process.py

#!/usr/bin/env python3

import os
project_id = os.environ['COCALC_PROJECT_ID']

pre = "/projects/{project_id}/files/{path}/".format(
    project_id=project_id, path=os.path.abspath('.')[len(os.environ["HOME"])+1:])
post = "?fullscreen=kiosk"


def transform(path):
    return pre + path + post


def process(x):
    i = x.find('src="')
    j = x.find('"', i + 5)
    if i == -1 or j == -1:
        return x
    return x[:i + 5] + transform(x[i + 5:j]) + x[j:]


s = open('slides.md').read()

while True:
    i = s.find("<cocalc")
    if i == -1:
        break
    j = s.find('/>', i)
    # blah
    x = s[i + len('<cocalc') + 1:j]
    s = s[:i] + "<iframe " + process(x) + "/>" + s[j + 2:]

open('.slides.md', 'w').write(s)