Can Qt Slot Return Value

The QSpinBox class provides a spin box widget. More...

I'd like to get a return value that tells me in the main/GUI thread if everything is ok. I know that slots should be void and should not be used for this, even if they seem to do it. Emit a signal back seems easy. However, I'd like to know the result immediately. Or should I just call it as regular function instead of using the signal/slot.

1. In contrast to the previous example, our new class also uses a return value for the increment slot. No further adjustments are required to receive the return value in QML. Qt automatically maps basic C types to QML types for all method parameters and return values.
2. PyQt is more versatile than C/Qt in this regard, because we can connect not just to slots, but also to any callable, and from PyQt 4.2, it is possible to dynamically add 'predefined' signals and slots to QObjects. Let's see how signals and slots works in practice with the Signals and Slots program shown in Figure 4.6.
3. 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.

Properties

• 12 properties inherited from QAbstractSpinBox
• 59 properties inherited from QWidget
• 1 property inherited from QObject

Public Functions

• 30 public functions inherited from QAbstractSpinBox
• 214 public functions inherited from QWidget
• 32 public functions inherited from QObject
• 14 public functions inherited from QPaintDevice

Public Slots

• 4 public slots inherited from QAbstractSpinBox
• 19 public slots inherited from QWidget
• 1 public slot inherited from QObject

Signals

• 1 signal inherited from QAbstractSpinBox
• 3 signals inherited from QWidget
• 2 signals inherited from QObject

Protected Functions

Reimplemented Protected Functions

• 20 protected functions inherited from QAbstractSpinBox
• 35 protected functions inherited from QWidget
• 9 protected functions inherited from QObject
• 1 protected function inherited from QPaintDevice

Additional Inherited Members

• 5 static public members inherited from QWidget
• 11 static public members inherited from QObject
• 1 protected slot inherited from QWidget

Detailed Description

The QSpinBox class provides a spin box widget.

QSpinBox is designed to handle integers and discrete sets of values (e.g., month names); use QDoubleSpinBox for floating point values.

QSpinBox allows the user to choose a value by clicking the up/down buttons or pressing up/down on the keyboard to increase/decrease the value currently displayed. The user can also type the value in manually. The spin box supports integer values but can be extended to use different strings with validate(), textFromValue() and valueFromText().

Every time the value changes QSpinBox emits two valueChanged() signals, one providing an int and the other a QString. The QString overload provides the value with both prefix() and suffix(). The current value can be fetched with value() and set with setValue().

Clicking the up/down buttons or using the keyboard accelerator's up and down arrows will increase or decrease the current value in steps of size singleStep(). If you want to change this behaviour you can reimplement the virtual function stepBy(). The minimum and maximum value and the step size can be set using one of the constructors, and can be changed later with setMinimum(), setMaximum() and setSingleStep().

Most spin boxes are directional, but QSpinBox can also operate as a circular spin box, i.e. if the range is 0-99 and the current value is 99, clicking 'up' will give 0 if wrapping() is set to true. Use setWrapping() if you want circular behavior.

The displayed value can be prepended and appended with arbitrary strings indicating, for example, currency or the unit of measurement. See setPrefix() and setSuffix(). The text in the spin box is retrieved with text() (which includes any prefix() and suffix()), or with cleanText() (which has no prefix(), no suffix() and no leading or trailing whitespace).

It is often desirable to give the user a special (often default) choice in addition to the range of numeric values. See setSpecialValueText() for how to do this with QSpinBox.

Subclassing QSpinBox

If using prefix(), suffix(), and specialValueText() don't provide enough control, you subclass QSpinBox and reimplement valueFromText() and textFromValue(). For example, here's the code for a custom spin box that allows the user to enter icon sizes (e.g., '32 x 32'):

See the Icons example for the full source code.

See also QDoubleSpinBox, QDateTimeEdit, QSlider, and Spin Boxes Example.

Property Documentation

cleanText : const QString

This property holds the text of the spin box excluding any prefix, suffix, or leading or trailing whitespace.

Access functions:

See also text, QSpinBox::prefix, and QSpinBox::suffix.

displayIntegerBase : int

This property holds the base used to display the value of the spin box

The default displayIntegerBase value is 10.

This property was introduced in Qt 5.2.

Access functions:

See also textFromValue() and valueFromText().

maximum : int

This property holds the maximum value of the spin box

When setting this property the minimum is adjusted if necessary, to ensure that the range remains valid.

The default maximum value is 99.

Access functions:

See also setRange() and specialValueText.

minimum : int

This property holds the minimum value of the spin box

When setting this property the maximum is adjusted if necessary to ensure that the range remains valid.

The default minimum value is 0.

Access functions:

See also setRange() and specialValueText.

prefix : QString

This property holds the spin box's prefix

The prefix is prepended to the start of the displayed value. Typical use is to display a unit of measurement or a currency symbol. For example:

To turn off the prefix display, set this property to an empty string. The default is no prefix. The prefix is not displayed when value() minimum() and specialValueText() is set.

If no prefix is set, prefix() returns an empty string.

Access functions:

See also suffix(), setSuffix(), specialValueText(), and setSpecialValueText().

singleStep : int

This property holds the step value

When the user uses the arrows to change the spin box's value the value will be incremented/decremented by the amount of the singleStep. The default value is 1. Setting a singleStep value of less than 0 does nothing.

Access functions:

suffix : QString

This property holds the suffix of the spin box

The suffix is appended to the end of the displayed value. Typical use is to display a unit of measurement or a currency symbol. For example:

To turn off the suffix display, set this property to an empty string. The default is no suffix. The suffix is not displayed for the minimum() if specialValueText() is set.

If no suffix is set, suffix() returns an empty string.

Access functions:

See also prefix(), setPrefix(), specialValueText(), and setSpecialValueText().

value : int

This property holds the value of the spin box

setValue() will emit valueChanged() if the new value is different from the old one. The value property has a second notifier signal which includes the spin box's prefix and suffix.

Access functions:

Notifier signal:

Member Function Documentation

QSpinBox::QSpinBox(QWidget *parent = Q_NULLPTR)

Constructs a spin box with 0 as minimum value and 99 as maximum value, a step value of 1. The value is initially set to 0. It is parented to parent.

See also setMinimum(), setMaximum(), and setSingleStep().

QSpinBox::~QSpinBox()

Destructor.

[override virtual protected] bool QSpinBox::event(QEvent *event)

Reimplemented from QObject::event().

[override virtual protected] void QSpinBox::fixup(QString &input) const

Reimplemented from QAbstractSpinBox::fixup().

void QSpinBox::setRange(intminimum, intmaximum)

Convenience function to set the minimum, and maximum values with a single function call.

is equivalent to:

See also minimum and maximum.

[virtual protected] QString QSpinBox::textFromValue(intvalue) const

This virtual function is used by the spin box whenever it needs to display the given value. The default implementation returns a string containing value printed in the standard way using QWidget::locale().toString(), but with the thousand separator removed unless setGroupSeparatorShown() is set. Reimplementations may return anything. (See the example in the detailed description.)

Note: QSpinBox does not call this function for specialValueText() and that neither prefix() nor suffix() should be included in the return value.

If you reimplement this, you may also need to reimplement valueFromText() and validate()

See also valueFromText(), validate(), and QLocale::groupSeparator().

[override virtual protected] QValidator::State QSpinBox::validate(QString &text, int &pos) const

Reimplemented from QAbstractSpinBox::validate().

[signal] void QSpinBox::valueChanged(inti)

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This signal is emitted whenever the spin box's value is changed. The new value's integer value is passed in i.

Note: Signal valueChanged is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

Note: Notifier signal for property value.

[signal] void QSpinBox::valueChanged(const QString &text)

This is an overloaded function.

The new value is passed in text with prefix() and suffix().

Note: Signal valueChanged is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

Note: Notifier signal for property value.

[virtual protected] int QSpinBox::valueFromText(const QString &text) const

This virtual function is used by the spin box whenever it needs to interpret text entered by the user as a value.

Subclasses that need to display spin box values in a non-numeric way need to reimplement this function.

Note: QSpinBox handles specialValueText() separately; this function is only concerned with the other values.

See also textFromValue() and validate().

© 2020 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

This page describes the use of signals and slots in Qt for Python.The emphasis is on illustrating the use of so-called new-style signals and slots, although the traditional syntax is also given as a reference.

The main goal of this new-style is to provide a more Pythonic syntax to Python programmers.

• 2New syntax: Signal() and Slot()

Traditional syntax: SIGNAL () and SLOT()

QtCore.SIGNAL() and QtCore.SLOT() macros allow Python to interface with Qt signal and slot delivery mechanisms.This is the old way of using signals and slots.

The example below uses the well known clicked signal from a QPushButton.The connect method has a non python-friendly syntax.It is necessary to inform the object, its signal (via macro) and a slot to be connected to.

New syntax: Signal() and Slot()

The new-style uses a different syntax to create and to connect signals and slots.The previous example could be rewritten as:

Using QtCore.Signal()

Signals can be defined using the QtCore.Signal() class.Python types and C types can be passed as parameters to it.If you need to overload it just pass the types as tuples or lists.

In addition to that, it can receive also a named argument name that defines the signal name.If nothing is passed as name then the new signal will have the same name as the variable that it is being assigned to.

The Examples section below has a collection of examples on the use of QtCore.Signal().

Note: Signals should be defined only within classes inheriting from QObject.This way the signal information is added to the class QMetaObject structure.

Can Qt Slot Return Value Guide

Using QtCore.Slot()

Slots are assigned and overloaded using the decorator QtCore.Slot().Again, to define a signature just pass the types like the QtCore.Signal() class.Unlike the Signal() class, to overload a function, you don't pass every variation as tuple or list.Instead, you have to define a new decorator for every different signature.The examples section below will make it clearer.

Another difference is about its keywords.Slot() accepts a name and a result.The result keyword defines the type that will be returned and can be a C or Python type.name behaves the same way as in Signal().If nothing is passed as name then the new slot will have the same name as the function that is being decorated.

Examples

The examples below illustrate how to define and connect signals and slots in PySide2.Both basic connections and more complex examples are given.

• Hello World example: the basic example, showing how to connect a signal to a slot without any parameters.

• Next, some arguments are added. This is a modified Hello World version. Some arguments are added to the slot and a new signal is created.

• Add some overloads. A small modification of the previous example, now with overloaded decorators.

• An example with slot overloads and more complicated signal connections and emissions (note that when passing arguments to a signal you use '[]'):

• An example of an object method emitting a signal:

• An example of a signal emitted from another QThread:

• Signals are runtime objects owned by instances, they are not class attributes:

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