FAQ: Clarify poetry2nix does not use nixpkgs for Python packages

[asymmetric]

I'm sure this could be expanded upon, and maybe this PR can provide the impetus for that.

[takeda]

This is not strictly true. Many packages that aren't explicitly defined in the lock file are actually obtained from nixpkgs.

This mostly applies to overrides, things like when package needs setuptools, flit, poetry, hatch and of course non python dependencies like openssl etc.

[asymmetric]

Interesting, I wrote this up after a comment by @adisbladis, but I might've interpreted it too broadly.

But in the end, this was my goal with this PR, to help spell out the architecture so we can document it somewhere visible.

Many packages that aren't explicitly defined in the lock file are actually obtained from nixpkgs.

@takeda can you point to locations where this is handled?

[takeda]

I see. The context matters. The question was about package (pillow) listed as a dependency to author's application. In that context pillow defined in poetry is used after applying overrides. The nixpkgs version is totally tossed away.

But then to install pillow poetry2nix adds setuptools dependency:

poetry2nix/overrides/build-systems.json

Lines 11369 to 11371 in a98ecb4

	"pillow": [
	"setuptools"
	],

That dependency is not defined in poetry.lock file and is pulled from nixpkgs.

You might think "oh this is a special case and it only happens at build time", but then there are broken packages like ansible, where all its dependencies are pulled from nixpkgs because it doesn't define any:

poetry2nix/overrides/default.nix

Lines 111 to 129 in a98ecb4

	ansible = super.ansible.overridePythonAttrs (
	old: {
	# Inputs copied from nixpkgs as ansible doesn't specify it's dependencies
	# in a correct manner.
	propagatedBuildInputs = (old.propagatedBuildInputs or [ ]) ++ [
	self.pycrypto
	self.paramiko
	self.jinja2
	self.pyyaml
	self.httplib2
	self.six
	self.netaddr
	self.dnspython
	self.jmespath
	self.dopy
	self.ncclient
	];
	}
	);

So the answer to this question is much more complex.

[asymmetric]

So a better answer is that everything declared in a poetry.lock is built without relying on nixpkgs definitions, with the exception of under-specified packages, like e.g. ansible, or build backends, as specified in build-systems.json.

Is this accurate?

[takeda]

The thing is that what is clear to me often is not clear to others, but this is what I would write:

poetry2nix overlays packages taken from poetry.lock on top of nixpkgs (in such way that overwritten packages in nixpkgs are completely ignored). Any package that is used, that isn't in the poetry.lock file (most commonly build dependencies) is taken from nixpkgs.