Abstract

This PSEP describes the support of PyQt's new-style signals and slots in PySide. Traditionally PyQt, and by extension PySide, has required the use of C++ function signature strings in connecting signals to slots. This has been widely regarded as unwieldy and non-Pythonic. A new-style signal and slot support was introduced in PyQt 4.5, in which the signals and slots may be manipulated using native Python objects and methods. This new syntax will be adopted to PySide with minor modifications.

Rationale

The PyQt's traditional syntax for signals and slots requires the use of explicit C++ style signal signatures [1]. This has been regarded as unwieldy and non-Pythonic feature and an opportunity for improvement in the syntax.

A new-style signal and slot syntax has been introduced in PyQt 4.5 [2]. The new style provides a clean syntax to connect signals and slots using Python objects and methods instead of C++ style signature strings. An example of connecting and emitting signals using the new-style syntax is given below (adapted from [2]):

class Foo(QtCore.QObject):

    # Define a new signal called 'trigger' that has no arguments.
    trigger = QtCore.pyqtSignal()

    def connect_and_emit_trigger(self):
        # Connect the trigger signal to a slot.
        self.trigger.connect(self.handle_trigger)

        # Emit the signal.
        self.trigger.emit()

    def handle_trigger(self):
        # Show that the slot has been called.

        print "trigger signal received"

PyQt's new-style signal and slot mechanism provides a clean, concise, and Pythonic syntax for defining, connecting, disconnecting, and emitting signals. To maintain compatibility with PyQt, their new-style signal and slot mechanism will be adopted to PySide with minor modifications only.

New Signal and Slot Syntax

PySide will adopt PyQt's new signal and slot syntax as-is. The PySide implementation will be functionally compatible with the PyQt 4.5 one, with the exceptions listed in the next Section. As PyQt compatibility will be the main guideline, the syntax specifics will not be described here. Instead, PyQt behaviour and documentation [2] shall be used as implementation guidelines.

Differences from PyQt's syntax

PyQt's new signal and slot style utilizes method and decorator names specific to their implementation. These will be generalized according to the table below:

In addition to the generalized class names, PyQt factory names will be provided as a compatibility feature. They will print out a deprecation warning, however.

On the mailing list discussion, concerns were raised about the proposed naming scheme. One issue was the risk of Qt implementing similarly named C++ classes, resulting in name clashes in PySide. However, the risk may be regarded minor during the lifespan of Qt version 4, especially now that the signals and slots are classes and are written in the capitalized form. Even in the case this would happen, the issue could be circumvented in PySide by renaming the new C++ classes suitably.

Another issue raised was the similarity of Signal and SIGNAL (as well as Slot and SLOT) class and function names in QtCore. While the similarity is undeniably an issue, it could still be argued that the case difference is substantial enough to minimize any risk of confusion in practice. It could be argued that the non-Pythonic all-caps spelling refers to the old-style signals and slots, while the standard spelling refers to the Pythonic new-style signals.

Compatibility

Since the new signal and slot syntax does not break compatibility with the previous syntax, it will be implemented both as part of the original and the possible future PySide API.

References

Contributors

Hugo Lima, Mark Summerfield, Ville M. Vainio, Paul A. Giannaros, and Douglas Soares de Andrade have contributed in the initial discussion of this PSEP.

After having the PSEP already accepted, Hugo Lima pointed out a naming scheme inconsistency, which caused the PSEP to be redrawn to the draft status to have the classes properly capitalized.

Copyright

This document has been placed in the public domain.