Pyqt4 documentation: Signals and Slots. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events. Each PyQt widget, which is derived from QObject class, is designed to emit signal in response to one or more events. The signal on its own does not perform any action. Old-style Signal and Slot Support. The returned value is normally passed to the QtCore.QObject.connect method. PyQt4 allows any Python callable to be used as a slot, not just Qt slots. This is done by simply referencing the callable. Because Qt slots are implemented as class methods they are also available as Python callables. I am trying to learn PyQt from rapid gui programming with python and qt and currently learning Signals and Slots. Below is a short snippet of my code: What I am trying to achieve here is call 2 SLOTS at once that is spinbox.setValue and getValuedial for dial object as soon as ValueChanged(int) signal is emitted. Feb 08, 2008 Whenever a signal is emitted, by default PyQt simply throws it away! To take notice of a signal we must connect it to a slot. In C/Qt, slots are methods that must be declared with a special syntax; but in PyQt, they can be any callable we like (e.g., any function or method), and no special syntax is required when defining them.
This section describes the older style for connecting signals and slots. Ituses the same API that a C++ application would use. This has a number ofadvantages.
An introduction to creating PySide/PyQt signals and slots, using QObject. How signals and slots are useful, and what they can do when developing in PySide/PyQt. We need to connect its punched signal to a slot that does something. The same signal can be emitted in multiple places. Now, let's define some slots that can be connected to the. 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.
- It is well understood and documented.
- Any future changes to the C++ API should be easily included.
It also has a number of disadvantages.
![Pyqt Pyqt](/uploads/1/2/5/2/125208457/445418155.png)
- It requires knowledge of the C++ types of signal arguments.
- It is error prone in that if you mis-type the signal name or signature thenno exception is raised, either when the signal is connected or emitted.
- It is verbose.
- It is not Pythonic.
This older style of connecting signals and slots will continue to be supportedthroughout the life of PyQt4.
PyQt4 Signals and Qt Signals¶
Qt signals are statically defined as part of a C++ class. They are referencedusing the
QtCore.SIGNAL()
function. This method takes a single stringargument that is the name of the signal and its C++ signature. For example:The returned value is normally passed to the
QtCore.QObject.connect()
method.PyQt4 allows new signals to be defined dynamically. The act of emitting aPyQt4 signal implicitly defines it. PyQt4 signals are also referenced usingthe
QtCore.SIGNAL()
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The PyQt_PyObject
Signal Argument Type¶
It is possible to pass any Python object as a signal argument by specifying
PyQt_PyObject
as the type of the argument in the signature. For example:While this would normally be used for passing objects like lists anddictionaries as signal arguments, it can be used for any Python type. Itsadvantage when passing, for example, an integer is that the normal conversionsfrom a Python object to a C++ integer and back again are not required.
The reference count of the object being passed is maintained automatically.There is no need for the emitter of a signal to keep a reference to the objectafter the call to
QtCore.QObject.emit()
, even if a connection is queued.Short-circuit Signals¶
There is also a special form of a PyQt4 signal known as a short-circuit signal.Short-circut signals implicitly declare each argument as being of type
PyQt_PyObject
.Short-circuit signals do not have a list of arguments or the surroundingparentheses.
Short-circuit signals may only be connected to slots that have been implementedin Python. They cannot be connected to Qt slots or the Python callables thatwrap Qt slots.
PyQt4 Slots and Qt Slots¶
Qt slots are statically defined as part of a C++ class. They are referencedusing the
QtCore.SLOT()
function. This method takes a single stringargument that is the name of the slot and its C++ signature. For example:The returned value is normally passed to the
QtCore.QObject.connect()
method.PyQt4 allows any Python callable to be used as a slot, not just Qt slots. Thisis done by simply referencing the callable. Because Qt slots are implementedas class methods they are also available as Python callables. Therefore it isnot usually necessary to use
QtCore.SLOT()
for Qt slots. However, doing sois more efficient as it avoids a conversion to Python and back to C++.Pyqt Signal Slots
Qt allows a signal to be connected to a slot that requires fewer arguments thanthe signal passes. The extra arguments are quietly discarded. PyQt4 slots canbe used in the same way.
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Note that when a slot is a Python callable its reference count is notincreased. This means that a class instance can be deleted without having toexplicitly disconnect any signals connected to its methods. However, if a slotis a lambda function or a partial function then its reference count isautomatically incremented to prevent it from being immediately garbagecollected.
Pyqt5 Signal Slot
Connecting Signals and Slots¶
Connections between signals and slots (and other signals) are made using the
QtCore.QObject.connect()
method. For example:Disconnecting signals works in exactly the same way using the
QtCore.QObject.disconnect()
method. However, not all the variations ofthat method are supported by PyQt4. Signals must be disconnected one at atime.Emitting Signals¶
Any instance of a class that is derived from the
QtCore.QObject
class canemit a signal using its emit()
method. This takes a minimum of oneargument which is the signal. Any other arguments are passed to the connectedslots as the signal arguments. For example:The QtCore.pyqtSignature()
Decorator¶
Pyqt Signal Slot
The
QtCore.pyqtSignature()
serves the same purpose as thepyqtSlot()
decorator but has a less Pythonic API.