But basically you want something like

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable-small";
    flake-utils.url = "github:numtide/flake-utils";
    poetry2nix = {
      url = "github:nix-community/poetry2nix";
      inputs.nixpkgs.follows = "nixpkgs";
      inputs.flake-utils.follows = "flake-utils";
    };
  };

  outputs = { self, nixpkgs, flake-utils, poetry2nix }:
    flake-utils.lib.eachDefaultSystem
      (system:
        let
          pkgs = import nixpkgs {
            overlays = [ poetry2nix.overlay ];
            inherit system;
          };
        in
        {
          packages.your-app = pkgs.poetry2nix.mkPoetryApplication {
               # stuff
          };
        }
      )
}

also expression in lambda passed to eachDefaultSystem contains this binding

  23           inherit system;
  24           overlays = [ self.overlay ];
  25         };

Does it import nixpkgs with revision locked by flake, or just version defined as channel?

also expression in lambda passed to eachDefaultSystem contains this binding

  22         pkgs = import nixpkgs {
  23           inherit system;
  24           overlays = [ self.overlay ];
  25         };

Does it import nixpkgs with revision locked by flake, or just version defined as channel?

looks like i almost get my first working flake with dev environment but ended up with code duplication to solve external library build dependency on poetry. And i can not think of good solution to extract duplication. Maybe anybody can suggest good idiomatic solution?

  description = "Exyaru (Atushka) project"; inputs.flake-utils.url = "github:numtide/flake-utils";
  inputs.nixpkgs.url = "github:NixOS/nixpkgs";
  inputs.poetry2nix.url = "github:nix-community/poetry2nix";

  outputs = { self, nixpkgs, flake-utils, poetry2nix }: {
      # Nixpkgs overlay providing the application
      overlay = nixpkgs.lib.composeManyExtensions [ poetry2nix.overlay
        (final: prev: {
          # The application
          exyaru = prev.poetry2nix.mkPoetryApplication {
            projectDir = ./.;
            overrides = prev.poetry2nix.defaultPoetryOverrides.extend ( self: super: {
              uengine = super.uengine.overridePythonAttrs (
                old: {
                  buildInputs = (old.buildInputs or [ ]) ++ [ self.poetry ];
                }
              );
            });
          };
        })
      ];
    } // (flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs {
          inherit system;
          overlays = [ self.overlay ];
        };
      in
      rec {
        apps = {
          exyaru = pkgs.exyaru;
        };

        defaultApp = pkgs.exyaru;

        packages = {
          exyaru = pkgs.exyaru;
        };

        defaultPackage = pkgs.exyaru;

        devShell = (pkgs.poetry2nix.mkPoetryEnv {
          projectDir = ./.;
          overrides = pkgs.poetry2nix.defaultPoetryOverrides.extend ( self: super: {
            uengine = super.uengine.overridePythonAttrs (
              old: {
                buildInputs = (old.buildInputs or [ ]) ++ [ self.poetry ];
              }
            );
          });
          editablePackageSources = {
            exyaru = ./.;
          };
        }).env.overrideAttrs (oldAttrs: {
          buildInputs = [ pkgs.poetry ];
        });
      }));
}