View source for Documentation/4.5/Developers/FAQ/Python Scripting

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== How to systematically execute custom python code at startup ? ==

Each time Slicer starts, it will look up for a file named <code>.slicerrc.py</code> into your HOME folder.

See [[Documentation/{{documentation/version}}/FAQ/General#What_is_my_HOME_folder_.3F|What is my HOME folder ?]]

== How to save an image/volume using python ? ==

The module <code>slicer.util</code> provides methods allowing to save either a node or an entire scene:
* saveNode
* saveScene

For more details see:
* https://github.com/Slicer/Slicer/blob/master/Base/Python/slicer/util.py#L229-267
* https://github.com/Slicer/Slicer/blob/master/Base/Python/slicer/tests/test_slicer_util_save.py


== How to assign a volume to a Slice view ? ==

Assuming the <code>MRHead</code> sample data has been loaded, you could do the following:

<pre>
red_logic = slicer.app.layoutManager().sliceWidget("Red").sliceLogic()
red_cn = red_logic.GetSliceCompositeNode()
red_logic.GetSliceCompositeNode().SetBackgroundVolumeID(slicer.util.getNode('MRHead').GetID())
</pre>

Discussion: http://slicer-devel.65872.n3.nabble.com/Assign-volumes-to-views-tt4028694.html

== How to access vtkRenderer in Slicer 3D view ? ==


<pre>
renderer = slicer.app.layoutManager().threeDWidget(0).threeDView().renderWindow().GetRenderers().GetFirstRenderer()
</pre>

== How to access displayable manager associated with a Slicer 2D or 3D view ? ==

As originally explained [http://slicer-devel.65872.n3.nabble.com/How-to-get-the-point-of-a-3D-model-based-on-the-fiducial-position-td4031760.html#a4031762 here], you could use the method <code>getDisplayableManagers()</code> available in any [{{doxygen-class-url|qMRMLThreeDView}} qMRMLThreeDView] and [{{doxygen-class-url|qMRMLSliceView}} qMRMLSliceView].

<pre>
lm = slicer.app.layoutManager()
for v in range(lm.threeDViewCount):
  td = lm.threeDWidget(v)
  ms = vtk.vtkCollection()
  td.getDisplayableManagers(ms)
  for i in range(ms.GetNumberOfItems()):
   m = ms.GetItemAsObject(i)
   if m.GetClassName() == "vtkMRMLModelDisplayableManager":
     print(m)
</pre>

== How to center the 3D view on the scene ? ==

<pre>
layoutManager = slicer.app.layoutManager()
threeDWidget = layoutManager.threeDWidget(0)
threeDView = threeDWidget.threeDView()
threeDView.resetFocalPoint()
</pre>

== Should I use 'old style' or 'new style' python classes in my scripted module ? ==

When python classes have no superclass specified they are 'old style' as described here [http://docs.python.org/2/reference/datamodel.html#new-style-and-classic-classes].

In general it doesn't matter for the classes in a scripted module, since they won't be subclassed either old or new style should be the same.

For other python code in slicer where you might be subclassing, it's better to use new style classes.  See the class hierarchies in the [https://github.com/Slicer/Slicer/tree/master/Modules/Scripted/EditorLib EditorLib] and the [https://github.com/Slicer/Slicer/tree/master/Modules/Scripted/DICOM/DICOMLib DICOMLib] for examples.

== How to harden a transform ? ==

<pre>
>>> n = getNode('Bone')
>>> logic = slicer.vtkSlicerTransformLogic()
>>> logic.hardenTransform(n)
</pre>

Discussion: http://slicer-devel.65872.n3.nabble.com/Scripting-hardened-transforms-tt4029456.html

== Where can I find example scripts? ==

Have a look at [[Documentation/{{documentation/version}}/ScriptRepository]].

== How can I use a visual debugger for step-by-step debugging ==

===Debugging using PyCharm or PyDev===
Visual debugging (setting breakpoints, execute code step-by-step, view variables, stack, etc.) of Python scripted module is possible by using [https://www.jetbrains.com/pycharm/ PyCharm] or [http://pydev.org/ PyDev] by using the ''Python debugger'' module [[Documentation/{{documentation/version}}/Extensions/DebuggingTools|Debugging tools]] extension.

'''See detailed instructions at the [[Documentation/{{documentation/version}}/Extensions/DebuggingTools|Debugging tools extension page]].'''
[[File:PyDevRemoteDebugSlicer.png|800px|thumb|center|Visual debugging of Python modules in Slicer]]

===Debugging using Visual Studio===
On Windows, [https://github.com/Microsoft/PTVS Python Tools for Visual Studio] (PTVS) enables debugging Python inside Visual Studio. Its remote debugging capability allows attaching the debugger to Slicer's embedded Python environment.

See [[Documentation/{{documentation/version}}/Developers/Tutorials/Debugging_Python_in_Visual_Studio | Debugging Python in Visual Studio]] for details.

===Debugging using remote-pdb===

A command line debugging session with a [https://docs.python.org/3/library/pdb.html pdb] interface can be achieved with [https://github.com/ionelmc/python-remote-pdb python-remote-pdb].

Install python-remote-pdb into Slicer's Python:

  git clone https://github.com/ionelmc/python-remote-pdb.git
  cd python-remote-pdb
  /path/to/Slicer-build/Slicer-build/Slicer ./setup.py install

Then, call ''set_trace()'' where you want to start the debugger.

  from remote_pdb import set_trace; set_trace()

In the console where Slicer was started, a message like the following will be printed:

  RemotePdb session open at 127.0.0.1:1234, waiting for connection

In another terminal, connect with telnet:

  telnet 127.0.0.1 1234

or socat (has history, readline support):

  socat readline tcp:127.0.0.1:1234

== Why can't I access my C++ Qt class from python ==
* Python wrapping of a Qt class requires a Qt style constructor with QObject as argument (it can be defaulted to null though), which is public. If one of these are missing, python wrapping will fail for that class
* [Other reasons go here]

== How can I access callData argument in a VTK object observer callback function ==

To get notification about an event emitted by a VTK object you can simply use the AddObserver method, for example:
<pre>
def sceneModifiedCallback(caller, eventId):
  print("Scene modified")
  print("There are {0} nodes in the scene". format(slicer.mrmlScene.GetNumberOfNodes()))

sceneModifiedObserverTag = slicer.mrmlScene.AddObserver(vtk.vtkCommand.ModifiedEvent, sceneModifiedCallback)
</pre>

If an event also contains additional information as CallData then the type of this argument has to be specified as well, for example:

<pre>
@vtk.calldata_type(vtk.VTK_OBJECT)
def nodeAddedCallback(caller, eventId, callData):
  print("Node added")
  print("New node: {0}".format(callData.GetName()))

nodeAddedModifiedObserverTag = slicer.mrmlScene.AddObserver(slicer.vtkMRMLScene.NodeAddedEvent, nodeAddedCallback)
</pre>

Note: @vtk.calldata_type is a Python decorator, which modifies properties of a function that is declared right after the decorator. The decorator is defined in VTK (in Wrapping\Python\vtk\util\misc.py).

Usage from a class requires an extra step of creating the callback in the class __init__ function, as Python2 by default does some extra wrapping (http://stackoverflow.com/questions/9523370/adding-attributes-to-instance-methods-in-python):
<pre>
class MyClass:
  def __init__(self):
    from functools import partial
    def nodeAddedCallback(self, caller, eventId, callData):
      print("Node added")
      print("New node: {0}".format(callData.GetName()))
    self.nodeAddedCallback = partial(nodeAddedCallback, self)
    self.nodeAddedCallback.CallDataType = vtk.VTK_OBJECT
  def registerCallbacks(self):
    self.nodeAddedModifiedObserverTag = slicer.mrmlScene.AddObserver(slicer.vtkMRMLScene.NodeAddedEvent, self.nodeAddedCallback)
  def unregisterCallbacks(self):
    slicer.mrmlScene.RemoveObserver(self.nodeAddedModifiedObserverTag)
        
myObject = MyClass()
myObject.registerCallbacks()
</pre>

Allowed CallDataType values:  VTK_STRING, VTK_OBJECT, VTK_INT, VTK_LONG, VTK_DOUBLE, VTK_FLOAT, "string0". See more information here:
https://github.com/Kitware/VTK/blob/master/Wrapping/PythonCore/vtkPythonCommand.cxx

A simplified syntax is available by using a mix-in:

<pre>
from slicer.util import VTKObservationMixin

class MyClass(VTKObservationMixin):
  def __init__(self):
    VTKObservationMixin.__init__(self)
    self.addObserver(slicer.mrmlScene, slicer.vtkMRMLScene.NodeAddedEvent, self.nodeAddedCallback)
  
  @vtk.calldata_type(vtk.VTK_OBJECT)
  def nodeAddedCallback(self, caller, eventId, callData):
    print("Node added")
    print("New node: {0}".format(callData.GetName()))

myObject = MyClass()
</pre>

Note: VTKObservationMixin is a Python mix-in that allows adding a set of methods to a class by inheritance. VTKObservationMixin includes addObserver, hasObserver, observer, removeObserver, removeObservers methods, defined in Slicer (in Base\Python\slicer\util.py). For example of usage, see [https://github.com/Slicer/Slicer/blob/master/Base/Python/slicer/tests/test_slicer_util_VTKObservationMixin.py test_slicer_util_VTKObservationMixin.py]

== How to run CLI module from Python? ==

See [[Documentation/{{documentation/version}}/Developers/Python_scripting#Running_a_CLI_from_Python|here]].

== How can I run slicer operations from a batch script? ==


<pre>
Slicer --no-main-window --python-script /tmp/test.py
</pre>

Contents of /tmp/test.py

<pre>
# use a slicer scripted module logic
from SampleData import SampleDataLogic
SampleDataLogic().downloadMRHead()
head = slicer.util.getNode('MRHead')

# use a vtk class
threshold = vtk.vtkImageThreshold()
threshold.SetInputData(head.GetImageData())
threshold.ThresholdBetween(100, 200)
threshold.SetInValue(255)
threshold.SetOutValue(0)

#  use a slicer-specific C++ class
erode = slicer.vtkImageErode()
erode.SetInputConnection(threshold.GetOutputPort())
erode.SetNeighborTo4()  
erode.Update()          

head.SetAndObserveImageData(erode.GetOutputDataObject(0))

slicer.util.saveNode(head, "/tmp/eroded.nrrd")

exit()
</pre>

== How can I run Slicer on a headless compute node? ==

Many cluster nodes are installed with minimal linux systems that don't include X servers.  X servers, particularly those with hardware acceleration traditionally needed to be installed with root privileges, making it impossible to run applications that rendered using X or OpenGL.

But there is a workaround which allows everything in slicer to work normally so you could even do headless rendering.

You can use a modern version of X that supports running a dummy framebuffer.  This can be installed in user mode so you don't even need to have root on the system.

See [https://www.xpra.org/trac/wiki/Xdummy] for details.

There's a thread here with more discussion: [http://massmail.spl.harvard.edu/public-archives/slicer-devel/2015/017317.html]

Here is a working example of the approach running on a headless compute node running CTK tests (which also use Qt and VTK)

[https://github.com/pieper/CTK/blob/master/.travis.yml]