They are quite similar, but there is one key difference:

request.POST['username'] will throw a KeyError if the username key is missing from the POST data.
request.POST.get('username') will simply return None if the key is missing, and won't throw an error.

So request.POST.get('username') is a safer choice if the username field is optional, or may be missing for some reason.
It's generally a good idea to use request.POST.get() to retrieve form data, to avoid potential KeyError exceptions being thrown.
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