Cleanup error handling

shell.nix

@@ -1,28 +1,27 @@
-{pkgs ? import <nixpkgs> {}}:
-pkgs.mkShell {
-  packages = with pkgs; [
-    nodejs-16_x
-    nodePackages.yarn
-    (with dotnetCorePackages;
-      combinePackages [
-        sdk_6_0
-        aspnetcore_6_0
-      ])
-    python3
-    python3Packages.venvShellHook
-  ];
-
-  # Run this command, only after creating the virtual environment
+{pkgs ? import <nixpkgs> {}}: let
   venvDir = "./.venv";
-  postVenvCreation = ''
-    unset SOURCE_DATE_EPOCH
-    pip install -r ./scanner/requirements.txt
-  '';
+  pythonPkgs = ./scanner/requirements.txt;
+in
+  pkgs.mkShell {
+    packages = with pkgs; [
+      nodejs-16_x
+      nodePackages.yarn
+      (with dotnetCorePackages;
+        combinePackages [
+          sdk_6_0
+          aspnetcore_6_0
+        ])
+      python3
+    ];
 
-  # Now we can execute any commands within the virtual environment.
-  # This is optional and can be left out to run pip manually.
-  postShellHook = ''
-    # allow pip to install wheels
-    unset SOURCE_DATE_EPOCH
-  '';
-}
+    shellHook = ''
+      # Install python modules
+      SOURCE_DATE_EPOCH=$(date +%s)
+      if [ ! -d "${venvDir}" ]; then
+        ${pkgs.python3} -m venv "${venvDir}"
+      fi
+      source "${venvDir}/bin/activate"
+	  export PIP_DISABLE_PIP_VERSION_CHECK=1
+      pip install -r ${pythonPkgs}
+    '';
+  }