which.py

"""
Python implementation of the `which` program. (Searches $PATH for the first file with a given name)
"""

from typing import List, Iterator

from os import environ as env
from operator import methodcaller

import os


class Dir:
    def __init__(self, path):
        self.path = path
        self.contents = os.listdir(path)

    def __contains__(self, file_name):
        return file_name in self.contents

    def join(self, path):
        return os.path.join(self.path, path)


class ProgramNotFoundError(ValueError):
    """ Raised when program could not be found in path """


def which(prog_name) -> str:
    path = env['PATH']
    paths = path.split(':')
    valid_paths = (
        path for path in paths
        if os.path.isdir(path)
    )
    dirs = (Dir(path) for path in valid_paths)
    dirs_containing_prog = (
        directory for directory in dirs
        if prog_name in directory
    )
    matches = (dir.join(prog_name) for dir in dirs_containing_prog)

    try:
        return next(matches)
    except StopIteration:
        raise ProgramNotFoundError(f'no {prog_name} in {path}')