[petsc-dev] Problem pip installing petsc4py on existing PETSc installation

[Satish Balay]

On Thu, 28 Aug 2025, Samar Khatiwala via petsc-dev wrote:

> Hello,
> 
> I’m trying to install petsc4py with pip on a machine with an existing PETSc (3.21.1) (installed without pets4py). When I do:
> 
> pip install -v petsc4py==3.21.1
> 
> the installation fails. (If I don’t specify the version pip just downloads the latest version and the installation fails in a different way, but that’s not surprising.)
> 
> I’ve attached the build log.
> 
> If I don’t specify PETSC_DIR etc and just do pip install petsc4py then that works. But I would like to use the existing installed PETSc (yes, I know it is quite old but please bear with me!).

>>>
  Successfully installed cython-3.1.3 numpy-2.0.2 setuptools-80.9.0
<snip>
  using Cython 3.1.3
  cythonizing 'petsc4py/PETSc.pyx' -> 'petsc4py/PETSc.c'
  warning: petsc4py/PETSc/petscis.pxi:196:4: '__getsegcount__' relates to the old Python 2 buffer protocol and is no longer used.
  warning: petsc4py/PETSc/petscis.pxi:201:4: '__getreadbuffer__' relates to the old Python 2 buffer protocol and is no longer used.
  warning: petsc4py/PETSc/petscvec.pxi:585:4: '__getsegcount__' relates to the old Python 2 buffer protocol and is no longer used.
  warning: petsc4py/PETSc/petscvec.pxi:590:4: '__getreadbuffer__' relates to the old Python 2 buffer protocol and is no longer used.
  warning: petsc4py/PETSc/petscvec.pxi:595:4: '__getwritebuffer__' relates to the old Python 2 buffer protocol and is no longer used.

  Error compiling Cython file:
  ------------------------------------------------------------
  ...

      @classmethod
      def Print(
          cls,
          *args: Any,
          sep: str = ' ',
                     ^
  ------------------------------------------------------------

  petsc4py/PETSc/Sys.pyx:159:19: Compiler crash in ExpressionWriter


  Compiler crash traceback from this point on:
    File "Cython/Compiler/Visitor.py", line 174, in Cython.Compiler.Visitor.TreeVisitor._visit
    File "/private/var/folders/d3/w861tc0j755f19272xfkq56w0000gn/T/pip-install-1e_zq9u1/petsc4py_c977438422fb480b9f145e0099e7cf3e/conf/cyautodoc.py", line 28, in visit_UnicodeNode
      self.emit_string(node, '')
  AttributeError: 'ExpressionWriter' object has no attribute 'emit_string'
  error: Cython failure: 'petsc4py/PETSc.pyx' -> 'petsc4py/PETSc.c'
<<<

Perhaps you can try installing/using an older cython version - say 3.0.12 - and see if that works.

Satish