Python Slots Class Attribute

This section is brief over view of Object Oriented Python.

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Here we will explore building blocks of OOP.

Python is a pure Object Oriented Programming language.

Everything in Python is an object.

That means even numbers, strings, functions, modules, classes etc are all objects.

Programming languages like C,C++, Java have primitive data types, which are not objects.

What is an object ?

An object is a unit of data of a class or type.

Object have one or more attributes and functionality.

Attributes of an objects are also called properties,

and functionality are implemented in methods.

If any of the above lines making no sense, don’t worry,

You will get there, when we look at the, concrete examples of objects.

For the moment, understand objects in Python are no different than, real life objects,

like , Car, Person, Laptop, House, Office, etc.

What every object has ?


Every object has,

  1. a Type
  2. an Id
  3. Attribute(s), some of them are Method(s)
  4. Value(s), of attributes.

If you have done at least a bit of Python programming,

then you have already used objects, without knowing them.

Let’s explore some built-in objects in Python and try to understand.

Example:


Here my_int is a variable, which is pointing to place in memory which holds value 10.

We can check what type of object my_int is referring to by using built-in function type()

Also each object has a unique ID, that can be checked by another built-in function id(),

In CPython implementation id is nothing but memory address of the object.


“my_int” is referencing an object of class int, int is short hand for integer.

it has value 10,

its id is 1488478656

and other properties of my_int can be obtained by another built-in function dir()

dir(obj) built in function lists the properties of a given object.

Check yourself these, by typing above code in Python interpreter.

The below table has few other built-in objects.

For simplicity, only type and Id are shown.

Like above check, type and id of these objects in Python interpreter.

ObjectTypeId
my_int = 10class 'int'1488478656
my_string = 'Welcome'class 'str'1488772652424
my_list = ['Apple', 'Orange','Grapes']class 'list'1488772571848
my_bool = Trueclass 'bool'1487986848
my_tuple = cclass 'tuple'1488772335560
my_dict ={'name' :'User'}class 'dict'
1488772640704
my_type = type(type(10))class 'type'
1488040944

Please note even “type” is an object,


which is of class ‘type’ with a unique id.

Until now, we discussed about objects which are built-in Python.

Let’s look at, how to create user defined class or types.

Classes

Python Get All Class Attributes


The user defined objects are created by using the ‘class’ keyword.

The ‘class’ is like a blueprint,

that defines the nature of a future object.

By using classes we construct instances.

In Python we have two types of syntax for class creation,

1. Old or classic style

2. New style

Below you will see examples for both styles,

but for the rest of the course, we will fallow New style syntax.

Syntax:

Old Style:


New style:


Now lets create a new user defined object type called MyClass,


By convention class names starts with a capital letter.

In the above example MyClass is class name,

the class statement binds the classname and class body.

Currently the class body, we don’t have anything. So MyClass is of no use.

A class can have arbitrary named attributes.

Body of a class


Body of a class where you normally specify the attributes of a class.

Attributes

Inside MyClass, we currently just have pass.

Which means, this class don’t have any implementation, and is useless.

Let’s add some implementation by adding attributes.

An attribute is a characteristic of an object.

Attributes can be divided into states and behaviors.

Functions can be an attributes of a class, these are called methods.

Behaviors are implemented by using methods,

and states are defined using variables .

The terms attributes and properties are interchangeable.

Method


Methods, are functions which are defined inside a class.

An operation, we can perform with the object are implemented as methods.

Method is also know as “callable attribute” of the class.

Let’s get a better understanding of attributes through an example.

The syntax for adding an attribute to class is:

x is an attribute of a class, which is defined,
inside class body and it refers value 10.

We can access the attribute of a class by using dot notation.

Dot notation


A Python attribute can be accessed by using ( . ) dot notation as below.

The ( . ) dot notation can be used to set and get the attributes.

Dot is used after the class name or object name fallowed by attribute name.

Python Slots Class Attribute


we can print and check what is x holding.


In the above example we have defined attributes inside class body.

But in Python attributes can also be added dynamically,

outside the class body, by using dot notation.

There is no difference between defining attributes inside or outside of class body.

Function as attribute


We can also define inside as class, which is called as method.

Lets define a say_hello() method inside MyClass.


As function is also an attribute, we call it by using dot notation.


When we use class statement, Python implicitly adds some useful attributes to class,

like __name__ and __bases__

Python Check Class Attribute

We can check these on our MyClass class.


Notice __name__ returns name of the class,

and bases returns the inherited class name.

You will learn more about inheritance in later section.

class also has another useful attribute called __dict__

the dict object holds other attributes of the class.

Python Class Add Attribute

Private attributes.


An identifier starting with two underscores inside a class, is private member.


We cannot access private members directly by using a dot notation.

See in the below example, x is not a private member,

__num_elements is private member. only x can be accessed with dot notation.

But private attributes can be accessed by using methods.

Creating Instance from user defined class


To create an instance, use class as if it is function.


Crating your second object.


In above example MyClass(),
each time constructs an object and returns a reference to that object.

obj_1 points to an object at 0x0000026F6DD9DB00 in memory location.

obj_2 points to an object at 0x0000026F6DDA85C0 in memory location.

Module Vs Class

We will explore more about attributes in the next lesson,

But before that lets understand the difference between classes and modules.

Let’s understand.

What is the difference between modules and classes ?

Python modules are files containing Python code.

Modules can be executed individually, or imported in some other module and get executed.

A module may contain, classes, properties or functions any of these or all of these.

Where as a class is Python code inside a module.

Let’s look at an example of accessing a class from a module.

Example:

Output:

4.0

Here Decimal is class inside decimal module,

which is part of standard Python installation.

in the above example, Decimal class is imported from decimal module.

In the next section we will look at some more examples of class and objects.

last modified July 16, 2020

In this part of the PyQt5 programming tutorial, we explore events and signalsoccurring in applications.

Events in PyQt5

GUI applications are event-driven. Events are generated mainly by theuser of an application. But they can be generated by other means as well; e.g. anInternet connection, a window manager, or a timer.When we call the application's exec_() method, the application entersthe main loop. The main loop fetches events and sends them to the objects.

In the event model, there are three participants:

  • event source
  • event object
  • event target

The event source is the object whose state changes. It generates events.The event object (event) encapsulates the state changes in the event source.The event target is the object that wants to be notified. Event source objectdelegates the task of handling an event to the event target.

PyQt5 has a unique signal and slot mechanism to deal with events.Signals and slots are used for communication between objects. A signalis emitted when a particular event occurs. A slot can be any Python callable.A slot is called when its connected signal is emitted.

PyQt5 signals and slots

This is a simple example demonstrating signals and slots in PyQt5.

signals_slots.py

In our example, we display a QtGui.QLCDNumberand a QtGui.QSlider. We change the lcdnumber by dragging the slider knob.

Here we connect a valueChanged signal of the slider to thedisplay slot of the lcd number.

The sender is an object that sends a signal. The receiveris the object that receives the signal. The slot is the method thatreacts to the signal.

PyQt5 reimplementing event handler

Events in PyQt5 are processed often by reimplementing event handlers.

In our example, we reimplement the keyPressEvent() event handler.

If we click the Escape button, the application terminates.

Event object in PyQt5

Event object is a Python object that contains a number of attributesdescribing the event. Event object is specific to the generated eventtype.

event_object.py

In this example, we display the x and ycoordinates of a mouse pointer in a label widget.

The x and y coordinates are displayd in a QLabelwidget.

Mouse tracking is disabled by default, so the widget only receives mouse moveevents when at least one mouse button is pressed while the mouse is being moved.If mouse tracking is enabled, the widget receives mouse move events evenif no buttons are pressed.

The e is the event object; it contains data about the eventthat was triggered; in our case, a mouse move event. With the x()and y() methods we determine the x and y coordinates ofthe mouse pointer. We build the string and set it to the label widget.

PyQt5 event sender

Sometimes it is convenient to know which widget is the sender of a signal.For this, PyQt5 has the sender method.

event_sender.py

We have two buttons in our example. In the buttonClicked methodwe determine which button we have clicked by calling thesender() method.

Both buttons are connected to the same slot.

We determine the signal source by calling the sender() method.In the statusbar of the application, we show the labelof the button being pressed.

PyQt5 emitting signals

Objects created from a QObject can emit signals.The following example shows how we to emit custom signals.

We create a new signal called closeApp. This signal isemitted during a mouse press event. The signal is connected to theclose() slot of the QMainWindow.

A signal is created with the pyqtSignal() as a class attributeof the external Communicate class.

The custom closeApp signal is connected to the close()slot of the QMainWindow.

When we click on the window with a mouse pointer, the closeApp signalis emitted. The application terminates.

In this part of the PyQt5 tutorial, we have covered signals and slots.