Constructors in kivy - python

I am quite new to kivy. While practicing with online examples, I noticed that many of their widget classes don't have constructors. I just want to ask what is significance of constructors in kivy widgets, n when to use them.

You don't need to declare a constructor unless you need to do something specific. For example, our Widget class have a constructor that does lot of stuff, like applying the properties, create the widget canvas, apply kv rules, bind the construtor events if passed etc.
If you create a widget, but have nothing to do, there is no need to declare a constructor, the MRO will determinate which one to use on the subclasses. This is the same for all Python classes.

Related

How to redesign an instance where common functionality (in a parent class) warrants returning instances of the children?

As per the title, I know this is inherently terrible design, as a parent should know nothing about its children. However, I am in the scenario where
All children have reusable behaviour that could be derived from the parent class,
Those reusable methods that all subclasses need, warrant returning instances of the current sub classes!
I need all child classes to have the repeatable behaviour but also have the parent class capable of returning instances of the children, this will need a complete refactor, but how do I design it properly?
I have tried using composition instead here, the only problem being that all classes will have to have to declare an explicit public API for using the common functionality and each and every subclass in the future will need to declare it.
class BasePage(object):
# Some Very Common Behaviour goes here.
# Nothing product specific, just selenium specific
class ProductBasePage(BasePage):
# Reusable behaviour here for the product
# Some of this behaviour in the parent does web-app navigation
# which merits (quite rightly) to return instances of children
# pages, for example (CustomersPage, DashboardPage, ProductsPage)
# but this is clearly flawed and inheritance is not the answer?
# but how do I keep the functionality that all subclasses need
# without then declaring it explicitly in every one of them?
class CustomersPage(ProductBasePage):
# Customer specific behaviour
# Should have access the common functionality implicitly from the parent
Customer Page and all other pages which extend the product page should have the ability to use a lot of common functionality that all of the subclasses of ProductBasePage should have, without having each and every subclass to define that behaviour explicitly. The re-usable methods however, return instances of ProductBasePages sub classes as their actions warrant this behaviour.
So how do I achieve this? I think Inheritance is not the answer here, but then how do I get reusability of the common functionality without declaring it explicitly in every class?
The solution to this problem should avoid circular Python import dependencies as well.
Just put those reusable methods into a separated class, and make them static. Then the subclasses can call those methods implicitly.
If you don't want to use static methods, go with plain functions.

Only one Class possible for GUI programming

I'm new to GUI-programming and using now tkinter for python.
In the past my "non-GUI" programs always consisted out of a few classes but if I look to the examples with a GUI it appears that only one class is used. All functions are included in this one class. Is this the normal way or is it possible to write a gui class which "calls" functions from other classes?
As I look at it now it seems the concept of object oriented programming dissapears by implementing the GUI in an OOP manner
It is definitely possible to use multiple classes in GUI apps.
For example you can have one class which defines and layouts GUI elements (like buttons, text fields, scrollbars etc.) and the second class would subclass it adding some functionality on top of it.

How to organize classes in pyqt GUI programming

I have constructed a main window GUI using qt designer and pyqt. As the program grows more complex, using only one class may result in too many methods and attributes to manage. I figured that I should construct new classes in order to make things more manageable.
My first question is, how do I know when do I add a new class for my application? Is there any rule of thumb as a general guide? Is it a good idea to add a new class for new windows/tabs?
My second question is, if I added new classes for my application, how do my new class gain access to the Ui_MainWindow design file that I designed in Qt designer? Below is the sample code for my main window. Its a simple clock which displays the current time. Lets say if I would like to create a class for the clock display itself, how can I rewrite the code using OOP efficiently?
from PyQt4 import QtGui
from myMainWindowUI import Ui_MainWindow
class MyMainWindow(QtGui.QMainWindow, Ui_MainWindow):
def __init__(self, *args, **kwargs):
super(MyMainWindow, self).__init__(*args, **kwargs)
self.setupUi(self)
self.timer = QtCore.QTimer(self)
self.timer.timeout.connect(self.Time)
self.timer.timeout.connect(self.Date)
self.timer.start(1000)
self.lcdNumber_time.setDigitCount(8)
self.lcdNumber_time.display(strftime("%H"+":"+"%M"+":"+"%S"))
self.label_dynamic_date.setText(strftime("%Y"+" "+"%B"+" "+"%d"+", "+"%A"))
def Time(self):
self.lcdNumber_time.display(strftime("%H"+":"+"%M"+":"+"%S"))
def Date(self):
self.label_dynamic_date.setText(strftime("%Y"+" "+"%B"+" "+"%d"+", "+"%A"))
app = QtGui.QApplication(sys.argv) # A new instance of QApplication
form = MyMainWindow()
form.show()
app.exec_()
In general, this isn't necessarily a Qt-specific problem. It isn't necessarily a python-specific problem either. You could extend this question to any language that supports class-based programming, or even any language with functions.
It makes sense to write a class when you want to encapsulate behavior. A class should usually provide a single purpose, and expose an interface to the outside that allows users of the class to interact with exactly the behavior you have designed. Once you have this single-purpose class, you now have reusable behavior. You could apply part of this reasoning to functions, where you say a given function has a specific purpose and once you design it to do the one bit of behavior, you now have a reusable function.
Applying this to Qt, it would make sense to move code into its own class when its logic is no longer trivial. An example would be where you are first creating and showing a QMessageBox from your main window. But then at some point you need to collect more types of information, and even pass some initial information in when you construct the dialog. While you could construct this on the fly in your main window, as needed, it would be better to move it into its own dialog class, with its own private logic of how to be constructed. Then you just instantiate one as needed, passing it the expected parameters to its constructor. Now your main window no longer has to also be concerned with constructing special dialogs.
Another point (which wasn't exactly clear from your question) is that Qt Designer UI files each represent a single class. It is expected that this UI definition will be applied to a single class in code. It would be bad design to have ClassA(UI_mainWindow) and ClassB, and have ClassB access members of ClassA or know anything about the internal implementation of ClassA. Back to our topic of "separation of concerns", ClassB should have its own purpose and interface, and be used by the ClassA(UI_mainWindow) to achieve that purpose. You don't want ClassB knowing anything about the main window or being able to do more than its designed purpose.
Let's assume your timer example was actually more trivial than you have shown it to be. If you moved it to another class, you should rely on custom signals to communicate intentions back to other classes like the main window. Or you could move each of the lcd and label widgets into their own custom classes with their own timer logic that made them self contained. Either way, signal/slot lets custom widgets connect with each other without needing to know anything about the other widgets.
In summary, you could say that it would be good to create individual classes in Qt when you are constructing any non-trivial UI elements in another class (when it requires many lines of code, or it requires a bunch of SLOT functions and wiring for internal logic). This will keep you from repeating your logic, and allow you to have reusable code. It will also keep your main window smaller and easier to debug, since you can keep a smaller mental model in your brain of what the main window is doing. If you try to keep your UI elements as single-purpose focused and generic as possible, you will end up having a lot of classes that can be reused.

Bolting on abstract behavior dependent upon the same base?

I'm hoping someone may be able to help me out with a design issue I'm dealing with. It's specifically in the game development domain, but I think it's really a broader issue that has probably been solved in an accepted way. I'm working in Python.
I have a GameObject class that holds the position of the object (and other general state attributes) and a reference to my Engine object, which holds information about the game world at large. GameObjects can be a categorized further: they can be VisibleGameObjects, PhysicalGameObjects (collidable), or both, in concrete form. For example, I could have an invisible boundary, which is physical, but does not have a visible representation.
VisibleGameObjects implement a draw() method that handles drawing functionality, delegating this through its parent's Engine reference. PhysicalGameObjects have bounding boxes, and define logic to handle collisions, also requiring access to GameObject attributes (acceleration, velocity, etc.)
The problem is, what happens when I'd like to define a concrete object that needs to inherit the behavior of both a VisibleGameObject, and a PhysicalGameObject (which both share a parent GameObject)? It's my understanding that this type of circular inheritance is a big-bad idea.
How can I refactor this to essentially bolt on the specific behaviors to a concrete child class (drawable, collidable) that depend on the state of the parent abstract class?
EDIT: My one thought was to assign them to concrete instances of GameObjects as components, favoring a has-a relationship over an is-a relationship. Even that doesn't seem so clean however; trying to check to see if an object is collidable by searching a "components" list for a collidable component doesn't seem great either.
It seems like you're looking for a trait
Unfortunately, python doesn't support traits natively, although there are multiple modules that try to implement the model.
My suggestion (unless you want to depend on the mentioned modules) would be to write abstract classes to expose the behaviour you want, but that don't inherit the main class - leaving that to a third class, which inherits both the main, and the behaviour-class.
It's probably less confusing with an example:
create a Visible abstract class that does not inherit from GameObject, and exposes all the intended behaviour/functions (as if it inherited from GameObject). Then, have VisibleGameObject inherit from both GameObject and Visible.
Obviously, you can only manage to write Visible on a dynamic language like python - otherwise the compiler would complain that it couldn't access inexistent fields.

pygtk: What class should my custom widgets inherit from?

When making a custom widget in pygtk, what class should it inherit from? I want to be able to put the widget inside other widgets, but I don't want other people to put stuff in mine. Usually I make my widgets inherit from gtk.HBox or gtk.VBox, and that works fine, but it is possible then for someone to do a pack_start() on my widget and cause strange things to happen. I'd inherit from gtk.Widget but then how do I add things to it? I'd inherit from gtk.Container or gtk.Bin but the docs say they are abstract classes.
If your custom widget contains other (probably standard) widgets, you could simply raise an exception in the overridden pack_ methods. That way, nobody can put stuff in it (easily). Inside your class, you then have to use super(...).pack_xxx instead of self.pack_xxx.
But it's probably better to derive from gtk.Container. Then you'll have to implement its abstract methods like do_add(self, widget).
In case you only draw custom content (no children), there's no need to derive from a container widget. See the tutorial on pygtk.org.

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