title: How to shutil.copyfile only if file differ?
body: <p><code>shutil.copyfile</code> is quite useful for copying files from a location to another. Unfortunately, it does copy the file even though it already exists. </p>
  
  <p>I find <code>rsync --checksum</code> quite convenient in this case, but I don't think it worth calling <code>rsync</code> from Python. </p>
  
  <p>What alternative can I use to copy a file only if it does not exist or it is not the same?</p>