#!/usr/bin/python3

"""
Input/Output utilities.
"""

import sys


def readvalue(msg, type_, emsg=None, fin=sys.stdin, fout=sys.stdout):
	"""
	Basically uses ``input(msg)`` until it has been able to convert 
	the input to ``type_``. Prints the error message ``emsg``
	if supplied, else the type error message.

	Reads from ``fin`` and writes to ``fout``. They default to ``sys.stdin``
	and ``sys.stdout``.
	"""
	x = None
	while(x is None):
		print(msg, file=fout, flush=True, end="")
		x_raw = fin.readline()
		try:
			# Remove the trailing \n.
			x = type_(x_raw[:-1])
		except ValueError as e:
			if(emsg):
				print(emsg, file=fout, flush=True)
			else:
				print(str(e), file=fout, flush=True)

	return x