Serving dynamic PDF through Flask, generated with pdfrw

Question

I'm trying to serve a dynamic PDF to my users through Flask. They fill a form to get some calculations presented, but they would like to auto-fill the values to a PDF-template and then print it on paper.

I got it working locally, just filling the values to a PDF-template, but I have no clue on how I'd work it into my Flask app. What I got locally is I've got a template PDF-file, a dictionary with the values, a function to fill the values to the template and write it out to a new PDF-file. Thou when hosting it with Flask, I want it to dynamically serve them the new PDF created by the function instead of writing it to a file.

I'm using the pdfrw library for the pdf-modification, I realise I might need to use make_reponse or IO or a combination. Just dont know how to get the result from pdfrw NOT to a file. Or where to start.

Here's the function:

    import os
    import pdfrw

    data_dict = #just a plain dict corresponding with the temp.pdf keys
    PDF_TEMPLATE_PATH = 'temp.pdf'
    PDF_OUT_PATH = 'out.pdf'


    ANNOT_KEY = '/Annots'
    ANNOT_FIELD_KEY = '/T'
    ANNOT_VAL_KEY = '/V'
    ANNOT_RECT_KEY = '/Rect'
    SUBTYPE_KEY = '/Subtype'
    WIDGET_SUBTYPE_KEY = '/Widget'


    def write_fillable_pdf(input_pdf_path, output_pdf_path, data_dict):
        template_pdf = pdfrw.PdfReader(input_pdf_path)
        annotations = template_pdf.pages[0][ANNOT_KEY]
        for annotation in annotations:
            if annotation[SUBTYPE_KEY] == WIDGET_SUBTYPE_KEY:
                if annotation[ANNOT_FIELD_KEY]:
                    key = annotation[ANNOT_FIELD_KEY][1:-1]
                    if key in data_dict.keys():
                        annotation.update(
                            pdfrw.PdfDict(V='{}'.format(data_dict[key]))
                        )
        pdfrw.PdfWriter().write(output_pdf_path, template_pdf)

    if __name__ == '__main__':
        write_fillable_pdf(PDF_TEMPLATE_PATH, PDF_OUT_PATH, data_dict)

So I guess, how do I get this line

pdfrw.PdfWriter().write(output_pdf_path, template_pdf)

to be able to serve with eg. make_reponse or IO or likely? I'll just make a button-href to a /pdf route in flask for this when I get it working.

Sorry I'm rather new to this, trying over my head to meet the wishes of my users.

Thank you for any guidance or direction.

Answer 1

Assuming you don't want to save the PDF to the server, and just keep it in memory, then serve it with Flask's send_file function, you could modify your write_fillable_pdf function to instead return a bytes type object. So instead of:

pdfrw.PdfWriter().write(output_pdf_path, template_pdf)

You could do the following:

buf = io.BytesIO()
pdfrw.PdfWriter().write(buf, template_pdf)
buf.seek(0)
return buf

remembering to import io at the top of the file!

Then in your flask code, the return value of write_fillable_pdf could be passed directly to the send file function:

from flask import send_file
data = write_fillable_pdf() # With some arguments
return send_file(data, mimetype='application/pdf')

You'd probably want to modifiy the arguments write_fillable_pdf accepts, as you no longer wish to supply the output_pdf_path. You'd also want to pass this your data_dict which could be constructed based on values supplied from the user or somewhere else.