Update python.md (#23669)

* Update python.md this makes it clear how to alter `attributes` by using `packageOverrides` * Update python.md * Update python.md * Update python.md * Update python.md * Update python.md * Update python.md
NixOS · Mar 20, 2017 · 91debcb · 91debcb
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diff --git a/doc/languages-frameworks/python.md b/doc/languages-frameworks/python.md
@@ -897,6 +897,27 @@ is executed it will attempt to download the python modules listed in
 requirements.txt. However these will be cached locally within the `virtualenv`
 folder and not downloaded again.
 
+### How to override a Python package from `configuration.nix`?
+
+If you need to change a package's attribute(s) from `configuration.nix` you could do:
+
+```nix
+  nixpkgs.config.packageOverrides = superP: {
+    pythonPackages = superP.pythonPackages.override {
+      overrides = self: super: {
+        bepasty-server = super.bepasty-server.overrideAttrs ( oldAttrs: {
+          src = pkgs.fetchgit {
+            url = "https://github.com/bepasty/bepasty-server";
+            sha256 = "9ziqshmsf0rjvdhhca55sm0x8jz76fsf2q4rwh4m6lpcf8wr0nps";
+            rev = "e2516e8cf4f2afb5185337073607eb9e84a61d2d";
+          };
+        });
+      };
+    };
+  };
+```
+
+If you are using the `bepasty-server` package somewhere, for example in `systemPackages` or indirectly from `services.bepasty`, then a `nixos-rebuild switch` will rebuild the system but with the `bepasty-server` package using a different `src` attribute. This way one can modify `python` based software/libraries easily. Using `self` and `super` one can also alter dependencies (`buildInputs`) between the old state (`self`) and new state (`super`). 
 
 ## Contributing