This allows one to always override the call to `buildPythonPackage`.
In the following example we create an environment where we have the `blaze` package using an older version of `pandas`. We override first the Python interpreter and pass `packageOverrides` which contains the overrides for packages in
the package set.
```
with import <nixpkgs> {};
(let
python = let
packageOverrides = self: super: {
pandas = super.pandas.overridePythonPackage(old: rec {
version = "0.19.1";
name = "pandas-${version}";
src = super.fetchPypi {
pname = "pandas";
inherit version;
sha256 = "08blshqj9zj1wyjhhw3kl2vas75vhhicvv72flvf1z3jvapgw295";
};
});
};
in pkgs.python3.override {inherit packageOverrides;};
in python.withPackages(ps: [ps.blaze])).env
```
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