BUG: Avoid accessing invalid page when inserting blank page under some conditions

pypdf/_writer.py

@@ -707,10 +707,12 @@ def insert_blank_page(
                 and previous page does not exist.
 
         """
-        if width is None or (height is None and index < self.get_num_pages()):
-            oldpage = self.pages[index]
-            width = oldpage.mediabox.width
-            height = oldpage.mediabox.height
+        if (width is None or height is None) and index < self.get_num_pages():
+            old_page = self.pages[index]
+            if width is None:
+                width = old_page.mediabox.width
+            if height is None:
+                height = old_page.mediabox.height
         page = PageObject.create_blank_page(self, width, height)
         self.insert_page(page, index)
         return page

tests/test_writer.py

@@ -238,6 +238,10 @@ def writer_operate(writer: PdfWriter) -> None:
     writer.page_mode = NameObject("/UseOC")
     assert writer._get_page_mode() == "/UseOC"
     writer.insert_blank_page(width=100, height=100)
+    page = writer.insert_blank_page(width=100)
+    assert page.mediabox.height == 100
+    page = writer.insert_blank_page(height=100)
+    assert page.mediabox.width == 100
     writer.insert_blank_page()  # without parameters
 
     writer.remove_images()
@@ -255,6 +259,24 @@ def writer_operate(writer: PdfWriter) -> None:
         assert k in objects_hash, f"Missing {v}"
 
 
+def test_insert_blank_page():
+    writer = PdfWriter(clone_from=RESOURCE_ROOT / "crazyones.pdf")
+
+    old_page = writer.pages[0]
+    page = writer.insert_blank_page(index=0)
+    assert page.mediabox.width == old_page.mediabox.width
+    assert page.mediabox.height == old_page.mediabox.height
+
+    old_page = writer.pages[0]
+    page = writer.insert_blank_page(width=50, index=0)
+    assert page.mediabox.width == 50
+    assert page.mediabox.height == old_page.mediabox.height
+
+    page = writer.insert_blank_page(width=10, height=20, index=0)
+    assert page.mediabox.width == 10
+    assert page.mediabox.height == 20
+
+
 @pytest.mark.parametrize(
     ("convert", "needs_cleanup"),
     [