BUG: Fix set_page_label TypeError when start=None (#4286)

* ROB: Tolerate extra operands before Tm/cm in layout mode

A matrix operator consumes the 6 operands nearest to it; values
left on the operand stack before those (malformed content streams)
overflowed TextStateManager.new_transform() and raised TypeError
in layout mode text extraction.

Complete the matrix with only the last 6 operands in
_complete_matrix and add_cm, mirroring the tolerance already in
place for missing/non-numeric TD and Tz operands (#4232, #4233).

Fixes #4283

Co-authored-by: rahul05ranjan <11793455+rahul05ranjan@users.noreply.github.com>

* Fix set_page_label TypeError when start=None

The start parameter is annotated Optional[int] and its validation
accepts None as "not set", but _set_page_label used ,
which treats None as a value to write and crashes in NumberObject.

Only write /St when a concrete non-zero start was provided.

Fixes #4285

Co-authored-by: rahul05ranjan <11793455+rahul05ranjan@users.noreply.github.com>

* Add tests for set_page_label with start=None

Co-authored-by: rahul05ranjan <11793455+rahul05ranjan@users.noreply.github.com>

* Update tests/test_writer.py

* Update tests/test_writer.py

---------

Co-authored-by: rahul05ranjan <11793455+rahul05ranjan@users.noreply.github.com>
Co-authored-by: Stefan <96178532+stefan6419846@users.noreply.github.com>

Author: rahul05ranjan

pypdf/_writer.py

@@ -3460,7 +3460,7 @@ def _set_page_label(
             new_page_label[NameObject("/S")] = NameObject(style)
         if prefix is not None:
             new_page_label[NameObject("/P")] = TextStringObject(prefix)
-        if start != 0:
+        if start is not None and start != 0:
             new_page_label[NameObject("/St")] = NumberObject(start)
 
         if NameObject(CatalogAttributes.PAGE_LABELS) not in self._root_object:

tests/test_writer.py

@@ -1769,6 +1769,38 @@ def test_set_page_label(pdf_file_path):
     writer.write(pdf_file_path)
 
 
+def test_set_page_label__start_none():
+    """`start=None` is a legitimate "not set" value and must not crash at write time (#4285).
+
+    It behaves exactly like the default `0`: no `/St` entry is written, so numbering
+    restarts at 1 per the PDF spec.
+    """
+    writer = PdfWriter()
+    for _ in range(4):
+        writer.add_blank_page(width=100, height=100)
+    writer.set_page_label(0, 3, "/D", None, None)
+
+    buffer = BytesIO()
+    writer.write(buffer)
+    buffer.seek(0)
+
+    assert PdfReader(buffer).page_labels == ["1", "2", "3", "4"]
+
+
+def test_set_page_label__prefix_only_without_start():
+    """A prefix-only label with `style=None` and `start=None` works (#4285)."""
+    writer = PdfWriter()
+    for _ in range(4):
+        writer.add_blank_page(width=100, height=100)
+    writer.set_page_label(0, 3, None, "P", None)
+
+    buffer = BytesIO()
+    writer.write(buffer)
+    buffer.seek(0)
+
+    assert PdfReader(buffer).page_labels == ["P", "P", "P", "P"]
+
+
 @pytest.mark.enable_socket
 def test_iss1601():
     url = "https://github.com/py-pdf/pypdf/files/10579503/badges-38.pdf"