# Data transfer between Source and python
**URL:** https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502
**Category:** ParaView Support
**Tags:** python
**Created:** [February 4, 2020, 10:40am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502 "2020-02-04T10:40:39Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 4, 2020, 10:40am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/1 "2020-02-04T10:40:40Z")
Dear Devs,
My source is defined by a set of parameters **x** , but also contains an inner scalar _q=f(x)_. I want to modify the **x** and dynamically get the _q_ within python. The end goal is to have a python script that loops over multiple **x** values, makes pictures of a source, but also gets and writes the _q_ value into a file as follows:
```
GetActiveSource().x = [1,2,3,4,5]
GetActiveSource().q # <--expecting new q value
```
Currently, if apply button is pressed, the correct q value is produced, but Render() + Update() in a python console do not have the same effect.
What is the correct python command for an apply button?
I’m currently trying to understand how the python interface for a source works. Particularly, what functions are called and when? Are data transferred only upon apply? Is it possible to transfer the data before apply is pressed?
Could you give a general direction towards how the python interface in PV is implemented, what classes are used etc.
Thank you very much,
Art
---
### Author:  [@cory.quammen](https://discourse.paraview.org/u/cory.quammen)
#### Post date: [February 4, 2020, 2:18pm UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/2 "2020-02-04T14:18:03Z")
Have you looked through the [ParaView Guide](https://www.paraview.org/paraview-guide/)? There are numerous Python examples throughout, and a skim of the guide should answer a number of questions you’ve posed here.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 4, 2020, 2:34pm UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/3 "2020-02-04T14:34:01Z")
Hi Cory,
Thanks a lot for the reply. I am reading it write now. =)
Sincerely,
Art
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 5, 2020, 2:36pm UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/4 "2020-02-05T14:36:57Z")
I am back. Reading the manual, though always useful, did not bring me any closer to the answer.
Let me reformulate the question a bit. I have a custom c++ plugin for a source, which has two widgets, A and B, which are linked. If A is modified, B will be modified as well. For simplicity, if A is set to 5, B will be set to 25 (i.e. A\*\*2). _I have achieved this already._
However, I would like to do the same in a python script. Namely,
```
mySource.A = 6 # <- new value
mySource.B # <- fetching B, expecting 36 (= 6**2)
```
Currently, this combination works
```
mySource.A = 6 # <- new value
# pressing apply with my mouse
mySource.B
# getting 36, which is correct
```
However this combination does not work
```
mySource.A = 6 # <- new value
UpdatePipeline()
Render()
mySource.B
# getting 25, which is the old value of B, which is wrong
```
So obviously, the connection between two widgets and the python interface is working. And yet, UpdatePipeline() + Render() is not equivalent to pressing the _Apply_ button, and I cannot figure out what I am doing wrong.
Possibly a more precise question, what **signals** are used to inform python about the changes in the widget values? Q\_PROPERTY’s NOTIFY signal didn’t do the trick.
Thanks a ton,
Art
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 5, 2020, 3:55pm UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/5 "2020-02-05T15:55:30Z")
Also, as an afterthought, can one get the _pqPropertiesPanel_ from within a plugin, and call the apply() method manually?
Dirty, but would do the trick.
Best,
Art
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [February 6, 2020, 2:34am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/6 "2020-02-06T02:34:31Z")
> [@siver](#):
>
> which has two widgets, A and B, which are linked.
How are they linked ? By Qt signals ? Then it breaks the ParaView server/manager mechanism.
> what **signals** are used to inform python about the changes in the widget values? Q\_PROPERTY’s NOTIFY signal didn’t do the trick.
None, it is all managed by properties as python can be used without Qt when using pvpython alone.
> can one get the _pqPropertiesPanel_ from within a plugin, and call the apply() method manually?
Yes, using pythonQt, but definitely a code that is not supported by paraview framework and can break at any update.
If you want a property to influence another property, then it is not supported by ParaView property mechanism.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 6, 2020, 9:43am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/7 "2020-02-06T09:43:30Z")
Hi, Mathieu. Thank you very much for your excellent answer.
I see the problem now. The source/client semantics of paraview is still quite new to me (we use PV only locally and never think of it as a source/client application), but I’m getting a better grasp of it now.
Are the following statements correct?
1. If I want one property to influence another, they should belong to two different filters (or sources) and be connected via a pipeline.
2. The only way to get some property ‘x’ exposed to python (i.e. be able to call _GetActiveSource().x_) is to have a Q\_PROPERTY on it in its c++ class.
Thank you very much,
Art
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [February 6, 2020, 9:47am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/8 "2020-02-06T09:47:31Z")
> 1. If I want one property to influence another, they should belong to two different filters (or sources) and be connected via a pipeline.
One property cannot influence another, except to compute its default value. Being in two filters is not related to that.
> The only way to get some property ‘x’ exposed to python (i.e. be able to call _GetActiveSource().x_ ) is to have a Q\_PROPERTY on it in its c++ class.
Not at all. To have a property exposed in python for a proxy, the property only needs to be declared in the xml of the proxy.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 6, 2020, 10:05am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/9 "2020-02-06T10:05:30Z")
I am properly confused. 🙂
> [@mwestphal](#):
>
> > The only way to get some property ‘x’ exposed to python (i.e. be able to call _GetActiveSource().x_ ) is to have a Q\_PROPERTY on it in its c++ class.
>
> Not at all. To have a property exposed in python for a proxy, the property only needs to be declared in the xml of the proxy.
Indeed, but doesn’t Q\_PROPERTY (and it’s READ WRITE fields) tell us which set/get methods to use for python calls (among other things)? Otherwise the xml property will return null and cannot be updated.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 6, 2020, 10:07am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/10 "2020-02-06T10:07:09Z")
_To summarize_, if the source is defined with a parameter **x** and we define a dependent variable _y_=f( **x** ), there is no way to updated _y_ dynamically for every time when **x** is modified (programmatically or otherwise), since it violates the server/client paradigm of PV.
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [February 6, 2020, 10:13am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/11 "2020-02-06T10:13:58Z")
> Q\_PROPERTY
Q\_PROPERTY is only use to create the link between the property and the Qt widget. unrelated to python.
ParaView can be compiled with only Qt and not python, or with only python and not Qt.
> _To summarize_ , if the source is defined with a parameter **x** and we define a dependent variable _y_ =f( **x** ), there is no way to updated _y_ dynamically for every time when **x** is modified (programmatically or otherwise), since it violates the server/client paradigm of PV.
Yes ! However, there is a few way around this to trick the user, using multiple properties and information property. I could explain in more details if you are interested.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 6, 2020, 10:36am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/12 "2020-02-06T10:36:39Z")
> [@mwestphal](#):
>
> I could explain in more details if you are interested.
Yes, absolutely!
Just to clarify, since my colleagues are even less skilled in paraview and programming, my hope was to have it like this:
```
mySource.x = 6
mySource.y
> 36
```
Or maybe to define my own function _f_ and have it like this
```
mySource.x = 6
mySource.f(mySource.x)
> 36
```
~~If there is a way to achieve this or similar~~ , I am all ears. I need to know!
Thank you very much,
Art
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [February 6, 2020, 10:39am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/13 "2020-02-06T10:39:30Z")
Using information\_properties, what you can do is :
```
mySource.x = 6
mySource.UpdatePipelineInformation()
mySource.yInfo
> 36
mySource.y = mySource.yInfo
mySource.Update()
```
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 6, 2020, 10:41am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/14 "2020-02-06T10:41:37Z")
how is “yInfo” different from y? Specifically in c++.
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [February 6, 2020, 10:44am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/15 "2020-02-06T10:44:30Z")
yInfo is another property XML, with a specific tag “information\_only”. C++ side, it is only a call to a Getter method, which would take care of the multiplication.
See Examples/Plugins/PropertyWidgets for an information\_only example. It does not contain your usecase exactly though.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 10, 2020, 12:39pm UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/16 "2020-02-10T12:39:47Z")
So I’ve tried the “information\_only” tag as follows:
xml:
```
```
c++:
```
public:
vtkStringArray* GetNewParameter(){
return NewParameter;
}
private:
vtkNew NewParameter;
```
But there is no `NewParameter` variable in python:
```
GetActiveSource().NewParameter # <- doesn't exist
```
Q1: How do I make `NewParameter` visible in python?
Q2: What is the alternative for double? Currently, I get this error:
```
No SIProperty for the following information helper: DoubleArrayHelper.
```
Q3: In the example, “GetShrinkFactor” method is called, although the plugin has no such method. How is this possible?
Thank you very much,
Art
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [February 11, 2020, 2:56am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/17 "2020-02-11T02:56:01Z")
> xml:
StringArrayHelper is deprecated. Use `si_class="vtkSIDataArrayProperty"` instead.
> How do I make `NewParameter` visible in python?
- You either set it visible in the xml with `panel_visibiliy="default"`
- or you use the more complex api : GetActiveSource().SMProxy.GetProperty(“NewParameter”)
> What is the alternative for double?
Same as for string if you need an array of double : `si_class="vtkSIDataArrayProperty"`
Or nothing if you only need a single value.
> In the example, “GetShrinkFactor” method is called, although the plugin has no such method. How is this possible?
It is a macro `vtkGetMacro("ShrinkFactor");` in the header.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 11, 2020, 10:40am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/18 "2020-02-11T10:40:29Z")
Thank you for the detailed answers, I feel like I’m close, but I’m still having issues. My current state is
xml:
```auto
```
c++:
```auto
public:
vtkDoubleArray* GetNewParameter(){
// shamelessly taken from a vtk example
newParameter->SetNumberOfComponents(1);
newParameter->SetNumberOfTuples(4);
std::array, 4> pts = {{ {{1.}}, {{2.}}, {{3.}}, {{4.}} }};
for (auto i = 0ul; i < pts.size(); ++i)
newParameter->SetTuple(i, pts[i].data());
return newParameter;
}
private:
vtkNew newParameter;
```
This works!
> `GetActiveSource().SMProxy.GetProperty("NewParameter").GetNumberOfElements() # > 4`
> `GetActiveSource().SMProxy.GetProperty("NewParameter").GetElement(0) # > 1`
But this unfortunately doesn’t:
1. NewParameter is still not visible in python:
> `GetActiveSource().NewParameter # doesn't exist`
1. I still have to press `Apply` to get an update, i.e. calling `UpdatePipeline()` from python CLI has no effect and `GetNewParameter()` is never called. How to ensure that `GetNewParameter()` is called every time the pipeline is updated? Example
> `UpdatePipeline()`
> `GetActiveSource().SMProxy.GetProperty("NewParameter").GetElement(0)`
> `> GetNewParameter() called, new values obtained`
1. Currently I generate the same data (newParameter = [1, 2, 3, 4]) every time. How do I fetch new python values (i.e. `GetActiveSource().Parameter = [2, 3]`) in c++?
Thank you so much,
Art
---
### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [February 11, 2020, 10:45am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/19 "2020-02-11T10:45:57Z")
> NewParameter is still not visible in python:
Are you using master ?
> How to ensure that `GetNewParameter()` is called every time the pipeline is updated? Example
`UpdatePipelineInformation()`
> How do I fetch new python values
Not sure what you mean, in your C++ class you can compute any values you want.
---
### Author:  [@siver](https://discourse.paraview.org/u/siver)
#### Post date: [February 11, 2020, 11:02am UTC](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502/20 "2020-02-11T11:02:18Z")
> Are you using master ?
I’m currently using ParaView 5.7.0, compiled from source, which comes from [https://www.paraview.org/download/](https://www.paraview.org/download/).
I guess the error `name 'UpdatePipelineInformation' is not defined` is related.
> in your C++ class you can compute any values you want
Yes of course, but what I mean is that upon `GetActiveSource().Parameter = [2, 3]` I update my widget values. I can then further parse the values around via `this->proxy()->GetProperty("Parameter")`, but this doesn’t seem to work for the plugin class itself.
I’m sorry if my questions are repetitive or trivial, I still have very little understanding of what I’m doing.
Thanks a lot,
Art
[Next page](https://discourse.paraview.org/t/data-transfer-between-source-and-python/3502.md?page=2)