With 1 change your code runs well:
#!/usr/bin/python3 def lcm(a,b): m = a*b while a != 0 and b != 0: if a > b: a %= b else: b %= a return m // (a+b) with open('2.txt') as f: a, b = [int(x) for x in next(f).split(',')] print("a=%d ; b=%d ; LCM=%d"%(a,b,lcm(a, b)))