There are plenty of examples in php out there, but I cannot find any in python. I reckon I can share my implementation here.

It is assuming you are using Google App Engine

    import webapp2
    import hashlib
    import hmac

    class MandrillWebhookHandler(webapp2.RequestHandler):

        def _calc_signature(self, raw, key):
            hashed = hmac.new(key, raw, hashlib.sha1)
            return hashed.digest().encode("base64").rstrip('\n')

        def verify_mandrill_signature(self):
            '''
            Mandrill includes an additional HTTP header with webhook POST requests,
                X-Mandrill-Signature, which will contain the signature for the request.
                To verify a webhook request, generate a signature using the same key
                that Mandrill uses and compare that to the value of the
                X-Mandrill-Signature header.
            :return: True if verified valid
            '''
            mandrill_signature = self.request.headers['X-Mandrill-Signature']
            mandrill_key = 'Your mandrill webhook key goes here'
            signed_data = self.request.path_url
            sorted_key = sorted(self.request.arguments())
            for k in sorted_key:
                signed_data += k
                signed_data += self.request.get(k)
            expected_signature = self._calc_signature(signed_data, mandrill_key)
            return expected_signature == mandrill_signature

        def head(self):
            '''
            Must return 200 so Mandrill knows it is a valid webhook

            http://help.mandrill.com/entries/22024856-Why-can-t-my-webhook-or-inbound-route-URL-be-verified-
            '''
            self.response.set_status(OK)

        def post(self):
            '''
            Mandrill sends data to our webhook by post
            '''
            if not self.verify_mandrill_signature():
                # check failed
                self.abort()

            # The rest of processing
            # ...