PDF Form Filling with pypdf

When to Use

When you need to programmatically fill in PDF forms (AcroForms) with text values and checkbox selections.

Setup

pip install pypdf

Key Concepts

Reading Form Fields

import pypdf
reader = pypdf.PdfReader('form.pdf')
fields = reader.get_fields()
for name, field in fields.items():
    print(f"{name} | type={field.get('/FT')} | states={field.get('/_States_', [])}")

• /FT = /Tx → text field
• /FT = /Btn → checkbox/button
• /_States_ shows valid values for checkboxes (e.g., ['/1', '/Off'])

Filling the Form

reader = pypdf.PdfReader('blank.pdf')
writer = pypdf.PdfWriter()
writer.append(reader)

# Build field data dict - keys are FULL field paths
field_data = {
    "Form[0].Page1[0].TextField[0]": "value",
}

# For checkboxes, use the "on" state value from _States_ (e.g., '/1', '/2')
field_data["Form[0].Checkbox[0]"] = "/1"  # checked
# Use '/Off' for unchecked (usually default)

# Update fields on specific pages
writer.update_page_form_field_values(writer.pages[page_idx], field_data)

with open('filled.pdf', 'wb') as f:
    writer.write(f)

Important Notes

• Field names are hierarchical paths like SC-100[0].Page2[0].List1[0].Item1[0].PlaintiffName1[0]
• update_page_form_field_values only updates fields present on the specified page
• For radio-button-style checkboxes (Yes/No pairs), each option has its own field with different "on" states (/1 for first, /2 for second)
• Always use the full field path as the key
• Auto-formatting may not work; set /NeedAppearances if text doesn't display