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doc/python: document fixed-point arguments (#271387) #477149

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doc/python: document fixed-point arguments (#271387)
doronbehar Jan 5, 2026
ad54fe3
python.section.md: List finalAttrs.passthru only attributes
doronbehar Jan 6, 2026
daf1d46
doc/python: explain better passthru note
doronbehar Jan 12, 2026
5ac2996
doc/python: simplify packageOverrides example
doronbehar Jan 12, 2026
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91 changes: 69 additions & 22 deletions doc/languages-frameworks/python.section.md
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Original file line number Diff line number Diff line change
Expand Up @@ -207,6 +207,61 @@ following are specific to `buildPythonPackage`:
* `setupPyGlobalFlags ? []`: List of flags passed to `setup.py` command.
* `setupPyBuildFlags ? []`: List of flags passed to `setup.py build_ext` command.

##### Using fixed-point arguments {#buildpythonpackage-fixed-point-arguments}

Both `buildPythonPackage` and `buildPythonApplication` support [fixed-point arguments](#chap-build-helpers-finalAttrs), similar to `stdenv.mkDerivation`.
This allows you to reference the final attributes of the derivation.

Instead of using `rec`:

```nix
buildPythonPackage rec {
pname = "pyspread";
version = "2.4";
src = fetchPypi {
inherit pname version;
hash = "sha256-...";
};
}
```

You can use the `finalAttrs` pattern:

```nix
buildPythonPackage (finalAttrs: {
pname = "pyspread";
version = "2.4";
src = fetchPypi {
pname = "pyspread";
inherit (finalAttrs) version;
Comment on lines +235 to +236
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@ambroisie ambroisie Jan 5, 2026

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I'd expect it to inherit pname as well, this is one of the few times I'd think using pname would be fine (as opposed to e.g: fetchFromGitHub's repo argument).

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@mweinelt mweinelt Jan 5, 2026

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It isn't really. We normalize pname (e.g. apply lowercasing), but the pypi name may not reflect that, so it is often untied.

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@Merikei Merikei Jan 11, 2026

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it is often untied

So this means while this documentation suggest untying it, tying pname is also accepted?

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@mweinelt mweinelt Jan 11, 2026

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Yeah, it might be stable, if the library name already is normalized, but uppercase, dashes, etc. will cause flapping every once in a while.

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@MattSturgeon MattSturgeon Jan 12, 2026 •
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Even if you do reuse the name for pname and src.pname, it's best to use a locally defined variable rather than finalAttrs.pname. Values in finalAttrs represent the final state, including any user defined overrides.

pname is an example of something an end-user may wish to override, e.g. pname = prevAttrs.pname + "-with-my-tweaks". Such an override shouldn't propagate to the src's project.

hash = "sha256-...";
};
})
```

See the [general documentation on fixed-point arguments](#chap-build-helpers-finalAttrs) for more details on the benefits of this pattern.

::: {.note}

Some `buildPythonPackage`/`buildPythonApplication` arguments are passed down indirectly to `stdenv.mkDerivation` via `passthru`.
Therefore the final state of these attributes can be accessed via `finalAttrs.passthru.${name}`.
[`<pkg>.overrideAttrs`](#sec-pkg-overrideAttrs) can override them using the `passthru = prevAttrs.passthru // { foo = "bar"; }` pattern.
Such arguments include:

- `disabled`
- `pyproject`
- `format`
- `build-system`, `dependencies` and `optional-dependencies`

<!--
TODO(@doronbehar): When [#258246][1] will be resolved, or when
`.overridePythonAttrs` will be removed, the above text might need to be revised.

- [1]: https://github.com/NixOS/nixpkgs/issues/258246

-->
:::

The [`stdenv.mkDerivation`](#sec-using-stdenv) function accepts various parameters for describing
build inputs (see "Specifying dependencies"). The following are of special
interest for Python packages, either because these are primarily used, or
Expand Down Expand Up @@ -237,29 +292,21 @@ the overrides for packages in the package set.
```nix
with import <nixpkgs> { };

(
let
python =
let
packageOverrides = self: super: {
pandas = super.pandas.overridePythonAttrs (old: rec {
version = "0.19.1";
src = fetchPypi {
pname = "pandas";
inherit version;
hash = "sha256-JQn+rtpy/OA2deLszSKEuxyttqBzcAil50H+JDHUdCE=";
};
});
let
python = pkgs.python3.override {
packageOverrides = self: super: {
pandas = super.pandas.overridePythonAttrs (old: {
version = "0.19.1";
src = fetchPypi {
pname = "pandas";
inherit version;
hash = "sha256-JQn+rtpy/OA2deLszSKEuxyttqBzcAil50H+JDHUdCE=";
};
in
pkgs.python3.override {
inherit packageOverrides;
self = python;
};

in
python.withPackages (ps: [ ps.blaze ])
).env
});
};
};
in
(python.withPackages (ps: [ ps.blaze ])).env
```

The next example shows a non trivial overriding of the `blas` implementation to
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3 changes: 3 additions & 0 deletions doc/redirects.json
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Expand Up @@ -3845,6 +3845,9 @@
"buildpythonpackage-parameters": [
"index.html#buildpythonpackage-parameters"
],
"buildpythonpackage-fixed-point-arguments": [
"index.html#buildpythonpackage-fixed-point-arguments"
],
"overriding-python-build-helpers": [
"index.html#overriding-python-build-helpers"
],
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