API call hooks are a technique that's reasonably well documented for App Engine - there's an article about it - but only lightly used. Today we'll cover some practical uses of API call hooks.
To start, let's define a simple logging handler. This can be useful when you're seeing some odd behaviour from the datastore, especially when you're using a library that may be modifying how it works. You can also use it to log any other API, such as the URLFetch or Task Queue APIs. For this, we just need a post-call hook:
def log_api_call(service, call, request, response): logging.debug("Call to %s.%s", service, call) logging.debug(request) logging.debug(response)
To install this for the datastore, for example, we call this:
apiproxy_stub_map.apiproxy.GetPostCallHooks().Append( 'log_api_call', log_api_call, 'datastore_v3')
The arguments to Append are, in order, a name for the hook, the hook function itself, and, optionally, the service you want to hook. If you leave out the last argument, the hook is installed for all API calls.
Once a hook is installed, it remains installed as long as the runtime is loaded - including across requests, and for requests to ...