# Load the source file
sourcePath = r'c:\path\to\my\file.cpp'
sourceText = codecs.open(sourcePath, encoding='utf-8').read()

# Find the generated section

startMarker = (
'''/*
 * Generated data tables - do not edit by hand!
 * To regenerate, run my_fancy_script.py.
 */
''')
startMarkerPos = sourceText.index(startMarker)

endMarker = (
'''/*
 * End of generated section
 */
''')
endMarkerPos = sourceText.index(endMarker)

# Rewrite the file with the new generated section,
# but only if it's different from the existing one.

generatedCodePrev = sourceText[startMarkerPos + len(startMarker) : endMarkerPos]
if generatedCode != generatedCodePrev:
    sourceTextNew = (sourceText[:startMarkerPos] +
                     startMarker +
                     generatedCode +
                     sourceText[endMarkerPos:])
    codecs.open(sourcePath, mode='w', encoding='utf-8').write(sourceTextNew)
    print('Updated %s with new data tables.' % sourcePath)
else:
    print('No changes to %s needed.' % sourcePath)