Check Prerequisite Commands Are Installed

Search an expanded PATH for each command name and return its full path, or raise when any name is missing.

import os
import shutil


class PrerequisiteCheckError(Exception):
    def __init__(self, errors: list[str]) -> None:
        super().__init__("\n".join(errors))
        self.errors = errors


def generate_expanded_path() -> str:
    current_path: str = os.environ.get("PATH", "")
    path_elements: list[str] = current_path.split(os.pathsep)
    expanded_path_elements: list[str] = [os.path.expanduser(path) for path in path_elements]
    return os.pathsep.join(expanded_path_elements)


def check_prerequisite(prerequisite_commands: list[str]) -> dict[str, str]:
    errors_verbose: list[str] = []
    command_paths: dict[str, str] = {}
    expanded_search_path: str = generate_expanded_path()

    for command in prerequisite_commands:
        full_path: str | None = shutil.which(command, path=expanded_search_path)
        if full_path is None:
            errors_verbose.append(f"{command} is not installed")
        else:
            command_paths[command] = full_path

    if errors_verbose:
        raise PrerequisiteCheckError(errors_verbose)

    return command_paths

Caveats

Raises PrerequisiteCheckError with one message per missing command. PATH entries that start with ~ are expanded before the search.