def is_file_input(im: str) -> bool:
	""" Determine user's input mode.
	
	Determine whether the user should provide the input using a file or a shell.
	Raises a ValueError if input_mode format is not expected.

	Params:
		im: A one-character string. 'f' for file input, 's' for shell.
	
	Returns:
		A boolean which is True if user's input is provided using a file,
		False otherwise.
	"""
	if len(im) > 1 or not (im == 'f' or im == 's'):
		raise ValueError("Please provide only one among the following characters: \
			'f' for file, 's' for shell.")
	else:
		return False or im == 'f'