Community content may not be verified or up-to-date. The MyMenuBar instance is known as a child of the MyMainWindow class MyMainWindow( QMainWindow): problem this is a problem on python GC its not work well with cyclic. The del statement does not necessarily call __del__ — it simply decrements the Once in a while Python runs an algorithm to detect such cycles, but the. If you want to force the issue, give your class a close method, as with __del__ method, closest to C++'s destructor) may not get called.
When a function is called as a method using the object. We call these static properties and static methods, respectively.
My class defines __del__ but it is not called when I delete the object.
There will only be one copy of a static property rather than a new copy per instance: Inheritance can avoid duplication and simplify the implementation of a class that needs the same data and functions as another class. Inheritance also allows us to substitute subclasses, relying only on the interface provided by a common superclass. Inherit to Avoid Duplication Consider this non-inheritance code: We could eliminate that repetition by having our Manager class inherit from the Employee class.
When we do, the state and behavior of the Employee class — its data and functions — will be incorporated into our Manager class. Notice the use of super: The difference between violating and adhering to this principle can sometimes be subtle.
A classic example of a subtle violation is a Rectangle superclass and a Square subclass: But there are disadvantages to inheritance too. Recall that we ensure valid state by manipulating data only through a small and fixed set of functions. But when we inherit, we increase the list of functions that can directly manipulate the data, and those additional functions are then also responsible for maintaining valid state.
If too many functions can directly manipulate the data, that data becomes nearly as bad as global variables. Too much inheritance creates monolithic classes that dilute encapsulation, are harder to make correct, and harder to reuse. Instead, prefer to design minimal classes that embody just one concept. Could we solve it without inheritance? An alternative approach is to connect objects through references to represent a part—whole relationship.
We call this composition. Instead, a manager is an ordinary Employee instance that a Group instance holds a reference to. If either inheritance or composition can reasonably express our program concepts and relationships, then prefer composition. Inherit to Substitute Subclasses Inheritance also allows different subclasses to be used interchangeably through the interface provided by a common superclass.
A function that expects a superclass instance as an argument can also be passed a subclass instance without the function having to know about any of the subclasses.
Substituting classes that have a common superclass is often called polymorphism: We faked inheritance of static properties by manually copying them. Now with ES6 classes, we get a real prototype link between a subclass constructor function and the superclass constructor: Arrays, for example, adjust their length property to be greater than the largest integer index. In ES5, when we tried to subclass Array, the new operator would allocate an ordinary object for our subclass, not the exotic object of our superclass: In ES5, objects were allocated before invoking the subclass constructor, and the subclass would pass that object to the superclass constructor.
Safely using destructors in Python - Eli Bendersky's website
Now with ES6 classes, objects are allocated before invoking the superclass constructor, and the superclass makes that object available to the subclass constructor. This lets Array allocate an exotic object even when we invoke new on our subclass. This protects against forgetting to invoke constructors with new.
However, it's important to know that this doesn't always work well. The nemesis of a reference-counting garbage collector is circular references.
Python 3 - Object Oriented
Foo 12 born Bar 12 born Ouch Where are the destructors? Here's what the Python documentation has to say on the matter: Python doesn't know the order in which it's safe to destroy objects that hold circular references to each other, so as a design decision, it just doesn't call the destructors for such methods!
Shouldn't we use destructors because of this deficiency? I'm very surprised to see that many Pythonistas think so, and recommend to use explicit close methods. But I disagree - explicit close methods are less safe, since they are easy to forget to call. Moreover, when exceptions can happen and in Python they happen all the timemanaging explicit closing becomes very difficult and burdensome. I actually think that destructors can and should be used safely in Python. With a couple of precautions, it's definitely possible.
python - del MyClass doesn't call object.__del__() - Stack Overflow
First and foremost, note that justified cyclic references are a rare occurrence. I say justified on purpose - a lot of uses in which cyclic references arise are an example of bad design and leaky abstractions. As a general rule of thumb, resources should be held by the lowest-level objects possible. Use an object to encapsulate the DB connection and close it safely in the destructor.
The DB object has no reason whatsoever to hold references to other objects in your code. If it does - it violates several good-design practices.
Sometimes Dependency Injection can help prevent cyclic references in complex code, but even in those rare few cases when you find yourself needing a true cyclic reference, there's a solution. Python provides the weakref module for this purpose.
The documentation quickly reveals that this is exactly what we need here: A weak reference to an object is not enough to keep the object alive: Here's the previous example rewritten with weakref: