# How to use signals and slots in Slicer 3d?
**URL:** https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013
**Category:** Support
**Created:** [October 13, 2020, 2:28pm UTC](https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013 "2020-10-13T14:28:14Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Chintha\_Siva\_Prasad](https://discourse.slicer.org/u/Chintha_Siva_Prasad)
#### Post date: [October 13, 2020, 2:28pm UTC](https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013/1 "2020-10-13T14:28:14Z")
I am familiar with pyqt. I want to use signals and slots mechanism in my slicer python script. For some reason when I am connecting the signal to the slot, slicer is exiting abnormally.
How to use signals and slots in slicer pythonqt?
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### Author:  [@jamesobutler](https://discourse.slicer.org/u/jamesobutler)
#### Post date: [October 13, 2020, 2:54pm UTC](https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013/2 "2020-10-13T14:54:08Z")
A previous post of mine mentioned some about basics of using Qt signals and slots with python.
> [@Dynamically Updating UI](https://discourse.slicer.org/t/dynamically-updating-ui/13801/6):
>
> Doing some like a=qt.QPushButton() is already the python interface that you can use in Slicer to interact with Qt which is C++ based. Slicer uses [PythonQt](https://mevislab.github.io/pythonqt/) and not [PyQt](https://riverbankcomputing.com/software/pyqt/intro). They are similar in syntax, but with different licensing. How are you currently connecting the button press to updating the label? I generally connect actions using the newer style of a=qt.QPushButton() a.clicked.connect(slotName)
**Essentially the syntax is something like {object}.{signal\_name}.connect({slot\_name}).**
So something that is based on QProgressBar will have [these signals](https://doc.qt.io/qt-5/qprogressbar.html#signals) such as valueChanged. Therefore
```python
def printMyNewValue(value):
print("The progress bar value is now: {}".format(value))
import qt
progress_bar = qt.QProgressBar()
progress_bar.setMaximum(10)
progress_bar.valueChanged.connect(printMyNewValue)
progress_bar.setValue(4) # will then print "The progress bar value is now: 4"
```
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### Author:  [@Chintha\_Siva\_Prasad](https://discourse.slicer.org/u/Chintha_Siva_Prasad)
#### Post date: [October 14, 2020, 2:20pm UTC](https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013/3 "2020-10-14T14:20:39Z")
I want to create a new signal and then connect it to a slot. I don’t want to use the pre-defined signals of widgets.
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### Author:  [@Chintha\_Siva\_Prasad](https://discourse.slicer.org/u/Chintha_Siva_Prasad)
#### Post date: [October 14, 2020, 2:23pm UTC](https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013/4 "2020-10-14T14:23:37Z")
When I am executing the above code slicer is exiting abnormally. How can I create a new signal in slicer that works fine?
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### Author:  [@lassoan](https://discourse.slicer.org/u/lassoan)
#### Post date: [October 15, 2020, 4:43am UTC](https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013/5 "2020-10-15T04:43:04Z")
This example works for me:
```python
class C(qt.QObject):
received = qt.Signal(object)
def __init__ (self):
super(C, self). __init__ (None)
def process(self, msg):
self.received.emit(msg)
class D(qt.QObject):
def __init__ (self, sender):
super(D, self). __init__ (None)
sender.received.connect(self.process)
def process(self, msg):
print("Processed this: "+repr(msg))
c = C()
d = D(c)
m = {'h': {'t': 's'}, 'c': {'e': 'i'}}
c.process(m)
```
(taken from these discussions: [Custom Signal/Slots with PythonQt - #6 by ihnorton](https://discourse.slicer.org/t/custom-signal-slots-with-pythonqt/3278/6) and [PythonQt / Discussion / Help: Segmentation fault after emiting a signal](https://sourceforge.net/p/pythonqt/discussion/631393/thread/bbc98b8f/))
However, normally there is no need to create Qt-based Python classes and define new signals and slots, as you can communicate via MRML nodes and VTK event observations:
> [@Custom Signal/Slots with PythonQt](https://discourse.slicer.org/t/custom-signal-slots-with-pythonqt/3278/8):
>
> We have signal/slot mechanism in Qt and observer mechanism in VTK, and you can implement custom plugin or callback mechanisms. These are all available to be used from both C++ and Python. Probably the most universal way (that is available from all Slicer classes) is to observe/invoke custom modified events on a MRML node.
What is your use case? What would you like to achieve with the custom signals/slots?
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### Author:  [@Chintha\_Siva\_Prasad](https://discourse.slicer.org/u/Chintha_Siva_Prasad)
#### Post date: [October 18, 2020, 3:18pm UTC](https://discourse.slicer.org/t/how-to-use-signals-and-slots-in-slicer-3d/14013/6 "2020-10-18T15:18:00Z")
I want to change the progress of a progressbar from another thread.
i want to create a signal(int) and connect it to progressBar.setValue(int).
But when I am connecting it is exiting…