Q&A: How to check if a QLineEdit is empty?
Empty strings are falsey in Python

A reader asked:

I just want to know, how do I check whether a QLineEdit is empty or not?

The QLineEdit class doesn't have an isEmpty() method which you can call to find out if the line edit is empty, but we don't need one! Instead we can get the current text using .text()) and then check if that value is empty. In the code below lineedit is our already created QLineEdit widget.

python
text = lineedit.text()
if text == '': # if the line edit is empty, .text() will return an empty string.
     # do something

We can simplify this further. In Python empty strings are falsey -- they are considered False values in conditional expressions. So instead of checking the string is empty, we can check if it is true (non-empty) or false (empty).

python
if lineedit.text():
     # do something if there is content in the line edit.

Or, to check if it is empty:

python
if not lineedit.text():
     # do something if the line edit is empty.

Below is a small demo application which updates a label to indicate if the line edit has text in it or not. In this we use Qt signals to send the current text to a slot method every time it is updated.

python
import sys

from PyQt5.QtWidgets import QApplication, QLabel, QLineEdit, QVBoxLayout, QWidget


class Window(QWidget):
    def __init__(self):
        super().__init__()

        self.lineedit = QLineEdit()
        self.lineedit.textChanged.connect(self.text_changed)

        self.label = QLabel()

        vlayout = QVBoxLayout()
        vlayout.addWidget(self.lineedit)
        vlayout.addWidget(self.label)

        self.setLayout(vlayout)

    def text_changed(self, s):

        # s contains the text of the line edit, we could also test self.lineedit.text()

        if s:
            self.label.setText("Not empty")

        else:
            self.label.setText("Empty")


app = QApplication(sys.argv)

w = Window()
w.show()

app.exec_()
python
import sys

from PyQt6.QtWidgets import QApplication, QLabel, QLineEdit, QVBoxLayout, QWidget


class Window(QWidget):
    def __init__(self):
        super().__init__()

        self.lineedit = QLineEdit()
        self.lineedit.textChanged.connect(self.text_changed)

        self.label = QLabel()

        vlayout = QVBoxLayout()
        vlayout.addWidget(self.lineedit)
        vlayout.addWidget(self.label)

        self.setLayout(vlayout)

    def text_changed(self, s):

        # s contains the text of the line edit

        if s:
            self.label.setText("Not empty")

        else:
            self.label.setText("Empty")


app = QApplication(sys.argv)

w = Window()
w.show()

app.exec()

python
import sys

from PySide2.QtWidgets import QApplication, QLabel, QLineEdit, QVBoxLayout, QWidget


class Window(QWidget):
    def __init__(self):
        super().__init__()

        self.lineedit = QLineEdit()
        self.lineedit.textChanged.connect(self.text_changed)

        self.label = QLabel()

        vlayout = QVBoxLayout()
        vlayout.addWidget(self.lineedit)
        vlayout.addWidget(self.label)

        self.setLayout(vlayout)

    def text_changed(self, s):

        # s contains the text of the line edit

        if s:
            self.label.setText("Not empty")

        else:
            self.label.setText("Empty")


app = QApplication(sys.argv)

w = Window()
w.show()

app.exec_()


python
import sys

from PySide6.QtWidgets import QApplication, QLabel, QLineEdit, QVBoxLayout, QWidget


class Window(QWidget):
    def __init__(self):
        super().__init__()

        self.lineedit = QLineEdit()
        self.lineedit.textChanged.connect(self.text_changed)

        self.label = QLabel()

        vlayout = QVBoxLayout()
        vlayout.addWidget(self.lineedit)
        vlayout.addWidget(self.label)

        self.setLayout(vlayout)

    def text_changed(self, s):

        # s contains the text of the line edit

        if s:
            self.label.setText("Not empty")

        else:
            self.label.setText("Empty")


app = QApplication(sys.argv)

w = Window()
w.show()

app.exec_()


Run the above and you'll see the label update as you add and remove text in the QLineEdit.

Empty lineedit

Lineedit with content

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PySide2 Widgets  pyside

In Qt (and most User Interfaces) ‘widget’ is the name given to a component of the UI that the user can interact with. User interfaces are made up of multiple widgets, arranged within the window. Qt comes with a large selection of widgets available, and even allows you to create your own custom and customized widgets. A quick demo First let's have a look at some of the most common PyQt widgets. The following code creates a range of PyQt widgets and adds them to a window layout so you can see them together. We'll cover how layouts work in Qt in the next tutorial. python import sys from PySide2.QtCore import Qt from PySide2.QtWidgets import ( QApplication, QCheckBox, QComboBox, QDateEdit, QDateTimeEdit, QDial, QDoubleSpinBox, QFontComboBox, QLabel, QLCDNumber, QLineEdit, QMainWindow, QProgressBar, QPushButton, QRadioButton, QSlider, QSpinBox, QTimeEdit, QVBoxLayout, QWidget, ) # Subclass QMainWindow to customize your application's main window class MainWindow(QMainWindow): def __init__(self): super().__init__() self.setWindowTitle("Widgets App") layout = QVBoxLayout() widgets = [ QCheckBox, QComboBox, QDateEdit, QDateTimeEdit, QDial, QDoubleSpinBox, QFontComboBox, QLCDNumber, QLabel, QLineEdit, QProgressBar, QPushButton, QRadioButton, QSlider, QSpinBox, QTimeEdit, ] for w in widgets: layout.addWidget(w()) widget = QWidget() widget.setLayout(layout) # Set the central widget of the Window. Widget will expand # to take up all the space in the window by default. self.setCentralWidget(widget) app = QApplication(sys.argv) window = MainWindow() window.show() app.exec_() Run it! You'll see a window appear containing all the widgets we've created. Big ol' list of widgets on Windows, Mac & Ubuntu Linux. Lets have a look at all the example widgets, from top to bottom: Widget What it does QCheckbox A checkbox QComboBox A dropdown list box QDateEdit For editing dates and datetimes QDateTimeEdit For editing dates and datetimes QDial Rotateable dial QDoubleSpinbox A number spinner for floats QFontComboBox A list of fonts QLCDNumber A quite ugly LCD display QLabel Just a label, not interactive QLineEdit Enter a line of text QProgressBar A progress bar QPushButton A button QRadioButton A toggle set, with only one active item QSlider A slider QSpinBox An integer spinner QTimeEdit For editing times There are far more widgets than this, but they don’t fit so well! You can see them all by checking the Qt documentation. Next, we'll step through some of the most commonly used widgets and look at them in more detail. To experiment with the widgets we'll need a simple application to put them in. Save the following code to a file named app.py and run it to make sure it's working. python import sys from PySide2.QtWidgets import ( QMainWindow, QApplication, QLabel, QCheckBox, QComboBox, QListBox, QLineEdit, QLineEdit, QSpinBox, QDoubleSpinBox, QSlider ) from PySide2.QtCore import Qt class MainWindow(QMainWindow): def __init__(self): super(MainWindow, self).__init__() self.setWindowTitle("My App") app = QApplication(sys.argv) w = MainWindow() w.show() app.exec_() In the code above we've imported a number of Qt widgets. Now we'll step through each of those widgets in turn, adding them to our application and seeing how they behave. QLabel We'll start the tour with QLabel, arguably one of the simplest widgets available in the Qt toolbox. This is a simple one-line piece of text that you can position in your application. You can set the text by passing in a str as you create it: python widget = QLabel("Hello") Or, by using the .setText() method: python widget = QLabel("1") # The label is created with the text 1. widget.setText("2") # The label now shows 2. You can also adjust font parameters, such as the size of the font or the alignment of text in the widget. python class MainWindow(QMainWindow): def __init__(self): super(MainWindow, self).__init__() self.setWindowTitle("My App") widget = QLabel("Hello") font = widget.font() font.setPointSize(30) widget.setFont(font) widget.setAlignment(Qt.AlignHCenter | Qt.AlignVCenter) self.setCentralWidget(widget) QLabel on Windows, Mac & Ubuntu Linux. Font tip Note that if you want to change the properties of a widget font it is usually better to get the current font, update it and then apply it back. This ensures the font face remains in keeping with the desktop conventions. The alignment is specified by using a flag from the Qt. namespace. The flags available for horizontal alignment are: Flag Behavior Qt.AlignLeft Aligns with the left edge. Qt.AlignRight Aligns with the right edge. Qt.AlignHCenter Centers horizontally in the available space. Qt.AlignJustify Justifies the text in the available space. The flags available for vertical alignment are: Flag Behavior Qt.AlignTop Aligns with the top. Qt.AlignBottom Aligns with the bottom. Qt.AlignVCenter Centers vertically in the available space. You can combine flags together using pipes (|), however note that you can only use vertical or horizontal alignment flag at a time. python align_top_left = Qt.AlignLeft | Qt.AlignTop Note that you use an OR pipe (`|`) to combine the two flags (not A & B). This is because the flags are non-overlapping bitmasks. e.g. Qt.AlignLeft has the hexadecimal value 0x0001, while Qt.AlignBottomis 0x0040. By ORing together we get the value 0x0041 representing 'bottom left'. This principle applies to all other combinatorial Qt flags. If this is gibberish to you, feel free to ignore and move on. Just remember to use | Finally, there is also a shorthand flag that centers in both directions simultaneously: Flag Behavior Qt.AlignCenter Centers horizontally and vertically Weirdly, you can also use QLabel to display an image using .setPixmap(). This accepts an pixmap, which you can create by passing an image filename to QPixmap. In the example files provided with this book you can find a file otje.jpg which you can display in your window as follows: python widget.setPixmap(QPixmap('otje.jpg')) "Otje" the cat. What a lovely face. By default the image scales while maintaining its aspect ratio. If you want it to stretch and scale to fit the window completely you can set .setScaledContents(True) on the QLabel. python widget.setScaledContents(True) QCheckBox The next widget to look at is QCheckBox() which, as the name suggests, presents a checkable box to the user. However, as with all Qt widgets there are number of configurable options to change the widget behaviors. python class MainWindow(QMainWindow): def __init__(self): super(MainWindow, self).__init__() self.setWindowTitle("My App") widget = QCheckBox() widget.setCheckState(Qt.Checked) # For tristate: widget.setCheckState(Qt.PartiallyChecked) # Or: widget.setTriState(True) widget.stateChanged.connect(self.show_state) self.setCentralWidget(widget) def show_state(self, s): print(s == Qt.Checked) print(s) QCheckBox on Windows, Mac & Ubuntu Linux. You can set a checkbox state programmatically using .setChecked or .setCheckState. The former accepts either True or False representing checked or unchecked respectively. However, with .setCheckState you also specify a particular checked state using a Qt. namespace flag: Flag Behavior Qt.Unchecked Item is unchecked Qt.PartiallyChecked Item is partially checked Qt.Checked Item is unchecked A checkbox that supports a partially-checked (Qt.PartiallyChecked) state is commonly referred to as 'tri-state', that is being neither on nor off. A checkbox in this state is commonly shown as a greyed out checkbox, and is commonly used in hierarchical checkbox arrangements where sub-items are linked to parent checkboxes. If you set the value to Qt.PartiallyChecked the checkbox will become tristate. You can also .setTriState(True) to set tristate support on a You can also set a checkbox to be tri-state without setting the current state to partially checked by using .setTriState(True) You may notice that when the script is running the current state number is displayed as an int with checked = 2, unchecked = 0, and partially checked = 1. You don’t need to remember these values, the Qt.Checked namespace variable == 2 for example. This is the value of these state's respective flags. This means you can test state using state == Qt.Checked. QComboBox The QComboBox is a drop down list, closed by default with an arrow to open it. You can select a single item from the list, with the currently selected item being shown as a label on the widget. The combo box is suited to selection of a choice from a long list of options. You have probably seen the combo box used for selection of font faces, or size, in word processing applications. Although Qt actually provides a specific font-selection combo box as QFontComboBox. You can add items to a QComboBox by passing a list of strings to .addItems(). Items will be added in the order they are provided. python class MainWindow(QMainWindow): def __init__(self): super(MainWindow, self).__init__() self.setWindowTitle("My App") widget = QComboBox() widget.addItems(["One", "Two", "Three"]) # The default signal from currentIndexChanged sends the index widget.currentIndexChanged.connect(self.index_changed) # The same signal can send a text string widget.currentTextChanged.connect(self.text_changed) self.setCentralWidget(widget) def index_changed(self, i): # i is an int print(i) def text_changed(self, s): # s is a str print(s) QComboBox on Windows, Mac & Ubuntu Linux. The .currentIndexChanged signal is triggered when the currently selected item is updated, by default passing the index of the selected item in the list. There is also a .currentTextChanged signal which instead provides the label of the currently selected item, which is often more useful. QComboBox can also be editable, allowing users to enter values not currently in the list and either have them inserted, or simply used as a value. To make the box editable: python widget.setEditable(True) You can also set a flag to determine how the insert is handled. These flags are stored on the QComboBox class itself and are listed below: Flag Behavior QComboBox.NoInsert No insert QComboBox.InsertAtTop Insert as first item QComboBox.InsertAtCurrent Replace currently selected item QComboBox.InsertAtBottom Insert after last item QComboBox.InsertAfterCurrent Insert after current item QComboBox.InsertBeforeCurrent Insert before current item QComboBox.InsertAlphabetically Insert in alphabetical order To use these, apply the flag as follows: python widget.setInsertPolicy(QComboBox.InsertAlphabetically) You can also limit the number of items allowed in the box by using .setMaxCount, e.g. python widget.setMaxCount(10) For a more in-depth look at the QComboBox take a look at my QComboBox documentation. QListWidget Next QListWidget. It’s very similar to QComboBox, differing mainly in the signals available. python class MainWindow(QMainWindow): def __init__(self): super(MainWindow, self).__init__() self.setWindowTitle("My App") widget = QListWidget() widget.addItems(["One", "Two", "Three"]) # In QListWidget there are two separate signals for the item, and the str widget.currentItemChanged.connect( self.index_changed ) widget.currentTextChanged.connect( self.text_changed ) self.setCentralWidget(widget) def index_changed(self, i): # Not an index, i is a QListItem print(i.text()) def text_changed(self, s): # s is a str print(s) QListWidget on Windows, Mac & Ubuntu Linux. QListWidget offers an currentItemChanged signal which sends the QListItem (the element of the list box), and a currentTextChanged signal which sends the text. QLineEdit The QLineEdit widget is a simple single-line text editing box, into which users can type input. These are used for form fields, or settings where there is no restricted list of valid inputs. For example, when entering an email address, or computer name. python class MainWindow(QMainWindow): def __init__(self): super(MainWindow, self).__init__() self.setWindowTitle("My App") widget = QLineEdit() widget.setMaxLength(10) widget.setPlaceholderText("Enter your text") #widget.setReadOnly(True) # uncomment this to make readonly widget.returnPressed.connect(self.return_pressed) widget.selectionChanged.connect(self.selection_changed) widget.textChanged.connect(self.text_changed) widget.textEdited.connect(self.text_edited) self.setCentralWidget(widget) def return_pressed(self): print("Return pressed!") self.centralWidget().setText("BOOM!") def selection_changed(self): print("Selection changed") print(self.centralWidget().selectedText()) def text_changed(self, s): print("Text changed...") print(s) def text_edited(self, s): print("Text edited...") print(s) QLineEdit on Windows, Mac & Ubuntu Linux. As demonstrated in the above code, you can set a maximum length for the text in a line edit. The QLineEdit has a number of signals available for different editing events including when return is pressed (by the user), when the user selection is changed. There are also two edit signals, one for when the text in the box has been edited and one for when it has been changed. The distinction here is between user edits and programmatic changes. The textEdited signal is only sent when the user edits text. Additionally, it is possible to perform input validation using an input mask to define which characters are supported and where. This can be applied to the field as follows: python widget.setInputMask('000.000.000.000;_') The above would allow a series of 3-digit numbers separated with periods, and could therefore be used to validate IPv4 addresses. QSpinBox and QDoubleSpinBox QSpinBox provides a small numerical input box with arrows to increase and decrease the value. QSpinBox supports integers while the related widget QDoubleSpinBox supports floats. python class MainWindow(QMainWindow): def __init__(self): super().__init__() self.setWindowTitle("My App") widget = QSpinBox() # Or: widget = QDoubleSpinBox() widget.setMinimum(-10) widget.setMaximum(3) # Or: widget.setRange(-10,3) widget.setPrefix("$") widget.setSuffix("c") widget.setSingleStep(3) # Or e.g. 0.5 for QDoubleSpinBox widget.valueChanged.connect(self.value_changed) widget.textChanged.connect(self.value_changed_str) self.setCentralWidget(widget) def value_changed(self, i): print(i) def value_changed_str(self, s): print(s) Run it and you'll see a numeric entry box. The value shows pre and post fix units, and is limited to the range +3 to -10. QSpinBox on Windows, Mac & Ubuntu Linux. The demonstration code above shows the various features that are available for the widget. To set the range of acceptable values you can use setMinimum and setMaximum, or alternatively use setRange to set both simultaneously. Annotation of value types is supported with both prefixes and suffixes that can be added to the number, e.g. for currency markers or units using .setPrefix and .setSuffix respectively. Clicking on the up and down arrows on the widget will increase or decrease the value in the widget by an amount, which can be set using .setSingleStep. Note that this has no effect on the values that are acceptable to the widget. Both QSpinBox and QDoubleSpinBox have a .valueChanged signal which fires whenever their value is altered. The raw .valueChanged signal sends the numeric value (either an int or a float) while .textChanged sends the value as a string, including both the prefix and suffix characters. QSlider QSlider provides a slide-bar widget, which functions internally much like a QDoubleSpinBox. Rather than display the current value numerically, it is represented by the position of the slider handle along the length of the widget. This is often useful when providing adjustment between two extremes, but where absolute accuracy is not required. The most common use of this type of widget is for volume controls. There is an additional .sliderMoved signal that is triggered whenever the slider moves position and a .sliderPressed signal that emits whenever the slider is clicked. python class MainWindow(QMainWindow): def __init__(self): super().__init__() self.setWindowTitle("My App") widget = QSlider() widget.setMinimum(-10) widget.setMaximum(3) # Or: widget.setRange(-10,3) widget.setSingleStep(3) widget.valueChanged.connect(self.value_changed) widget.sliderMoved.connect(self.slider_position) widget.sliderPressed.connect(self.slider_pressed) widget.sliderReleased.connect(self.slider_released) self.setCentralWidget(widget) def value_changed(self, i): print(i) def slider_position(self, p): print("position", p) def slider_pressed(self): print("Pressed!") def slider_released(self): print("Released") Run this and you'll see a slider widget. Drag the slider to change the value. QSlider on Windows, Mac & Ubuntu Linux. You can also construct a slider with a vertical or horizontal orientation by passing the orientation in as you create it. The orientation flags are defined in the Qt. namespace. For example -- python widget.QSlider(Qt.Vertical) Or -- python widget.QSlider(Qt.Horizontal) QDial Finally, the QDial is a rotatable widget that functions just like the slider, but appears as an analogue dial. This looks nice, but from a UI perspective is not particularly user friendly. However, they are often used in audio applications as representation of real-world analogue dials. python class MainWindow(QMainWindow): def __init__(self): super().__init__() self.setWindowTitle("My App") widget = QDial() widget.setRange(-10, 100) widget.setSingleStep(0.5) widget.valueChanged.connect(self.value_changed) widget.sliderMoved.connect(self.slider_position) widget.sliderPressed.connect(self.slider_pressed) widget.sliderReleased.connect(self.slider_released) self.setCentralWidget(widget) def value_changed(self, i): print(i) def slider_position(self, p): print("position", p) def slider_pressed(self): print("Pressed!") def slider_released(self): print("Released") Run this and you'll see a circular dial, rotate it to select a number from the range. QDial on Windows, Mac & Ubuntu Linux. The signals are the same as for QSlider and retain the same names (e.g. .sliderMoved). Conclusion This concludes our brief tour of the common widgets used in PySide applications. To see the full list of available widgets, including all their signals and attributes, take a look at the Qt documentation. More