BUG: Do not fail on choice field without /Opt key (#3540)

Closes #2838.

According to the PDF 2.0 specification, the `/Opt` key is not required for choice fields.

Author: jhuber-de

pypdf/_doc_common.py

@@ -599,7 +599,7 @@ def _build_field(
             fileobj.write("\n")
         retval[key] = Field(field)
         obj = retval[key].indirect_reference.get_object()  # to get the full object
-        if obj.get(FA.FT, "") == "/Ch":
+        if obj.get(FA.FT, "") == "/Ch" and obj.get(NameObject(FA.Opt)):
             retval[key][NameObject("/_States_")] = obj[NameObject(FA.Opt)]
         if obj.get(FA.FT, "") == "/Btn" and "/AP" in obj:
             #  Checkbox

tests/test_reader.py

@@ -418,7 +418,7 @@ def test_get_page_of_encrypted_file(pdffile, password, should_fail):
             "crazyones.pdf",
             {},
             None,
-        ),
+        )
     ],
 )
 def test_get_form(src, expected, expected_get_fields, txt_file_path):
@@ -448,6 +448,21 @@ def test_get_form(src, expected, expected_get_fields, txt_file_path):
             ]
 
 
+@pytest.mark.enable_socket
+def test_reading_choice_field_without_opt_key():
+    """Tests reading a choice field in a PDF without an /Opt key."""
+    url = "https://github.com/user-attachments/files/23853677/Musterservicevertrag-HNRAGB_Okt2022-Blanko.pdf"
+    reader = PdfReader(BytesIO(get_data_from_url(url, name="Musterservicevertrag-HNRAGB_Okt2022-Blanko.pdf")))
+    fields = reader.get_fields()
+
+    tn_anrede = fields.get("TN_Anrede")
+    assert tn_anrede is not None
+
+    # Ensure that parsing of a choice field without /Opt key worked
+    tn_anrede_opt = tn_anrede.get("/Opt")
+    assert tn_anrede_opt is None
+
+
 @pytest.mark.parametrize(
     ("src", "page_number"),
     [