I'm using Python [zeep](http://docs.python-zeep.org/en/master/) to use SOAP to interact with Salesforce's Metadata API.

I'm attempting to start with the simplest thing possible, just log in and read a metadata object.


    from zeep import Client as zClient
    
    client = zClient('sf_config/sf_metadata_40.xml')
    
    // readMetadata method exists, prints <zeep.client.OperationProxy object at ...>
    print client.service.readMetadata

    // errors because login method does not exist
    print client.service.login

I know the zeep client reads the XML because it correctly knows that `readMetadata` exists, but I'm confused about how I am supposed to log in with my Salesforce username and password. I expected a `login` method to exist.

Additionally, I tried adding username and password to the construction of the `client` object:

    from zeep.wsse.username import UsernameToken

    client = zClient('sf_config/sf_metadata_40.xml',
                     wsse=UsernameToken('bub@bob.com', 'scary_secretPassW*rd'))

but it was unsuccessful, in that, executing the `readMetadata` method caused this error:  
`Fault: UNKNOWN_EXCEPTION: Destination URL not reset. The URL returned from login must be set in the SforceService`

(The Java examples in Salesforce docs use a `connection` object that I'm not really sure what the Python zeep equivalent would be, so I don't have an example to go off of.)

UPDATE:  
That error brings this to mind, from the [docs](https://developer.salesforce.com/docs/atlas.en-us.api_meta.meta/api_meta/meta_calls_intro.htm):
> Retrieve the metadataServerUrl from the LoginResult returned by your SOAP API login() call.

But I'm not sure what the `login()` call is, since such a method does not exist on the `client.service`.