Pyqt Connect Signal To Slot
Pyqt Signal Slot

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A Pop-Up Alert in 25 Lines

The button click (signal) is connected to the action (slot). In this example, the method slotmethod will be called if the signal emits. This principle of connecting slots methods or function to a widget, applies to all widgets. New-style Signal and Slot Support¶. This section describes the new style of connecting signals and slots introduced in PyQt4 v4.5. One of the key features of Qt is its use of signals and slots to communicate between objects. Try commenting out the different signals and seeing the effect on what the slot prints. We start by creating a function that will behave as a ‘slot’ for our signals. Then we use.connect on the.windowTitleChanged signal. We pass the function that we want to be called with the signal data.

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This chapter covers three tiny yet useful GUI applications written in PyQt and discusses PyQt's 'signals and slots' mechanism--a high-level communication mechanism for responding to user interaction that lets you ignore irrelevant detail.

Anyway, its not really common to connect same signal to different slot in same class. You might as well fall foo2 and foo3 from foo1. You might as well fall foo2 and foo3 from foo1. But if the foox each lives in own classes, then its a pretty normal design.
Sending Python values with signals and slots On the #pyqt channel on Freenode, Khertan asked about sending Python values via Qt's signals and slots mechanism. The following example uses the PyQtPyObject value declaration with an old-style signal-slot connection, and again when the signal is emitted, to communicate a Python dictionary.

This chapter is from the book 

Rapid GUI Programming with Python and Qt: The Definitive Guide to PyQt Programming

This chapter is from the book

This chapter is from the book 

Rapid GUI Programming with Python and Qt: The Definitive Guide to PyQt Programming

A Pop-Up Alert in 25 Lines
An Expression Evaluator in 30 Lines
A Currency Converter in 70 Lines
Signals and Slots

In this chapter we begin with brief reviews of three tiny yet useful GUI applications written in PyQt. We will take the opportunity to highlight some of the issues involved in GUI programming, but we will defer most of the details to later chapters. Once we have a feel for PyQt GUI programming, we will discuss PyQt's 'signals and slots' mechanism—this is a high-level communication mechanism for responding to user interaction that allows us to ignore irrelevant detail.

Although PyQt is used commercially to build applications that vary in size from hundreds of lines of code to more than 100 000 lines of code, the applications we will build in this chapter are all less than 100 lines, and they show just how much can be done with very little code.

In this chapter we will design our user interfaces purely by writing code, but in Chapter 7, we will learn how to create user interfaces using Qt's visual design tool, Qt Designer.

Python console applications and Python module files always have a .py extension, but for Python GUI applications we use a .pyw extension. Both .py and .pyw are fine on Linux, but on Windows, .pyw ensures that Windows uses the pythonw.exe interpreter instead of python.exe, and this in turn ensures that when we execute a Python GUI application, no unnecessary console window will appear.^* On Mac OS X, it is essential to use the .pyw extension.

The PyQt documentation is provided as a set of HTML files, independent of the Python documentation. The most commonly referred to documents are those covering the PyQt API. These files have been converted from the original C++/Qt documentation files, and their index page is called classes.html; Windows users will find a link to this page in their Start button's PyQt menu. It is well worth looking at this page to get an overview of what classes are available, and of course to dip in and read about those classes that seem interesting.

The first application we will look at is an unusual hybrid: a GUI application that must be launched from a console because it requires command-line arguments. We have included it because it makes it easier to explain how the PyQt event loop works (and what that is), without having to go into any other GUI details. The second and third examples are both very short but standard GUI applications. They both show the basics of how we can create and lay out widgets ('controls' in Windows-speak)—labels, buttons, comboboxes, and other on-screen elements that users can view and, in most cases, interact with. They also show how we can respond to user interactions—for example, how to call a particular function or method when the user performs a particular action.

In the last section we will cover how to handle user interactions in more depth, and in the next chapter we will cover layouts and dialogs much more thoroughly. Use this chapter to get a feel for how things work, without worrying about the details: The chapters that follow will fill in the gaps and will familiarize you with standard PyQt programming practices.

A Pop-Up Alert in 25 Lines

Our first GUI application is a bit odd. First, it must be run from the console, and second it has no 'decorations'—no title bar, no system menu, no X close button. Figure 4.1 shows the whole thing.

To get the output displayed, we could enter a command line like this:

When run, the program executes invisibly in the background, simply marking time until the specified time is reached. At that point, it pops up a window with the message text. About a minute after showing the window, the application will automatically terminate.

The specified time must use the 24-hour clock. For testing purposes we can use a time that has just gone; for example, by using 12:15 when it is really 12:30, the window will pop up immediately (well, within less than a second).

Now that we know what it does and how to run it, we will review the implementation. The file is a few lines longer than 25 lines because we have not counted comment lines and blank lines in the total—but there are only 25 lines of executable code. We will begin with the imports.

We import the sys module because we want to access the command-line arguments it holds in the sys.argv list. The time module is imported because we need its sleep() function, and we need the PyQt modules for the GUI and for the QTime class.

We begin by creating a QApplication object. Every PyQt GUI application must have a QApplication object. This object provides access to global-like information such as the application's directory, the screen size (and which screen the application is on, in a multihead system), and so on. This object also provides the event loop, discussed shortly.

When we create a QApplication object we pass it the command-line arguments; this is because PyQt recognizes some command-line arguments of its own, such as -geometry and -style, so we ought to give it the chance to read them. If QApplication recognizes any of the arguments, it acts on them, and removes them from the list it was given. The list of arguments that QApplication recognizes is given in the QApplication's initializer's documentation.

At the very least, the application requires a time, so we set the due variable to the time right now. We also provide a default message. If the user has not given at least one command-line argument (a time), we raise a ValueError exception. This will result in the time being now and the message being the 'usage' error message.

If the first argument does not contain a colon, a ValueError will be raised when we attempt to unpack two items from the split() call. If the hours or minutes are not a valid number, a ValueError will be raised by int(), and if the hours or minutes are out of range, due will be an invalid QTime, and we raise a ValueError ourselves. Although Python provides its own date and time classes, the PyQt date and time classes are often more convenient (and in some respects more powerful), so we tend to prefer them.

If the time is valid, we set the message to be the space-separated concatenation of the other command-line arguments if there are any; otherwise, we leave it as the default 'Alert!' that we set

Pyqt Connect Signal To Multiple Slots