traceback: Optional[TracebackType],
     ) -> None:
         """Write data to the fileobj."""
-        if self.fileobj:
+        if self.fileobj and not self.cloned:
             self.write(self.fileobj)
-            close_attr = getattr(self.fileobj, "close", None)
-            if callable(close_attr):
-                self.fileobj.close()  # type: ignore[attr-defined]
 
     def _repr_mimebundle_(
         self,

tests/test_writer.py

@@ -2485,9 +2485,27 @@ def test_append_pdf_with_dest_without_page(caplog):
 def test_stream_not_closed():
     """Tests for #2905"""
     src = RESOURCE_ROOT / "pdflatex-outline.pdf"
-    with NamedTemporaryFile() as tmp:
+    with NamedTemporaryFile(suffix=".pdf") as tmp:
         with PdfReader(src) as reader, PdfWriter() as writer:
-            for i in range(4):
-                writer.add_page(reader.pages[i])
+            writer.add_page(reader.pages[0])
             writer.write(tmp)
         assert not tmp.file.closed
+
+    with NamedTemporaryFile(suffix=".pdf") as target:
+        with PdfWriter(target.file) as writer:
+            writer.add_blank_page(100, 100)
+        assert not target.file.closed
+
+    with open(src, "rb") as fileobj:
+        with PdfWriter(fileobj) as writer:
+            pass
+        assert not fileobj.closed
+
+
+def test_auto_write():
+    """Another test for #2905"""
+    with NamedTemporaryFile(suffix=".pdf", delete_on_close=False) as tmp:
+        tmp.close()
+        with PdfWriter(tmp.name) as writer:
+            writer.add_blank_page(100, 100)
+        assert Path(tmp.name).stat().st_size > 0