Google App Engine Python adding

Question

class Config(db.Model):

    Latest = db.IntegerProperty()

    class New(webapp.RequestHandler): 
        def get(self): 
            config = Config()
            Last = Config.Latest
            t = Last + 1

returns

t = LastUUID + ADDNUM
TypeError: unsupported operand type(s) for +: 'IntegerProperty' and 'Int'

What im trying to do is get the int from the datastore and app 1 to it. Then reassign the int in the datastore. I have no clue why it is throwing these errors. I even tried t = int(Last +1). UPDATE: This is what i needed and solved my problem. http://code.google.com/appengine/articles/sharding_counters.html

Integers get auto-promoted to longs in python. And that has nothing to do with the error. — Falmarri
– Falmarri, Commented Jan 5, 2011 at 23:45
What are you trying to achieve? Aside from the errors pointed out by Thomas, trying to use global counters is usually a bad idea in App Engine (or any other distributed system). — Nick Johnson
– Nick Johnson, Commented Jan 6, 2011 at 2:39

Thomas K · Accepted Answer · 2011-01-06 00:05:18Z

3

The changes you need are more than tweaking a single line.

You need some sort of initialisation function which will create a Config() instance, assign Latest an initial value (like 1 or 0) and store it in the datastore.

Then your RequestHandler needs to execute a query to retrieve the relevant Config instance. Finally, update Latest, and save the instance into the datastore again.

answered Jan 6, 2011 at 0:05

Thomas K

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5 Comments

Nick Johnson Over a year ago

This is correct. In fact, it's the only complete and correct answer yet posted, so I'm not sure why it was voted down.

Chris B. Over a year ago

Because it's completely wrong. You don't need an initialization function, you don't need to execute a query or request an object from the datastore, and you don't need to resave the instance. The ONLY thing you need to do is tweak a single line, and the code works as written.

Thomas K Over a year ago

@Chris: It might run with a simple tweak, but it won't achieve what the poster is trying to do. The question specifically describes getting a number from the datastore, adding one, and putting it back in the datastore.

Chris B. Over a year ago

When someone asks "How do I drive to the store to buy milk? My car won't start", it's not helpful to provide a map and turn-by-turn directions if they're missing spark plugs.

Thomas K Over a year ago

@Chris: The code has multiple issues. The distinction between Config and config is the most obvious, because it raises an error. But the lack of any code to store to or retrieve from the datastore is really the most significant problem, so it's wrong to suggest that it's a single line fix. To stretch the analogy, it's like fitting a spark plug to a car while ignoring the lack of, say, wheels.

Chris B. · Accepted Answer · 2011-01-06 00:29:00Z

1

This has nothing to do with Google App Engine or Django. In the following code:

class X(object):
    @property
    def y(self):
        return 5

x = X()
print X.y + 5

... you get the same error. X.y refers to the unbound property on the class object. x.y refers to the bound property, and indeed in the above example print x.y + 5 prints "10".

Change the line to Last = config.Latest and it should work. And I strongly recommend you take up the recommendations in PEP 8, particularly under "Prescriptive: Naming Conventions". Generally speaking, in Python classes should use CapWords, while functions and variables should use lowercase_with_underscores.

edited Jan 6, 2011 at 0:29

answered Jan 6, 2011 at 0:22

Chris B.

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Comments

Falmarri · Accepted Answer · 2011-01-05 23:47:02Z

0

The syntax you're looking for is

t = int(Last) +1

However, I'm not sure that's what you want to do. I don't know django or whatever framework this is so it's hard to say. But in python, types must be compatible for you to add them.

answered Jan 5, 2011 at 23:47

Falmarri

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2 Comments

badp Over a year ago

That's not true of numbers. 0.1 + 2 adds just fine.

Falmarri Over a year ago

I said compatible type. float and ints are compatible.

python appengine · Accepted Answer · 2011-01-07 01:52:49Z

0

http://code.google.com/appengine/articles/sharding_counters.html

This is a basic conter that can be updated multiple times a second. Works perfect

answered Jan 7, 2011 at 1:52

python appengine

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