Fill Fillable Fields

import json import sys

from pypdf import PdfReader, PdfWriter

from extract_form_field_info import get_field_info

Fills fillable form fields in a PDF. See forms.md.

def fill_pdf_fields(input_pdf_path: str, fields_json_path: str, output_pdf_path: str): with open(fields_json_path) as f: fields = json.load(f) # Group by page number. fields_by_page = {} for field in fields: if "value" in field: field_id = field["field_id"] page = field["page"] if page not in fields_by_page: fields_by_page[page] = {} fields_by_page[page][field_id] = field["value"]

reader = PdfReader(input_pdf_path)

has_error = False
field_info = get_field_info(reader)
fields_by_ids = {f["field_id"]: f for f in field_info}
for field in fields:
    existing_field = fields_by_ids.get(field["field_id"])
    if not existing_field:
        has_error = True
        print(f"ERROR: `{field['field_id']}` is not a valid field ID")
    elif field["page"] != existing_field["page"]:
        has_error = True
        print(f"ERROR: Incorrect page number for `{field['field_id']}` (got {field['page']}, expected {existing_field['page']})")
    else:
        if "value" in field:
            err = validation_error_for_field_value(existing_field, field["value"])
            if err:
                print(err)
                has_error = True
if has_error:
    sys.exit(1)

writer = PdfWriter(clone_from=reader)
for page, field_values in fields_by_page.items():
    writer.update_page_form_field_values(writer.pages[page - 1], field_values, auto_regenerate=False)

# This seems to be necessary for many PDF viewers to format the form values correctly.
# It may cause the viewer to show a "save changes" dialog even if the user doesn't make any changes.
writer.set_need_appearances_writer(True)

with open(output_pdf_path, "wb") as f:
    writer.write(f)

def validation_error_for_field_value(field_info, field_value): field_type = field_info["type"] field_id = field_info["field_id"] if field_type == "checkbox": checked_val = field_info["checked_value"] unchecked_val = field_info["unchecked_value"] if field_value != checked_val and field_value != unchecked_val: return f'ERROR: Invalid value "{field_value}" for checkbox field "{field_id}". The checked value is "{checked_val}" and the unchecked value is "{unchecked_val}"' elif field_type == "radio_group": option_values = [opt["value"] for opt in field_info["radio_options"]] if field_value not in option_values: return f'ERROR: Invalid value "{field_value}" for radio group field "{field_id}". Valid values are: {option_values}' elif field_type == "choice": choice_values = [opt["value"] for opt in field_info["choice_options"]] if field_value not in choice_values: return f'ERROR: Invalid value "{field_value}" for choice field "{field_id}". Valid values are: {choice_values}' return None

pypdf (at least version 5.7.0) has a bug when setting the value for a selection list field.

In _writer.py around line 966:

if field.get(FA.FT, "/Tx") == "/Ch" and field_flags & FA.FfBits.Combo == 0:

txt = "\n".join(annotation.get_inherited(FA.Opt, []))

The problem is that for selection lists, get_inherited returns a list of two-element lists like

[["value1", "Text 1"], ["value2", "Text 2"], ...]

This causes join to throw a TypeError because it expects an iterable of strings.

The horrible workaround is to patch get_inherited to return a list of the value strings.

We call the original method and adjust the return value only if the argument to get_inherited

is FA.Opt and if the return value is a list of two-element lists.

def monkeypatch_pydpf_method(): from pypdf.generic import DictionaryObject from pypdf.constants import FieldDictionaryAttributes

original_get_inherited = DictionaryObject.get_inherited

def patched_get_inherited(self, key: str, default = None):
    result = original_get_inherited(self, key, default)
    if key == FieldDictionaryAttributes.Opt:
        if isinstance(result, list) and all(isinstance(v, list) and len(v) == 2 for v in result):
            result = [r[0] for r in result]
    return result

DictionaryObject.get_inherited = patched_get_inherited

if name == "main": if len(sys.argv) != 4: print("Usage: fill_fillable_fields.py [input pdf] [field_values.json] [output pdf]") sys.exit(1) monkeypatch_pydpf_method() input_pdf = sys.argv[1] fields_json = sys.argv[2] output_pdf = sys.argv[3] fill_pdf_fields(input_pdf, fields_json, output_pdf)