As you've seen throughout this chapter, SOAP involves several layers. There's the HTTP layer, since SOAP is sending XML documents to, and receiving XML documents from, an HTTP server. So all the debugging techniques you learned in Chapter 11, HTTP Web Services come into play here. You can import httplib and then set httplib.HTTPConnection.debuglevel = 1 to see the underlying HTTP traffic.

Beyond the underlying HTTP layer, there are a number of things that can go wrong. SOAPpy does an admirable job hiding the SOAP syntax from you, but that also means it can be difficult to determine where the problem is when things don't work.

Here are a few examples of common mistakes that I've made in using SOAP web services, and the errors they generated.

Example 12.15. Calling a Method With an Incorrectly Configured Proxy

>>> from SOAPpy import SOAPProxy
>>> url = 'http://services.xmethods.net:80/soap/servlet/rpcrouter'
>>> server = SOAPProxy(url)                                        
>>> server.getTemp('27502')                                        
<Fault SOAP-ENV:Server.BadTargetObjectURI:
Unable to determine object id from call: is the method element namespaced?>
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 453, in __call__
    return self.__r_call(*args, **kw)
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 475, in __r_call
    self.__hd, self.__ma)
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 389, in __call
    raise p
SOAPpy.Types.faultType: <Fault SOAP-ENV:Server.BadTargetObjectURI:
Unable to determine object id from call: is the method element namespaced?>

Did you spot the mistake? You're creating a SOAPProxy manually, and you've correctly specified the service URL, but you haven't specified the namespace. Since multiple services may be routed through the same service URL, the namespace is essential to determine which service you're trying to talk to, and therefore which method you're really calling.

The server responds by sending a SOAP Fault, which SOAPpy turns into a Python exception of type SOAPpy.Types.faultType. All errors returned from any SOAP server will always be SOAP Faults, so you can easily catch this exception. In this case, the human-readable part of the SOAP Fault gives a clue to the problem: the method element is not namespaced, because the original SOAPProxy object was not configured with a service namespace.

Misconfiguring the basic elements of the SOAP service is one of the problems that WSDL aims to solve. The WSDL file contains the service URL and namespace, so you can't get it wrong. Of course, there are still other things you can get wrong.

Example 12.16. Calling a Method With the Wrong Arguments

>>> wsdlFile = 'http://www.xmethods.net/sd/2001/TemperatureService.wsdl'
>>> server = WSDL.Proxy(wsdlFile)
>>> temperature = server.getTemp(27502)                                
<Fault SOAP-ENV:Server: Exception while handling service request:
services.temperature.TempService.getTemp(int) -- no signature match>   
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 453, in __call__
    return self.__r_call(*args, **kw)
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 475, in __r_call
    self.__hd, self.__ma)
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 389, in __call
    raise p
SOAPpy.Types.faultType: <Fault SOAP-ENV:Server: Exception while handling service request:
services.temperature.TempService.getTemp(int) -- no signature match>

Did you spot the mistake? It's a subtle one: you're calling server.getTemp with an integer instead of a string. As you saw from introspecting the WSDL file, the getTemp() SOAP function takes a single argument, zipcode, which must be a string. WSDL.Proxy will not coerce datatypes for you; you need to pass the exact datatypes that the server expects.

Again, the server returns a SOAP Fault, and the human-readable part of the error gives a clue as to the problem: you're calling a getTemp function with an integer value, but there is no function defined with that name that takes an integer. In theory, SOAP allows you to overload functions, so you could have two functions in the same SOAP service with the same name and the same number of arguments, but the arguments were of different datatypes. This is why it's important to match the datatypes exactly, and why WSDL.Proxy doesn't coerce datatypes for you. If it did, you could end up calling a completely different function! Good luck debugging that one. It's much easier to be picky about datatypes and fail as quickly as possible if you get them wrong.

It's also possible to write Python code that expects a different number of return values than the remote function actually returns.

Example 12.17. Calling a Method and Expecting the Wrong Number of Return Values

>>> wsdlFile = 'http://www.xmethods.net/sd/2001/TemperatureService.wsdl'
>>> server = WSDL.Proxy(wsdlFile)
>>> (city, temperature) = server.getTemp(27502)  
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: unpack non-sequence

Did you spot the mistake? server.getTemp only returns one value, a float, but you've written code that assumes you're getting two values and trying to assign them to two different variables. Note that this does not fail with a SOAP fault. As far as the remote server is concerned, nothing went wrong at all. The error only occurred after the SOAP transaction was complete, WSDL.Proxy returned a float, and your local Python interpreter tried to accomodate your request to split it into two different variables. Since the function only returned one value, you get a Python exception trying to split it, not a SOAP Fault.

What about Google's web service? The most common problem I've had with it is that I forget to set the application key properly.

Example 12.18. Calling a Method With An Application-Specific Error

>>> from SOAPpy import WSDL
>>> server = WSDL.Proxy(r'/path/to/local/GoogleSearch.wsdl')
>>> results = server.doGoogleSearch('foo', 'mark', 0, 10, False, "", 
...     False, "", "utf-8", "utf-8")
<Fault SOAP-ENV:Server:                                              
 Exception from service object: Invalid authorization key: foo:
 <SOAPpy.Types.structType detail at 14164616>:
 {'stackTrace':
  'com.google.soap.search.GoogleSearchFault: Invalid authorization key: foo
   at com.google.soap.search.QueryLimits.lookUpAndLoadFromINSIfNeedBe(
     QueryLimits.java:220)
   at com.google.soap.search.QueryLimits.validateKey(QueryLimits.java:127)
   at com.google.soap.search.GoogleSearchService.doPublicMethodChecks(
     GoogleSearchService.java:825)
   at com.google.soap.search.GoogleSearchService.doGoogleSearch(
     GoogleSearchService.java:121)
   at sun.reflect.GeneratedMethodAccessor13.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
   at java.lang.reflect.Method.invoke(Unknown Source)
   at org.apache.soap.server.RPCRouter.invoke(RPCRouter.java:146)
   at org.apache.soap.providers.RPCJavaProvider.invoke(
     RPCJavaProvider.java:129)
   at org.apache.soap.server.http.RPCRouterServlet.doPost(
     RPCRouterServlet.java:288)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:760)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
   at com.google.gse.HttpConnection.runServlet(HttpConnection.java:237)
   at com.google.gse.HttpConnection.run(HttpConnection.java:195)
   at com.google.gse.DispatchQueue$WorkerThread.run(DispatchQueue.java:201)
Caused by: com.google.soap.search.UserKeyInvalidException: Key was of wrong size.
   at com.google.soap.search.UserKey.<init>(UserKey.java:59)
   at com.google.soap.search.QueryLimits.lookUpAndLoadFromINSIfNeedBe(
     QueryLimits.java:217)
   ... 14 more
'}>
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 453, in __call__
    return self.__r_call(*args, **kw)
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 475, in __r_call
    self.__hd, self.__ma)
  File "c:\python23\Lib\site-packages\SOAPpy\Client.py", line 389, in __call
    raise p
SOAPpy.Types.faultType: <Fault SOAP-ENV:Server: Exception from service object:
Invalid authorization key: foo:
<SOAPpy.Types.structType detail at 14164616>:
{'stackTrace':
  'com.google.soap.search.GoogleSearchFault: Invalid authorization key: foo
   at com.google.soap.search.QueryLimits.lookUpAndLoadFromINSIfNeedBe(
     QueryLimits.java:220)
   at com.google.soap.search.QueryLimits.validateKey(QueryLimits.java:127)
   at com.google.soap.search.GoogleSearchService.doPublicMethodChecks(
     GoogleSearchService.java:825)
   at com.google.soap.search.GoogleSearchService.doGoogleSearch(
     GoogleSearchService.java:121)
   at sun.reflect.GeneratedMethodAccessor13.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
   at java.lang.reflect.Method.invoke(Unknown Source)
   at org.apache.soap.server.RPCRouter.invoke(RPCRouter.java:146)
   at org.apache.soap.providers.RPCJavaProvider.invoke(
     RPCJavaProvider.java:129)
   at org.apache.soap.server.http.RPCRouterServlet.doPost(
     RPCRouterServlet.java:288)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:760)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
   at com.google.gse.HttpConnection.runServlet(HttpConnection.java:237)
   at com.google.gse.HttpConnection.run(HttpConnection.java:195)
   at com.google.gse.DispatchQueue$WorkerThread.run(DispatchQueue.java:201)
Caused by: com.google.soap.search.UserKeyInvalidException: Key was of wrong size.
   at com.google.soap.search.UserKey.<init>(UserKey.java:59)
   at com.google.soap.search.QueryLimits.lookUpAndLoadFromINSIfNeedBe(
     QueryLimits.java:217)
   ... 14 more
'}>

	Can you spot the mistake? There's nothing wrong with the calling syntax, or the number of arguments, or the datatypes. The problem is application-specific: the first argument is supposed to be my application key, but foo is not a valid Google key.
	The Google server responds with a SOAP Fault and an incredibly long error message, which includes a complete Java stack trace. Remember that all SOAP errors are signified by SOAP Faults: errors in configuration, errors in function arguments, and application-specific errors like this. Buried in there somewhere is the crucial piece of information: Invalid authorization key: foo.