doesnt_work.nix

let
  nixpkgsSHA = "4fed373652870addb0b0cf809899bd0d49cfaf59"; # 2020-07-18
  pkgs = import (fetchTarball
    "https://github.com/tbenst/nixpkgs/archive/${nixpkgsSHA}.tar.gz") {
      system = builtins.currentSystem;
      overlays = [ (self: super: {
        hdf5 = super.hdf5.override { mpi = super.mpich; };
        pythonOverrides = python-self: python-super: {
          mpi4py = python-super.mpi4py.override {
            mpi = super.mpich;
          };
        };
        python3 = super.python3.override {
          packageOverrides = self.pythonOverrides;
        };
      }) ];
      
    };
in
pkgs.stdenv.mkDerivation {
    name = "circus";
    buildInputs = [
      pkgs.pythonPackages.h5py
    ];
}

works.nix

let
  nixpkgsSHA = "4fed373652870addb0b0cf809899bd0d49cfaf59"; # 2020-07-18
  pkgs = import (fetchTarball
    "https://github.com/tbenst/nixpkgs/archive/${nixpkgsSHA}.tar.gz") {
      system = builtins.currentSystem;
      overlays = [ (self: super:   {
        hdf5 = super.hdf5.override { mpi = super.openmpi; };
      }) ];
      
  };
in
pkgs.stdenv.mkDerivation {
    name = "circus";
    buildInputs = [
      pkgs.pythonPackages.h5py
    ];
}