I have a class A that has a db.ReferenceProperty referencing a type of entity that is defined later in the file. How do I refer to B from A?
If you can't move B above A, then you can use the following workaround:
foo = db.ReferenceProperty(reference_class=db.Model)
This essentially allows you to reference any class that extends db.Model. You can add validation logic if needed.
You also cannot use the collection_name attribute to back reference A from B.
Related
Repeatedly inheriting from 2 classes having the same parent, I fall into the classic case of inheriting 2 times of the same attribute. I'd like to merge the 2 attributes into one and tried to do it with an undefine, but it gets me a compile error.
The other solution I see is renaming the attribute from one of both parents, but as I understand each instance of my D class would have an useless attribute which is not what I want...
Error: Undefine subclause lists name of frozen feature or attribute or
C external.
What to do: unless you can change the status of the feature in the parent,
remove its name from Undefine subclause since it cannot be undefined.
How to merge 2 attributes from repeatedly inherited classes
class A
serial: STRING
end -- class A
class B
inherit
A
end -- class B
class C
inherit
A
end -- class C
class D
inherit
B
undefine
serial -- error seems to appear here in that case
end
C
end -- class D
In case it's two unrelated attributes (not coming from the same parent) that you want to merge, you should redefine both of them:
class A
feature
serial: STRING
end
class B
feature
serial: STRING
end
class C
inherit
A
redefine
serial
end
B
redefine
serial
end
feature
serial: STRING
end
As you already saw, the compiler will not let you undefine an attribute, even when the goal is to merge it with another attribute.
There is no reason to undefine a feature that is going to be merged with the same version coming from a different inheritance path. In the example, the attribute serial is not changed in B, C, and D. Therefore, inheriting from B and C without any adaptation is OK:
class D inherit
B
C
end
How can we access a property value within a #classmethod? For example:
class Account(polymodel.PolyModel):
someprop = ndb.StringProperty(required=True)
#classmethod
def get_or_create_someprop(cls):
if not cls.someprop:
# create someprop
else:
return cls.someprop
In this example code above, I am trying create someprop if it doesn’t exist, or return it if it already exists. I assumed that the above code would achieve this. However, the first step I need to do is access the someprop value from within the classmethod. Using cls.someprop does not actually return the value of someprop but instead returns StringProperty('state').
I have tried to use this and self which are undefined.
So, is it possible to access a property value of an entity using a classmethod? If so, how?
In general you cannot do this from a class method because a property belongs to an object, i.e. an instance of the class (the class is just the object generator). In other words you need the self argument to refer to the object and its properties.
In your particular case the class is an entity model (the blueprint for creating entities), not an entity and you can only refer to a property of an entity itself.
But you should be able to achieve what you seek simply by not declaring it a class method - then it becomes a method of the object/entity and in that case you can reference the entity's property via self instead of cls: self.someprop.
I'd make the check a bit more specific, though, to cover the case in which the property has a value like 0 or an empty string which is interpreted by python as False in a logical check: if self.someprop is None instead of if not self.someprop.
I have a struct:
struct Order
{
public string orderNumber;
public string orderDetail;
}
I then assign some values in Form1 and try to pass them by reference (ref) to Form2:
(Form1)
Order order = new Order();
order.orderNumber = "1234";
order.orderDetail = "Widgets";
Form2 frm2 = new Form2(ref order);
Is it possible to store the values in Form2 so that when Form2 is completed processing the values it will return the updated struct values to Form1?
In this scenario there would be a button that would close the form after validating the data.
One pattern that's sometimes useful is to define a class something like:
class Holder<T> {public T value;}
Such a class makes it possible to pass and mutate value types with code that requires reference types. Using such an approach, a routine which accepted a structure by reference and was supposed to pop up a modal dialog and fill in the structure from it, could create a Holder<thatStructType>, pass that to the form, and then copy the data from that Holder back to the passed-in reference. While in your particular scenario, it may be better to have the data-holding thing simply be a class, structures have the advantage that one can know that no outstanding references to them exist; if a routine declares a structure and passes it by reference to some outside code, then once that code returns the values in that structure won't change unless or until the routine writes them itself or passes the structure by reference to some other code. By contrast, if a routine exposes a class reference to outside code, there's no telling what that code may do with it.
Incidentally, the Holder class is also useful in a number of other scenarios. For example, if one has a Dictionary<String, Holder<Integer>> myDict, one may use Threading.Interlocked.Increment(myDict(myKey).Value)) to perform a thread-safe increment of the indicated item, much more efficiently than would be possible with a Dictionary<String, Integer>.
What I think you're asking is if Form2 can store a reference to the order structure that was passed in the constructor. The answer is no. If you want to store references, use a reference type (a class).
I have a GObject "A" which creates an instance of another GObject "B" in its constructor.
The "B" object needs to be passed several construction-only properties. Now when creating an instance of object "A" I want to allow passing values for these properties through the constructor of object "A" on to the constructor of object "B".
The only way I have found to do that was to create identical properties for object "A" and pass their values on to the constructor of "B". These properties would have no further meaning to "A" so this seems like a kludge.
Is there a better way to do what I want?
Have A inherit from B. Then A has all of B's properties automatically.
Don't use properties in A, but instead pass B's properties (or even better, an already-constructed B object) as parameters to A's constructor.
Delay construction of B until A can figure out how it nees to configure B. Add a private flag to A, b_initialized or something, that tells you whether A's internal pointer to B is valid.
Some more clarification on the second suggestion:
A's stuff is constructed in the a_init() function that is provided for by the G_DEFINE_TYPE() macro. But that's not how you get an instance of A. It's usual to write a function, which is part of the public interface of A, like this:
A *a_new()
{
return (A *)g_object_new(TYPE_A, NULL);
}
You can easily extend this to include other parameters:
A *a_new(int b_param_1, int b_param_2)
{
A *a = (A *)g_object_new(TYPE_A, NULL);
a->priv->b = b_new(b_param_1, b_param_2);
return a;
}
This has the disadvantage of leaving your A object in an invalid state (i.e., without a B) if you construct it using g_object_new, for example if you're trying to build it from a GtkBuilder file. If that's a problem, I still strongly suggest refactoring.
Use dependency injection, pass an already initialized object of type B to the constructor of A.
That way the client that is using your class can decide whether to pass in different kinds of Bs (if it makes sense you can even use an interface instead of a class as the B type, writing code against interfaces is generally better than writing code against implementations).
Deriving A from B only makes sense if it really is a specialization of it's parent class.
From the question it isn't clear if derivation makes sense, but it's an often overused method for composition.
If a class A is in composition relationship with class B , does it mean that the specific instance of B got via A should only be modifiable through class A not to break the data encapsulation of class A? Or, does composition imply only life-time bind, not the data encapsulation ?
class A
{
B itsB;
B* getB() {return &itsB);
}
void AnotherClass::anyOperation()
{
itsA->getB()->function(); // is this legal ?
}
Composition does not strictly specify the rules concerning whether a composed object should be allowed to be modified outside the composing class or not.
Encapsulation has to do with controlling access to the members defined in a class to outside world. In general ,fields of a class should not be directly accessible by outside code , if this tenet is followed , then the question of allowing it to be modifeid by outside code does not arise. Encapsulation and composition are not related in principle , so in your example , the fact that B is a composed into A itself does not dictate the rule that itsB should not be modifiable outside A.
However , you should think in terms of 1. Who owns the object itsB ? 2. Is it thread-safe to allow it to be modified outside A ? Is it breaking encapsulation ?
If B has public setter functions, the only way in C++ to ensure that nobody retrieves A's B through getB() and changes its value is for getB() to return a constant reference. If it returned a constant pointer, that just ensures that you don't change the pointer; you can still change the values inside the B that the pointer points to.
To mention my problem with different words:
The class A has mB and mC members , where A and B, A and C are in composition relationships.
B and C classes are in association relationship.
What would you say, if I want to link specific instance "mB" to specific instance "mC" (of class A), allowing them to communicate between themselves.
Would it mean to break the encapsulation provided by A for mB and mC ?