How to use the twisted.internet.defer.fail function in Twisted

def reverse_resolve(self, target_ip):
        for name, candidate_ip in self.dns.items():
            if not isIPAddress(name) and candidate_ip == target_ip:
                return defer.succeed(name)
        return defer.fail(socket.herror())

for group in groups:
            try:
                g = self.dbm['groups'][group]
            except KeyError:
                pass
            else:
                index = g.maxArticle + 1
                g.maxArticle += 1
                g.articles[index] = article
                xref.append((group, str(index)))
                self.dbm['groups'][group] = g

        if not xref:
            return defer.fail(NewsServerError("No groups carried: " + ' '.join(groups)))

        article.putHeader('Xref', '%s %s' % (socket.gethostname().split()[0], ' '.join(map(lambda x: ':'.join(x), xref))))
        self.dbm['Message-IDs'][article.getHeader('Message-ID')] = xref
        return defer.succeed(None)

        @type contextFactory: L{NoneType } or
            L{ClientContextFactory }
        @param contextFactory: The context factory with which to negotiate TLS.
            If not provided, try to create a new one.

        @rtype: L{Deferred } which successfully results in
            L{dict} mapping L{bytes} to L{list} of L{bytes} and/or L{bytes} to
            L{NoneType } or fails with L{TLSError}
        @return: A deferred which fires when the transport has been
            secured according to the given context factory with the server
            capabilities, or which fails with a TLS error if the transport
            cannot be secured.
        """
        tls = interfaces.ITLSTransport(self.transport, None)
        if tls is None:
            return defer.fail(TLSError(
                "POP3Client transport does not implement "
                "interfaces.ITLSTransport"))

        if contextFactory is None:
            contextFactory = self._getContextFactory()

        if contextFactory is None:
            return defer.fail(TLSError(
                "POP3Client requires a TLS context to "
                "initiate the STLS handshake"))

        d = self.capabilities()
        d.addCallback(self._startTLS, contextFactory, tls)
        return d

        @param path: the directory or file to check.
        @type path: C{str}

        @param keys: the list of desired metadata
        @type keys: C{list} of C{str}
        """
        filePath = self._path(path)
        if filePath.isdir():
            entries = filePath.listdir()
            fileEntries = [filePath.child(p) for p in entries]
        elif filePath.isfile():
            entries = [os.path.join(*filePath.segmentsFrom(self.filesystemRoot))]
            fileEntries = [filePath]
        else:
            return defer.fail(FileNotFoundError(path))

        results = []
        for fileName, filePath in zip(entries, fileEntries):
            ent = []
            results.append((fileName, ent))
            if keys:
                try:
                    ent.extend(self._statNode(filePath, keys))
                except (IOError, OSError), e:
                    return errnoToFailure(e.errno, fileName)
                except:
                    return defer.fail()

        return defer.succeed(results)

def receive(self, path):
        path = self._path(path)
        return defer.fail(AnonUserDeniedError())

def claim_renew_all_before_expiration(self, height):
        return defer.fail(NotImplementedError())

def ftp_RNTO(self, toName):
        fromName = self._fromName
        del self._fromName
        self.state = self.AUTHED

        try:
            fromsegs = toSegments(self.workingDirectory, fromName)
            tosegs = toSegments(self.workingDirectory, toName)
        except InvalidPath:
            return defer.fail(FileNotFoundError(fromName))
        return self.shell.rename(fromsegs, tosegs).addCallback(lambda ign: (REQ_FILE_ACTN_COMPLETED_OK,))

def rename(self, fromPath, toPath):
        return defer.fail(AnonUserDeniedError())

def push(self, namespace):
        try:
            package = pickle.dumps(namespace, 2)
        except:
            return defer.fail(failure.Failure())
        else:
            if isinstance(package, failure.Failure):
                return defer.fail(package)
            else:
                d = self.callRemote('push', package)
                return d.addCallback(self.checkReturnForFailure)

def ftp_RNTO(self, toName):
        fromName = self._fromName
        del self._fromName
        self.state = self.AUTHED

        try:
            fromsegs = toSegments(self.workingDirectory, fromName)
            tosegs = toSegments(self.workingDirectory, toName)
        except InvalidPath:
            return defer.fail(FileNotFoundError(fromName))
        return self.shell.rename(fromsegs, tosegs).addCallback(lambda ign: (REQ_FILE_ACTN_COMPLETED_OK,))