Bonjour, 
I begin in wonderful world of Nix! 
I’m happy to create and use a flake that I use without problem. But, now, I want to integrate a simple thing. Create a python venv with shell commands. But I’m stuck…
It’s part of my flake that works:
{
description = "Fred Darwin system flake";
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
nix-darwin.url = "github:LnL7/nix-darwin";
nix-darwin.inputs.nixpkgs.follows = "nixpkgs";
nix-homebrew.url = "github:zhaofengli-wip/nix-homebrew";
flake-utils.url = "github:numtide/flake-utils";
};
outputs = inputs@{ self, nix-darwin, nixpkgs, nix-homebrew, flake-utils }:
let
configuration = { pkgs, config, ... }: {
nixpkgs.config.allowUnfree = true;
environment.systemPackages =
[
pkgs.audacity
[..]
pkgs.vim
];
};
onActivation.autoUpdate = true;
onActivation.upgrade = true;
};
system.activationScripts.applications.text = let
env = pkgs.buildEnv {
name = "system-applications";
paths = config.environment.systemPackages;
pathsToLink = "/Applications";
};
in
pkgs.lib.mkForce ''
# Set up applications.
echo "setting up /Applications..." >&2
rm -rf /Applications/Nix\ Apps
mkdir -p /Applications/Nix\ Apps
find ${env}/Applications -maxdepth 1 -type l -exec readlink '{}' + |
while read src; do
app_name=$(basename "$src")
echo "copying $src" >&2
${pkgs.mkalias}/bin/mkalias "$src" "/Applications/Nix Apps/$app_name"
done
'';
system.defaults = {
dock.autohide = true;
dock.orientation = "bottom";
dock.magnification = false;
dock.mineffect = "genie";
dock.persistent-apps = [
"/System/Applications/Launchpad.app"
"/Applications/Firefox.app"
[...]
"/System/Applications/Utilities/Screenshot.app"
];
dock.tilesize = 30;
finder.FXPreferredViewStyle = "clmv";
finder.AppleShowAllExtensions = true;
[...]
trackpad.TrackpadThreeFingerTapGesture = 2;
};
# Auto upgrade nix package and the daemon service.
services.nix-daemon.enable = true;
# nix.package = pkgs.nix;
# Necessary for using flakes on this system.
nix.settings.experimental-features = "nix-command flakes";
# Create /etc/zshrc that loads the nix-darwin environment.
programs.zsh.enable = true; # default shell on catalina
# programs.fish.enable = true;
# Set Git commit hash for darwin-version.
system.configurationRevision = self.rev or self.dirtyRev or null;
# Used for backwards compatibility, please read the changelog before changing.
# $ darwin-rebuild changelog
system.stateVersion = 5;
# The platform the configuration will be used on.
nixpkgs.hostPlatform = "aarch64-darwin";
};
in
{
darwinConfigurations."Macbook-Fred" = nix-darwin.lib.darwinSystem {
modules = [
configuration
nix-homebrew.darwinModules.nix-homebrew
{
nix-homebrew = {
# Install Homebrew under the default prefix
enable = true;
# Apple Silicon Only: Also install Homebrew under the default Intel prefix for Rosetta 2
enableRosetta = true;
# User owning the Homebrew prefix
user = "fred";
# Automatically migrate existing Homebrew installations
autoMigrate = true;
};
}
];
};
# Expose the package set, including overlays, for convenience.
darwinPackages = self.darwinConfigurations."Macbook-Fred".pkgs;
};
}
And I would like to integrate, something simple, create a venv with shell command:
flake-utils.lib.eachDefaultSystem (system: let
pkgs = import nixpkgs {inherit system;};
in {
devShell = pkgs.mkShell {
buildInputs = with pkgs; [
python311
python311Packages.virtualenv
];
shellHook = ''
python3 -m venv /Users/fred/venv/default
'';
};
});
I tried to put in many locations in flake.
Before:
flake-utils.lib.eachDefaultSystem (system: let
pkgs = import nixpkgs {inherit system;};
in {
devShell = pkgs.mkShell {
buildInputs = with pkgs; [
python311
python311Packages.virtualenv
];
shellHook = ''
python3 -m venv /Users/fred/venv/default
'';
};
});
system.activationScripts.applications.text = let
env = pkgs.buildEnv {
name = "system-applications";
paths = config.environment.systemPackages;
pathsToLink = "/Applications";
[...]
But I get this error:
darwin-rebuild switch --flake .#Macbook-Fred
error: syntax error, unexpected '(', expecting '.' or '='
at /nix/store/76452zsv9iv89hl07aqinbd2l9ghrpml-source/flake.nix:106:41:
105|
106| flake-utils.lib.eachDefaultSystem (system: let
| ^
107| pkgs = import nixpkgs {inherit system;};
I tried:
flake-utils.lib.eachDefaultSystem = let
pkgs = import nixpkgs ;
in {
devShell = pkgs.mkShell {
buildInputs = with pkgs; [
python311
python311Packages.virtualenv
];
shellHook = ''
python3 -m venv /Users/fred/venv/default
'';
};
};
Error:
darwin-rebuild switch --flake .#Macbook-Fred
building the system configuration...
error:
… while evaluating the attribute 'config.system.build.toplevel'
at /nix/store/69bhrhj8imx84l6zvdz0zmh3qc8ykwvd-source/lib/modules.nix:334:9:
333| options = checked options;
334| config = checked (removeAttrs config [ "_module" ]);
| ^
335| _module = checked (config._module);
… while calling the 'seq' builtin
at /nix/store/69bhrhj8imx84l6zvdz0zmh3qc8ykwvd-source/lib/modules.nix:334:18:
333| options = checked options;
334| config = checked (removeAttrs config [ "_module" ]);
| ^
335| _module = checked (config._module);
… while evaluating the error message for definitions for `flake-utils', which is an option that does not exist
… while evaluating a definition from `<unknown-file>'
… while evaluating an attribute `lib`
… while evaluating an attribute `eachDefaultSystem`
… while evaluating an attribute `devShell`
(stack trace truncated; use '--show-trace' to show the full, detailed trace)
error: expected a set but found a function: «lambda @ /nix/store/69bhrhj8imx84l6zvdz0zmh3qc8ykwvd-source/pkgs/top-level/impure.nix:14:1»
I’m sorry… I don’t understand… 
Help ! 