docs: def find_executable(filename, environ=None):
      """Find an executable by searching the user's $PATH."""
      if environ is None:
          environ = os.environ
      path = environ.get("PATH", "/usr/local/bin:/usr/bin:/bin").split(":")
      for dirpath in path:
          dirpath = os.path.abspath(dirpath.strip())
          filepath = os.path.normpath(os.path.join(dirpath, filename))
          if os.path.exists(filepath):
              return filepath
      return None