[vtk-developers] Speeding up `import vtk`

[Jean-Christophe Fillion-Robin]

Hi Ben,

Here is an updated version of the comment I originally posted on MR 3674
<https://gitlab.kitware.com/vtk/vtk/merge_requests/3674#note_355306>.


Short version:

Having the dependency graph of VTK kits available in python is needed to
generate cross platform distributable wheels.


Long version:

The introduction vtkmodules enables granular import.

In a install or build tree where the PATH, LD_LIBRARY_PATH, ... are set or
if RPATH is available, importing any specific module will work because the
dynamic library loader will find the correct dependencies and load them.
(e.g ParaView, Slicer, ...)

To support generation of distributable python wheels and avoid user to
compile VTK each time they want to use it, a given compiled python module
can NOT depends on shared libraries and expect the library loader to find
them unless:

* the dependent libraries are system ones
* the compiled python module explicitly loads them in order


What does this mean ?

* We need to support building any given kit as standalone kit without any
dependencies beside of system libraries and symbols from other kit
* We need to make sure dependent kit are explicitly imported within python
in the right order


Hence the need to have the dependency graph available from python.

There is also other challenges like:

* sharing symbol across python modules (e.g GlobalTimeStamp need to be the
same for all modules). To address this, we use Python Capsule C/API in ITK.


For these reasons, we need to have the dependency graph available.

And once, we have the dependency graph ... supporting lazy loading wouldn't
be too much work.


In the future, support for Native libraries in wheels would alleviate some
of these issues. In the mean time, it is a necessary evil:

*
https://github.com/jcfr/wheel-builders/blob/058287254a139908578501eb70b602e33202720a/pynativelib-proposal.rst
* https://github.com/pypa/wheel-builders/pull/2




On Fri, Dec 15, 2017 at 12:02 PM, Utkarsh Ayachit <
utkarsh.ayachit at kitware.com> wrote:

> >>   from vtkmodules import vtkSomeFactory
> >>   vtkSomeFactory.SetDefaultClass("vtkNameOfSomeClass")
> >
> > This would be `from vtkmodules.vtkSomeModule import vtkSomeFactory`. The
> > point of vtkmodules is that the top-level package *doesn't* import
> > everything.
>
> You can also:
> > from vtk import vtkSomeFactory
> >  vtkSomeFactory.SetDefaultClass("vtkNameOfSomeClass")
>
> Utkarsh
>



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