.. index:: Plugins; User interaction
.. highlight:: python
:linenothreshold: 5
.. testsetup:: communicating
iface = start_qgis()
The code snippets on this page need the following imports if you're outside the pyqgis console:
.. testcode:: communicating
from qgis.core import (
QgsMessageLog,
QgsGeometry,
)
from qgis.gui import (
QgsMessageBar,
)
from qgis.PyQt.QtWidgets import (
QSizePolicy,
QPushButton,
QDialog,
QGridLayout,
QDialogButtonBox,
)
***************************
Communicating with the user
***************************
.. contents::
:local:
This section shows some methods and elements that should be used to communicate
with the user, in order to keep consistency in the User Interface.
Showing messages. The QgsMessageBar class
=========================================
Using message boxes can be a bad idea from a user experience point of view. For
showing a small info line or a warning/error messages, the QGIS message bar is
usually a better option.
Using the reference to the QGIS interface object, you can show a message in the
message bar with the following code
.. testcode:: communicating
from qgis.core import Qgis
iface.messageBar().pushMessage("Error", "I'm sorry Dave, I'm afraid I can't do that", level=Qgis.Critical)
.. testoutput:: communicating
Messages(2): Error : I'm sorry Dave, I'm afraid I can't do that
.. figure:: img/errorbar.png
:align: center
:width: 40em
QGIS Message bar
You can set a duration to show it for a limited time
.. testcode:: communicating
iface.messageBar().pushMessage("Ooops", "The plugin is not working as it should", level=Qgis.Critical, duration=3)
.. testoutput:: communicating
Messages(2): Ooops : The plugin is not working as it should
.. figure:: img/errorbar-timed.png
:align: center
:width: 40em
QGIS Message bar with timer
The examples above show an error bar, but the ``level`` parameter can be used
to creating warning messages or info messages, using the
:class:`Qgis.MessageLevel <qgis.core.Qgis.MessageLevel>` enumeration. You can use up to 4 different levels:
0. Info
1. Warning
2. Critical
3. Success
.. figure:: img/infobar.png
:align: center
:width: 40em
QGIS Message bar (info)
Widgets can be added to the message bar, like for instance a button to show
more info
.. testcode:: communicating
def showError():
pass
widget = iface.messageBar().createMessage("Missing Layers", "Show Me")
button = QPushButton(widget)
button.setText("Show Me")
button.pressed.connect(showError)
widget.layout().addWidget(button)
iface.messageBar().pushWidget(widget, Qgis.Warning)
.. testoutput:: communicating
Messages(1): Missing Layers : Show Me
.. figure:: img/bar-button.png
:align: center
:width: 40em
QGIS Message bar with a button
You can even use a message bar in your own dialog so you don't have to show a
message box, or if it doesn't make sense to show it in the main QGIS window
.. testcode:: communicating
class MyDialog(QDialog):
def __init__(self):
QDialog.__init__(self)
self.bar = QgsMessageBar()
self.bar.setSizePolicy( QSizePolicy.Minimum, QSizePolicy.Fixed )
self.setLayout(QGridLayout())
self.layout().setContentsMargins(0, 0, 0, 0)
self.buttonbox = QDialogButtonBox(QDialogButtonBox.Ok)
self.buttonbox.accepted.connect(self.run)
self.layout().addWidget(self.buttonbox, 0, 0, 2, 1)
self.layout().addWidget(self.bar, 0, 0, 1, 1)
def run(self):
self.bar.pushMessage("Hello", "World", level=Qgis.Info)
myDlg = MyDialog()
myDlg.show()
.. figure:: img/dialog-with-bar.png
:align: center
:width: 40em
QGIS Message bar in custom dialog
Showing progress
================
Progress bars can also be put in the QGIS message bar, since, as we have seen,
it accepts widgets. Here is an example that you can try in the console.
.. testcode:: communicating
import time
from qgis.PyQt.QtWidgets import QProgressBar
from qgis.PyQt.QtCore import *
progressMessageBar = iface.messageBar().createMessage("Doing something boring...")
progress = QProgressBar()
progress.setMaximum(10)
progress.setAlignment(Qt.AlignLeft|Qt.AlignVCenter)
progressMessageBar.layout().addWidget(progress)
iface.messageBar().pushWidget(progressMessageBar, Qgis.Info)
for i in range(10):
time.sleep(1)
progress.setValue(i + 1)
iface.messageBar().clearWidgets()
.. testoutput:: communicating
Messages(0): Doing something boring...
Also, you can use the built-in status bar to report progress, as in the next
example:
.. testcode:: communicating
vlayer = iface.activeLayer()
count = vlayer.featureCount()
features = vlayer.getFeatures()
for i, feature in enumerate(features):
# do something time-consuming here
print('.') # printing should give enough time to present the progress
percent = i / float(count) * 100
# iface.mainWindow().statusBar().showMessage("Processed {} %".format(int(percent)))
iface.statusBarIface().showMessage("Processed {} %".format(int(percent)))
iface.statusBarIface().clearMessage()
.. testoutput:: communicating
:hide:
.
Logging
=======
There are three different types of logging available in QGIS to log and save all
the information about the execution of your code. Each has its specific output
location. Please consider to use the correct way of logging for your purpose:
* :class:`QgsMessageLog <qgis.core.QgsMessageLog>` is for messages to communicate
issues to the user.
The output of the QgsMessageLog is shown in the Log Messages Panel.
* The python built in **logging** module is for debugging on the level of the QGIS
Python API (PyQGIS). It is recommended for Python script developers that need to
debug their python code, e.g. feature ids or geometries
* :class:`QgsLogger <qgis.core.QgsLogger>` is for messages for *QGIS internal*
debugging / developers (i.e. you suspect something is triggered by some broken code).
Messages are only visible with developer versions of QGIS.
Examples for the different logging types are shown in the following sections below.
.. warning::
Use of the Python ``print`` statement is unsafe to do in any code which may be
multithreaded and **extremely slows down the algorithm**. This includes **expression
functions**, **renderers**,
**symbol layers** and **Processing algorithms** (amongst others). In these
cases you should always use the python **logging** module or thread safe classes
(:class:`QgsLogger <qgis.core.QgsLogger>`
or :class:`QgsMessageLog <qgis.core.QgsMessageLog>`) instead.
QgsMessageLog
-------------
.. testcode:: communicating
# You can optionally pass a 'tag' and a 'level' parameters
QgsMessageLog.logMessage("Your plugin code has been executed correctly", 'MyPlugin', level=Qgis.Info)
QgsMessageLog.logMessage("Your plugin code might have some problems", level=Qgis.Warning)
QgsMessageLog.logMessage("Your plugin code has crashed!", level=Qgis.Critical)
.. testoutput:: communicating
MyPlugin(0): Your plugin code has been executed correctly
(1): Your plugin code might have some problems
(2): Your plugin code has crashed!
.. note::
You can see the output of the :class:`QgsMessageLog <qgis.core.QgsMessageLog>`
in the :ref:`log_message_panel`
The python built in logging module
------------------------------------
.. code-block:: python
import logging
formatter = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
logfilename=r'c:\temp\example.log'
logging.basicConfig(filename=logfilename, level=logging.DEBUG, format=formatter)
logging.info("This logging info text goes into the file")
logging.debug("This logging debug text goes into the file as well")
The basicConfig method configures the basic setup of the logging.
In the above code the filename, logging level and the format are defined.
The filename refers to where to write the logfile to, the logging level defines what
levels to output and the format defines the format in which each message is output.
.. code-block::
2020-10-08 13:14:42,998 - root - INFO - This logging text goes into the file
2020-10-08 13:14:42,998 - root - DEBUG - This logging debug text goes into the file as well
If you want to erase the log file every time you execute your script you can do something like:
.. code-block:: python
if os.path.isfile(logfilename):
with open(logfilename, 'w') as file:
pass
Further resources on how to use the python logging facility are available at:
* https://docs.python.org/3/library/logging.html
* https://docs.python.org/3/howto/logging.html
* https://docs.python.org/3/howto/logging-cookbook.html
.. warning::
Please note that without logging to a file by setting a filename the logging may be
multithreaded which heavily slows down the output.