Add support for page overlays to support non-trivial page headers/footers

[doekman]

Paged media CSS doesn't support more complex page header/footer requirements, like tables. A solution to this is to create a separate page with such a header, and merge this into the PDF on the required pages. A proof of concept was written by pikhovkin in this Gist.

I made a function to use this functionality with WeasyPrint, and would like to create a pull request to include this into Flask-WeayPrint, since it is such a common use-case.

(any comments about my coding is welcome, since I'm new to the Python language and community)

---

My API is currently like this:

first_page =     lambda page_nr: page_nr==0
not_first_page = lambda page_nr: page_nr!=0
odd_page =       lambda page_nr: page_nr%2==1
even_page =      lambda page_nr: page_nr%2==0

class Overlay(object):
    def __init__(self, template_name_or_list, fn_include_on_page=not_first_page, **kwargs):

def render_pdf_with_overlays(html, *overlays, stylesheets=None, download_filename=None):

#example:
html = flask.render_template('my_report.html')
first_page_overlay = Overlay('first_page_overlay.html', fn_include_on_page=first_page, left='10cm', top='5cm')
default_page_overlay = Overlay('default_page_overlay.html', fn_include_on_page=not_first_page)
pdf_response = render_pdf_with_overlays(HTML(string=html), first_page_overlay, default_page_overlay)

Overlay: Represents one HTML page to be overlaid on a PDF document.

• template_name_or_list: the name of the HTML template, which is resolved with jinja2's get_or_select_template method.
• fn_include_on_page: a function that takes a 0-based page number, and returns a boolean that indicates if it needs to be included on that page.
• kwargs: context used to render the overlay. Can also be used to dynamically position content.

render_pdf_with_overlays renders the PDF like Flask-WeasyPrint's render_pdf, but with overlays.

• stylesheets are also used to render the Overlay, so if any FontConfiguration is included, it is also included in the Overlays.
• the overlays templates are rendered to HTML with the supplied context, and this context is extended with the page and pages entries, indicating current 1-based page number and number of pages (CSS page and pages counters don't work with single page overlays)

[liZe]

Hi @doekman,

Thank you for your comment and for your idea, it looks like a great solution. Next version of WeasyPrint may rely on pdfrw, a library able to edit PDF files. It's opening a large set of new possible solutions like this one, with or without an API in WeasyPrint.

However, your solution doesn't really rely on the Flask part of Flask-WeasyPrint and would be better discussed in Kozea/WeasyPrint#92. Could you please copy your comment in Kozea/WeasyPrint#92?

Thank you!

[doekman]

My solution does rely on Flask a bit (actually Jinja2) for rendering of the page number (which obviously varies per page). But I get your point, some work should be done in WeasyPrint first.

I added my comment to WeasyPrint, and put my current solution in a gist, so others might use it.

[liZe]

My solution does rely on Flask a bit (actually Jinja2) for rendering of the page number (which obviously varies per page).

You can use CSS instead of Flask for page numbers 😉.

I added my comment to WeasyPrint, and put my current solution in a gist, so others might use it.

Thanks a lot!