What does fetch() fetch in GAE?

This is a follow up to my other question.

I thought that

    mylist = list(Rep().all().fetch(50))

makes mylist a list. But when I try to get its length I get the message

self.response.out.write(len(P))
TypeError: object of type 'Rep' has no len()

Can anyone explain what I am doing wrong?

    Rep().replist = L                                   
    Rep().put()                              
    mylis开发者_StackOverflow中文版t = list(Rep().all().fetch(50))
    P = mylist.pop()
    self.response.out.write(len(P))

UPDATE

As a reference for others who may encounter the same problem; I post the following table which was very helpful to me. (The original here)

Rep().........................Rep object 
Rep.all().....................Query object 
list(Rep.all())...............List of Rep objects. 
list(Rep.all())[0]............A single Rep object 
list(Rep.all())[0].replist....A list

Thanks for all the answers.

Instead of this:

Rep().replist = L                                    
Rep().put()                               
mylist = list(Rep().all().fetch(50)) 
P = mylist.pop() 
self.response.out.write(len(P)) 

Try something like this:

r = Rep()
r.replist = L                                    
r.put()                               
mylist = Rep.all().fetch(50)
P = mylist.pop() 
self.response.out.write(len(P.replist)) 

This code of yours:

Rep().replist = L                                    
Rep().put()                               

Is creating a Rep instance, then assigning its replist to L. Then it's creating another Rep, and calling put() on it. So the one you are writing to the datastore is a blank Rep - it won't have your list.

In this code:

mylist = list(Rep().all().fetch(50)) 

You are calling all() on an instance of Rep - you should instead call it directly on the class. Also you don't need to wrap the results in list(), as fetch() already returns a list.

Then below where you have this:

self.response.out.write(len(P)) 

You are trying to get the length of P (which is a Rep), not the length of P's replist.

Update:

In response to the first comment:

In this code:

r = Rep()

The Rep() is creating an instance of a Rep. The r = is then assigning that instance to the name r. So now the name r refers to that instance of a Rep.

Then in this code:

r.replist = L

It is assigning the replist property of r to refer to the list L.

You are correct, instead of those two lines you can do this:

r = Rep(replist = L)

What this does is pass L to the __init__ function of Rep, with the argument name replist. The __init__ function of Rep is inherited from the db.Model base class. This function assigns the value of any arguments provided to a property of the same name on the model. So in this case, it assigns L to the replist property. So it has the same effect as the original two lines of code, but it works a bit differently.

In response to the second comment:

The = operator in Python means assignment, which is not the same as mathematical equivalence.

So this code:

r = Rep()

Does not mean that r is now equivalent to Rep(), and that you can now use r and Rep() to mean the same thing.

What it means is that r is now equal to the result of Rep(). What Rep() does is allocate a new instance of a Rep. So that makes r a reference to a new Rep. To refer to that same instance later, you therefore need to use r, not Rep() (which would allocate a new instance each time you call it).

if you want to print the number of elements of replist property.

self.response.write(len(P.replist));

Hope i can help you :p

Try this:

self.response.out.write(len(mylist))