The status bar

The QGIS window has a status bar. You can use it to display the current status of your Python program, for example:

iface.mainWindow().statusBar().showMessage("Please wait...")

The status message will appear at the bottom of the window, like this:

As you can see, there isn't much room on the status bar, so you'll need to keep your status message short. To hide the message again, do the following:

iface.mainWindow().statusBar().clearMessage()

The message bar

A message bar appears within a window to display messages to the user, for example:

Message bars have several useful features:

Any window in QGIS can have its own message bar. The iface variable has a messageBar() method, which returns the message bar for the main QGIS window, but you can also add a message bar to your own custom windows if you wish.

To add a message to a message bar, you call the message bar's pushMessage() method. To create a message without a title, you use the following method signature:

messageBar.pushMessage(text, level=QsgMessageBar.INFO, duration=None)

For example:

from qgis.gui import *
iface.messageBar().pushMessage("Hello World",
         level=QgsMessageBar.INFO)

To include a title, use the following method signature:

messageBar.pushMessage(title, text, level=QgsMessageBar.INFO, duration=None)

In both cases, the level parameter can be set to QgsMessageBar.INFO, QgsMessageBar.WARNING, or QgsMessageBar.CRITICAL, and if the duration parameter is specified, it will be the number of seconds before the message is hidden.

To remove all the messages currently being shown, you can call the messageBar.clearWidgets() method.

Progress indicators

You can also make use of the message bar to display a Qt progress indicator. To do this, use the messageBar.createMessage() method to create a widget to display your message, then modify the widget to include additional Qt controls, and finally call the messageBar.pushWidget() method to display the message and the controls you added. For example:

progressMessage = iface.messageBar().createMessage("Please wait")
progressBar = QProgressBar()
progressBar.setMaximum(100)
progressBar.setAlignment(Qt.AlignLeft | Qt.AlignVCenter)
progressMessage.layout().addWidget(progressBar)
iface.messageBar().pushWidget(progressMessage)
...
progressBar.setValue(n)
...
iface.messageBar().clearWidgets()

QGIS logging

You can use the built-in logging facilities of QGIS to display the output in a separate window. For example:

for i in range(100):
    QgsMessageLog.logMessage("Message %d" % i)

The log messages will be shown in the log view, which you can show by navigating to View | Panels | Log Messages.

If you wish, you can change the importance of your message by adding a message level to the logMessage() call, for example:

QgsMessageLog.logMessage("Something is wrong",
                         level=QgsMessageLog.CRITICAL)

Rather than being mixed in with other QGIS messages, you can also choose to have all your log messages appear in a pane by themselves, by adding a tag to the logMessage() call as follows:

QgsMessageLog.logMessage("Test Message", tag="my panel")

Your log messages will then appear in a panel by themselves, like this:

Custom dialogs and windows

As QGIS is built on top of Qt, you can use the PyQt classes to create your own windows and dialog boxes, and display them directly from within your Python code. For example, here's a script that displays a custom dialog box that prompts the user to enter a latitude and longitude value:

from PyQt4.QtGui import *

class MyDialog(QDialog):
    def __init__(self):
        QDialog.__init__(self)
        self.setWindowTitle("Enter Coordinate")

        layout = QFormLayout(self)

        self.lat_label = QLabel("Latitude", self)
        self.lat_field = QLineEdit(self)

        self.long_label = QLabel("Longitude", self)
        self.long_field = QLineEdit(self)

        self.ok_btn = QPushButton("OK", self)
        self.ok_btn.clicked.connect(self.accept)

        self.cancel_btn = QPushButton("Cancel", self)
        self.cancel_btn.clicked.connect(self.reject)

        btn_layout = QHBoxLayout(self)
        btn_layout.addWidget(self.ok_btn)
        btn_layout.addWidget(self.cancel_btn)

        layout.addRow(self.lat_label, self.lat_field)
        layout.addRow(self.long_label, self.long_field)
        layout.addRow(btn_layout)

        self.setLayout(layout)

dialog = MyDialog()
if dialog.exec_() == QDialog.Accepted:
    lat = dialog.lat_field.text()
    long = dialog.long_field.text()
    print lat,long

Running this script will cause the following dialog box to be displayed:

If the user clicks on the OK button, the entered latitude and longitude values will be printed to the console. Of course, this is just a simple example—there's no error checking or conversion of the entered values from text back to numbers. However, this is just a simple example. There's a lot more that can be done using the PyQt libraries, and people have written entire books on the subject. However, the main thing to realize now is that, because QGIS is built on top of Qt, you can use all of the features of PyQt to build sophisticated user interfaces. You're certainly not limited to using the Python console to interact with the user.