Extract Form Field Info

---
description: Imported skill extract_form_field_info from anthropic
name: extract_form_field_info
signature: 9db1a2720cf54223cdc4bf797080c70f4e0d27288d9f400e066c14524519021d
source: /a0/tmp/skills_research/anthropic/skills/pdf/scripts/extract_form_field_info.py
---

import json
import sys

from pypdf import PdfReader


# Extracts data for the fillable form fields in a PDF and outputs JSON that
# Claude uses to fill the fields. See forms.md.


# This matches the format used by PdfReader `get_fields` and `update_page_form_field_values` methods.
def get_full_annotation_field_id(annotation):
    components = []
    while annotation:
        field_name = annotation.get('/T')
        if field_name:
            components.append(field_name)
        annotation = annotation.get('/Parent')
    return ".".join(reversed(components)) if components else None


def make_field_dict(field, field_id):
    field_dict = {"field_id": field_id}
    ft = field.get('/FT')
    if ft == "/Tx":
        field_dict["type"] = "text"
    elif ft == "/Btn":
        field_dict["type"] = "checkbox"  # radio groups handled separately
        states = field.get("/_States_", [])
        if len(states) == 2:
            # "/Off" seems to always be the unchecked value, as suggested by
            # https://opensource.adobe.com/dc-acrobat-sdk-docs/standards/pdfstandards/pdf/PDF32000_2008.pdf#page=448
            # It can be either first or second in the "/_States_" list.
            if "/Off" in states:
                field_dict["checked_value"] = states[0] if states[0] != "/Off" else states[1]
                field_dict["unchecked_value"] = "/Off"
            else:
                print(f"Unexpected state values for checkbox `${field_id}`. Its checked and unchecked values may not be correct; if you're trying to check it, visually verify the results.")
                field_dict["checked_value"] = states[0]
                field_dict["unchecked_value"] = states[1]
    elif ft == "/Ch":
        field_dict["type"] = "choice"
        states = field.get("/_States_", [])
        field_dict["choice_options"] = [{
            "value": state[0],
            "text": state[1],
        } for state in states]
    else:
        field_dict["type"] = f"unknown ({ft})"
    return field_dict


# Returns a list of fillable PDF fields:
# [
#   {
#     "field_id": "name",
#     "page": 1,
#     "type": ("text", "checkbox", "radio_group", or "choice")
#     // Per-type additional fields described in forms.md
#   },
# ]
def get_field_info(reader: PdfReader):
    fields = reader.get_fields()

    field_info_by_id = {}
    possible_radio_names = set()

    for field_id, field in fields.items():
        # Skip if this is a container field with children, except that it might be
        # a parent group for radio button options.
        if field.get("/Kids"):
            if field.get("/FT") == "/Btn":
                possible_radio_names.add(field_id)
            continue
        field_info_by_id[field_id] = make_field_dict(field, field_id)

    # Bounding rects are stored in annotations in page objects.

    # Radio button options have a separate annotation for each choice;
    # all choices have the same field name.
    # See https://westhealth.github.io/exploring-fillable-forms-with-pdfrw.html
    radio_fields_by_id = {}

    for page_index, page in enumerate(reader.pages):
        annotations = page.get('/Annots', [])
        for ann in annotations:
            field_id = get_full_annotation_field_id(ann)
            if field_id in field_info_by_id:
                field_info_by_id[field_id]["page"] = page_index + 1
                field_info_by_id[field_id]["rect"] = ann.get('/Rect')
            elif field_id in possible_radio_names:
                try:
                    # ann['/AP']['/N'] should have two items. One of them is '/Off',
                    # the other is the active value.
                    on_values = [v for v in ann["/AP"]["/N"] if v != "/Off"]
                except KeyError:
                    continue
                if len(on_values) == 1:
                    rect = ann.get("/Rect")
                    if field_id not in radio_fields_by_id:
                        radio_fields_by_id[field_id] = {
                            "field_id": field_id,
                            "type": "radio_group",
                            "page": page_index + 1,
                            "radio_options": [],
                        }
                    # Note: at least on macOS 15.7, Preview.app doesn't show selected
                    # radio buttons correctly. (It does if you remove the leading slash
                    # from the value, but that causes them not to appear correctly in
                    # Chrome/Firefox/Acrobat/etc).
                    radio_fields_by_id[field_id]["radio_options"].append({
                        "value": on_values[0],
                        "rect": rect,
                    })

    # Some PDFs have form field definitions without corresponding annotations,
    # so we can't tell where they are. Ignore these fields for now.
    fields_with_location = []
    for field_info in field_info_by_id.values():
        if "page" in field_info:
            fields_with_location.append(field_info)
        else:
            print(f"Unable to determine location for field id: {field_info.get('field_id')}, ignoring")

    # Sort by page number, then Y position (flipped in PDF coordinate system), then X.
    def sort_key(f):
        if "radio_options" in f:
            rect = f["radio_options"][0]["rect"] or [0, 0, 0, 0]
        else:
            rect = f.get("rect") or [0, 0, 0, 0]
        adjusted_position = [-rect[1], rect[0]]
        return [f.get("page"), adjusted_position]
    
    sorted_fields = fields_with_location + list(radio_fields_by_id.values())
    sorted_fields.sort(key=sort_key)

    return sorted_fields


def write_field_info(pdf_path: str, json_output_path: str):
    reader = PdfReader(pdf_path)
    field_info = get_field_info(reader)
    with open(json_output_path, "w") as f:
        json.dump(field_info, f, indent=2)
    print(f"Wrote {len(field_info)} fields to {json_output_path}")


if __name__ == "__main__":
    if len(sys.argv) != 3:
        print("Usage: extract_form_field_info.py [input pdf] [output json]")
        sys.exit(1)
    write_field_info(sys.argv[1], sys.argv[2])