the `zapwallettxes.py` qa test is currently broken due to a syntax error:

```python
     assert_raises(JSONRPCException, self.nodes[0].gettransaction, [txid3])
     #there must be a expection because the unconfirmed wallettx0 must be gone by now
```

This doesn't actually test that the transaction is not in the wallet, because `gettransaction()` is being called incorrectly (with an array instead of a string).

Changing the test to `assert_raises(JSONRPCException, self.nodes[0].gettransaction, txid3)` causes the rpc to succeed and the test script to fail because `txid3` is still in the wallet.

This is because the mempool is now persistent and is saved to disk when bitcoind shuts down. zapwallettxes removes the unconfirmed transaction from the wallet but bitcoind will rescan the mempool.dat file on startup and add the unconfirmed transaction back into the wallet.

I've 'fixed' zapwallettxes.py in https://github.com/jnewbery/bitcoin/tree/zapwallettxes , but the question remains: how *should* -zapwallettxes interact with persistent mempool? The use case for zapwallettxes is explained here: https://github.com/bitcoin/bitcoin/pull/3659#issuecomment-34878999 . My view is that even with zapwallettxes, bitcoind should rescan the mempool, but there should be better documentation or a warning that unconfirmed transaction in your mempool will be re-added to the wallet even if zapwallettxes is used. If you really want to remove all unconfirmed transactions from your wallet, you can delete your mempool.dat file and then restart with zapwallettxes.
strUsage += HelpMessageOpt("-maxreceivebuffer=<n>", strprintf(_("Maximum per-connection receive buffer, <n>*1000 bytes (default: %u)"), DEFAULT_MAXRECEIVEBUFFER));
    strUsage += HelpMessageOpt("-maxsendbuffer=<n>", strprintf(_("Maximum per-connection send buffer, <n>*1000 bytes (default: %u)"), DEFAULT_MAXSENDBUFFER));
    strUsage += HelpMessageOpt("-maxtimeadjustment", strprintf(_("Maximum allowed median peer time offset adjustment. Local perspective of time may be influenced by peers forward or backward by this amount. (default: %u seconds)"), DEFAULT_MAX_TIME_ADJUSTMENT));
    strUsage += HelpMessageOpt("-onion=<ip:port>", strprintf(_("Use separate SOCKS5 proxy to reach peers via Tor hidden services (default: %s)"), "-proxy"));
    strUsage += HelpMessageOpt("-onlynet=<net>", _("Only connect to nodes in network <net> (ipv4, ipv6 or onion)"));
    strUsage += HelpMessageOpt("-permitbaremultisig", strprintf(_("Relay non-P2SH multisig (default: %u)"), DEFAULT_PERMIT_BAREMULTISIG));
    strUsage += HelpMessageOpt("-peerbloomfilters", strprintf(_("Support filtering of blocks and transaction with bloom filters (default: %u)"), DEFAULT_PEERBLOOMFILTERS));
    strUsage += HelpMessageOpt("-port=<port>", strprintf(_("Listen for connections on <port> (default: %u or testnet: %u)"), Params(CBaseChainParams::MAIN).GetDefaultPort(), Params(CBaseChainParams::TESTNET).GetDefaultPort()));
    strUsage += HelpMessageOpt("-proxy=<ip:port>", _("Connect through SOCKS5 proxy"));
    strUsage += HelpMessageOpt("-proxyrandomize", strprintf(_("Randomize credentials for every proxy connection. This enables Tor stream isolation (default: %u)"), DEFAULT_PROXYRANDOMIZE));
    strUsage += HelpMessageOpt("-rpcserialversion", strprintf(_("Sets the serialization of raw transaction or block hex returned in non-verbose mode, non-segwit(0) or segwit(1) (default: %d)"), DEFAULT_RPC_SERIALIZE_VERSION));
    strUsage += HelpMessageOpt("-seednode=<ip>", _("Connect to a node to retrieve peer addresses, and disconnect"));
    strUsage += HelpMessageOpt("-timeout=<n>", strprintf(_("Specify connection timeout in milliseconds (minimum: 1, default: %d)"), DEFAULT_CONNECT_TIMEOUT));
    strUsage += HelpMessageOpt("-torcontrol=<ip>:<port>", strprintf(_("Tor control port to use if onion listening enabled (default: %s)"), DEFAULT_TOR_CONTROL));
    strUsage += HelpMessageOpt("-torpassword=<pass>", _("Tor control port password (default: empty)"));
#ifdef USE_UPNP
#if USE_UPNP
    strUsage += HelpMessageOpt("-upnp", _("Use UPnP to map the listening port (default: 1 when listening and no -proxy)"));
#else
    strUsage += HelpMessageOpt("-upnp", strprintf(_("Use UPnP to map the listening port (default: %u)"), 0));
#endif
#endif
    strUsage += HelpMessageOpt("-whitebind=<addr>", _("Bind to given address and whitelist peers connecting to it. Use [host]:port notation for IPv6"));
    strUsage += HelpMessageOpt("-whitelist=<IP address or network>", _("Whitelist peers connecting from the given IP address (e.g. 1.2.3.4) or CIDR notated network (e.g. 1.2.3.0/24). Can be specified multiple times.") +
        " " + _("Whitelisted peers cannot be DoS banned and their transactions are always relayed, even if they are already in the mempool, useful e.g. for a gateway"));
    strUsage += HelpMessageOpt("-whitelistrelay", strprintf(_("Accept relayed transactions received from whitelisted peers even when not relaying transactions (default: %d)"), DEFAULT_WHITELISTRELAY));
    strUsage += HelpMessageOpt("-whitelistforcerelay", strprintf(_("Force relay of transactions from whitelisted peers even if they violate local relay policy (default: %d)"), DEFAULT_WHITELISTFORCERELAY));
    strUsage += HelpMessageOpt("-maxuploadtarget=<n>", strprintf(_("Tries to keep outbound traffic under the given target (in MiB per 24h), 0 = no limit (default: %d)"), DEFAULT_MAX_UPLOAD_TARGET));
#ifdef ENABLE_WALLET
    strUsage += CWallet::GetWalletHelpString(showDebug);