Need to replace a string with a file (HTML file)

[Larz60+]

the following assumes the actual text you want to replace is "Message"
wrote on the fly, I have not tested this code

import fileinput


with open('C:/01/File_replace/Email_Body.txt','r') as infile:
    replacement_text = infile.read()

with open('C:/01/File_replace/Template_EM.html') as currentfile, 
    open('C:/01/File_replace/Template_EM_New.html','w') as nf:

    for line in currentfile:
        if "Message" in line:
            line.replace("Message", replacement_text)
        nf.write(line)