pyflakes found the following issues in sendall():

Lib/test/mock_socket.py:95:21 undefined name 'data'
Lib/test/mock_socket.py:96:28 undefined name 'data'
Lib/test/mock_socket.py:97:20 undefined name 'data'

Code:

    def sendall(self, buffer, flags=None):
        self.last = data
        self.output.append(data)
        return len(data)

    def send(self, data, flags=None):
        self.last = data
        self.output.append(data)
        return len(data)

I guess that sendall() buffer parameter should be renamed to data.