from pdfrw import PdfReader
pdf = PdfReader('sample.pdf')
print(pdf.keys())
print(pdf.Info)
print(pdf.Root.keys())
print('PDF has {} pages'.format(len(pdf.pages)))

['/Root', '/Info', '/ID', '/Size']
{'/CreationDate': "(D:20180822164509+05'30')", '/Creator': '(Adobe LiveCycle Designer ES 10.4)', '/ModDate': "(D:20180822165611+05'30')", '/Producer': '(Adobe XML Form Module Library)'}
['/AcroForm', '/MarkInfo', '/Metadata', '/Names', '/NeedsRendering', '/Pages', '/Perms', '/StructTreeRoot', '/Type']
PDF has 1 pages

You can find the form fields here:

pdf.Root.AcroForm.Fields

or here

pdf.Root.Pages.Kids[page_index].Annots

This is a PdfArray object. Basically a List. The Name of the field is found here:

pdf.Root.AcroForm.Fields[field_index].T

Other keys include the value .V There's a bunch of display information, like the font etc under .AP.N.Resources

However, if you update the value for a field and output the pdf file. It might only display the value when the field has focus i.e is clicked on.

I haven't figured out how to fix that yet.

Use this to fill every fields if they are indexed.

template = PdfReader('template.pdf')
page_c = 0
while page_c < len(template.Root.Pages.Kids): #LOOP through pages
    annot_c = 0
    while annot_c < len(template.Root.Pages.Kids[page_c].Annots): #LOOP through fields
        template.Root.Pages.Kids[page_c].Annots[annot_c].update(PdfDict(V=str(annot_c)+'-'+str(page_c)))
        annot_c=annot_c+1
    page_c=page_c+1
PdfWriter().write('output.pdf', template)

I wrote a library built upon:'pdfrw', 'pdf2image', 'Pillow', 'PyPDF2' called fillpdf (pip install fillpdf and poppler dependency conda install -c conda-forge poppler)

Basic usage:

from fillpdf import fillpdfs

fillpdfs.get_form_fields("blank.pdf")

# returns a dictionary of fields
# Set the returned dictionary values a save to a variable
# For radio boxes ('Off' = not filled, 'Yes' = filled)

data_dict = {
'Text2': 'Name',
'Text4': 'LastName',
'box': 'Yes',
}

fillpdfs.write_fillable_pdf('blank.pdf', 'new.pdf', data_dict)

# If you want it flattened:
fillpdfs.flatten_pdf('new.pdf', 'newflat.pdf')

More info here: https://github.com/t-houssian/fillpdf

If some fields don't fill, you can use fitz (pip install PyMuPDF) and PyPDF2 (pip install PyPDF2) like the following altering the points as needed:

import fitz
from PyPDF2 import PdfFileReader

file_handle = fitz.open('blank.pdf')
pdf = PdfFileReader(open('blank.pdf','rb'))
box = pdf.getPage(0).mediaBox
w = box.getWidth()
h = box.getHeight()

# For images
image_rectangle = fitz.Rect((w/2)-200,h-255,(w/2)-100,h-118)
pages = pdf.getNumPages() - 1
last_page = file_handle[pages]
last_page._wrapContents()
last_page.insertImage(image_rectangle, filename=f'image.png')

# For text
last_page.insertText(fitz.Point((w/2)-247 , h-478), 'John Smith', fontsize=14, fontname="times-bold")
file_handle.save(f'newpdf.pdf')

AcroForm based Forms using PDFix SDK

def SetFormFieldValue(email, key, open_path, save_path):
    pdfix  = GetPdfix()
    if pdfix is None:
        raise Exception('Pdfix Initialization fail')
    if not pdfix.Authorize(pdfix_email, pdfix_license):
        raise Exception('Authorization fail : ' + pdfix.GetError())
    doc = pdfix.OpenDoc(open_path, "")
    if doc is None:
        raise Exception('Unable to open pdf : ' + pdfix.GetError())
    field = doc.GetFormFieldByName("Text1")
    if field is not None:
        value = field.GetValue()
        value = "New Value"
        field.SetValue(value)
    if not doc.Save(save_path, kSaveFull):
        raise Exception(pdfix.GetError())
    doc.Close()
    pdfix.Destroy()

A full solution was provided here: How to edit editable pdf using the pdfrw library?

The key part is the:

template_pdf.Root.AcroForm.update(pdfrw.PdfDict(NeedAppearances=pdfrw.PdfObject('true')))