# How is filter hierarchy handled in custom filter?

**URL:** https://discourse.paraview.org/t/how-is-filter-hierarchy-handled-in-custom-filter/8480
**Category:** ParaView Support
**Created:** [November 24, 2021, 12:44pm UTC](https://discourse.paraview.org/t/how-is-filter-hierarchy-handled-in-custom-filter/8480 "2021-11-24T12:44:15Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![pindakaas](https://discourse.paraview.org/letter_avatar_proxy/v4/letter/p/cab0a1/32.png) [@pindakaas](https://discourse.paraview.org/u/pindakaas)
#### Post date: [November 24, 2021, 12:44pm UTC](https://discourse.paraview.org/t/how-is-filter-hierarchy-handled-in-custom-filter/8480/1 "2021-11-24T12:44:15Z")

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Hello,

I am trying to create a custom filter which consists of _first_ extracting data from several locations and _then_ combining these extracted locations in a group. Here’s a sort of scheme of what I expect to happen:

Input - Extract Location 1 - Group Datasets (Output)  
- Extract Location 2 -^  
- Extract Location i -^

When doing this, warnings pop up that the group datasets proxy already exists, so I suspect what actually happens is that for each extracted location a group dataset filter is applied. Something like this:

Input - Extract Location 1 - Group Datasets (Output)  
- Extract Location 2 - Group Datasets (Output, replaces previous)  
- Extract Location i - Group Datasets (Output, replaces previous)

Is this correct? Is there a way to write a custom filter for what I intend to do?

Thank you in advance!  
Davis

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### Author: ![pavel](https://discourse.paraview.org/user_avatar/discourse.paraview.org/pavel/32/8287_2.png) [@pavel](https://discourse.paraview.org/u/pavel)
#### Post date: [December 30, 2021, 6:49am UTC](https://discourse.paraview.org/t/how-is-filter-hierarchy-handled-in-custom-filter/8480/2 "2021-12-30T06:49:38Z")

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Hi Davis,

Could you please clarify your question? Do you have several files on disk? Do you want to gather all the data from them in one dataset (vtkMultiBlockDataSet)?  
In case you have several files on disk, you can write simple .txt or .ini file with all the file names and read this .txt or .ini file by custom reader pyhton plugin to avoid applying plugin for each file independently.

Best Regards,  
Pavel