CODEWITHSUNDEEP
pythongoogle-app-engineapp-engine-ndbtasklet
Steve Sauder
Steve Sauder

Reputation: 146

Difficulty getting ndb.tasklets to work in Google App Engine

I'm having a bit of difficulty understanding what's going on with some tasklets that I've written to use with Google App Engine's NDB datastore's asynchronous API's. The overview is: I need to pull together a purchase history for a user, and the "UserPurchase" object references the "Product" or "Custom" item that was purchased, and the "Establishment" in which it was purchased, both using KeyProperty properties in the UserPurchase ndb.Model. I want to parallelize the various NDB lookups as much as possible, so I've stuck quite close to the NDB documentation (at https://cloud.google.com/appengine/docs/standard/python/ndb/async) and built 3 tasklets:

@ndb.tasklet
def get_establishment(purch):
    resto = yield purch.p_establishment.get_async()
    ret = {
        "rkey": resto.key.urlsafe(),
        "rname": resto.resto_name
    }
    raise ndb.Return(ret)

@ndb.tasklet
def get_item(purch):
    item = yield purch.p_item.get_async()
    if (purch.p_item.kind() == "Product"):
        ret = {
            "ikey": item.key.urlsafe(),
            "iname": item.name
        }
    else:
        ret = {
            "ikey": item.key.urlsafe(),
            "iname": item.cust_name
        }
    raise ndb.Return(ret)

@ndb.tasklet
def load_history(purch):
    at, item = yield get_establishment(purch), get_item(purch)
    ret = {
        "date": purch.p_datetime,
        "at_key": at.rkey,
        "at": at.rname,
        "item_key": item.ikey,
        "item": item.iname,
        "qty": purch.p_quantity,
        "type": purch.p_type
    }
    raise ndb.Return(ret)

Then, I make a call like so:

pq = UserPurchase.query().order(-UserPurchase.p_datetime)
phistory = pq.map(load_history, limit = 20)

to retrieve the most recent 20 purchases for display. When I run it, however, I get a strange exception that I'm not sure what to make of ... and I'm not familiar enough with tasklets and how they work to confidently say I know what's going on. I'd really appreciate it if someone could give me some pointers on what to look for...! Here's the exception I get when I run the above code:

      ...
       File "/Programming/VirtualCellar/server/virtsom.py", line 2348, in get
        phistory = pq.map(load_history, limit = 20)
      File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/ndb/utils.py", line 160, in positional_wrapper
        return wrapped(*args, **kwds)
      File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/ndb/query.py", line 1190, in map
        **q_options).get_result()
      File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/ndb/tasklets.py", line 383, in get_result
        self.check_success()
      File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/ndb/tasklets.py", line 624, in _finish
        result = [r.get_result() for r in self._results]
      File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/ndb/tasklets.py", line 383, in get_result
        self.check_success()
      File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/ndb/tasklets.py", line 430, in _help_tasklet_along
        value = gen.send(val)
      File "/Programming/VirtualCellar/server/virtsom.py", line 2274, in load_history
        "at_key": at.rkey,
    AttributeError: 'dict' object has no attribute 'rkey'

Based on the error, it almost seems like tasklets don't like returning dictionaries...

Upvotes: 1

Views: 155

Answers (1)

Dan Cornilescu
Dan Cornilescu

Reputation: 39824

You're misinterpreting the error message: a dict is returned, but you're trying to access an attribute of that dict which doesn't exist.

It appears you want to access a value inside that dict, not an atribute of the dict, if so then change inside load_history:

        "at_key": at.rkey,

to:

        "at_key": at.get('rkey'),

or:

        "at_key": at['rkey'],

You also want to revisit your other similar attempts in accessing other dict values in at and item dicts.

Upvotes: 1

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