def gcd(m, n):
    """Greatest Common Divisor (GCD) by Euclid's Algorithm.

    Time complexity: O(m%n).
    """
    while n != 0:
        m, n = n, m % n
    return m


def main():
    print(gcd(4, 2))
    print(gcd(2, 4))

    print(gcd(10, 4))
    print(gcd(4, 10))

    print(gcd(10, 1))
    print(gcd(1, 10))


if __name__ == '__main__':
    main()

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function


def gcd(m, n):
    """Greatest Common Divisor (GCD) by Euclid's Algorithm.

    Time complexity: O(m%n).
    """
    while n != 0:
        m, n = n, m % n
    return m


def main():
    print('gcd(4, 2): {}'.format(gcd(4, 2)))
    print('gcd(2, 4): {}'.format(gcd(2, 4)))

    print('gcd(10, 4): {}'.format(gcd(10, 4)))
    print('gcd(4, 10): {}'.format(gcd(4, 10)))

    print('gcd(3, 4): {}'.format(gcd(3, 4)))
    print('gcd(4, 3): {}'.format(gcd(4, 3)))


if __name__ == '__main__':
    main()

Add importing from __future__ & revise main()
Add importing from __future__ & revise main()

+ from __future__ import absolute_import
+ from __future__ import division
+ from __future__ import print_function
+ 
+ 
  def gcd(m, n):
      """Greatest Common Divisor (GCD) by Euclid's Algorithm.
  
      Time complexity: O(m%n).
      """
      while n != 0:
          m, n = n, m % n
      return m
  
  
  def main():
-     print(gcd(4, 2))
-     print(gcd(2, 4))
+     print('gcd(4, 2): {}'.format(gcd(4, 2)))
+     print('gcd(2, 4): {}'.format(gcd(2, 4)))
  
-     print(gcd(10, 4))
-     print(gcd(4, 10))
+     print('gcd(10, 4): {}'.format(gcd(10, 4)))
+     print('gcd(4, 10): {}'.format(gcd(4, 10)))
  
-     print(gcd(10, 1))
-     print(gcd(1, 10))
+     print('gcd(3, 4): {}'.format(gcd(3, 4)))
+     print('gcd(4, 3): {}'.format(gcd(4, 3)))
  
  
  if __name__ == '__main__':
      main()
  
Add importing from __future__ & revise main()
## Code Before:
def gcd(m, n):
    """Greatest Common Divisor (GCD) by Euclid's Algorithm.

    Time complexity: O(m%n).
    """
    while n != 0:
        m, n = n, m % n
    return m


def main():
    print(gcd(4, 2))
    print(gcd(2, 4))

    print(gcd(10, 4))
    print(gcd(4, 10))

    print(gcd(10, 1))
    print(gcd(1, 10))


if __name__ == '__main__':
    main()

## Instruction:
Add importing from __future__ & revise main()
## Code After:
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function


def gcd(m, n):
    """Greatest Common Divisor (GCD) by Euclid's Algorithm.

    Time complexity: O(m%n).
    """
    while n != 0:
        m, n = n, m % n
    return m


def main():
    print('gcd(4, 2): {}'.format(gcd(4, 2)))
    print('gcd(2, 4): {}'.format(gcd(2, 4)))

    print('gcd(10, 4): {}'.format(gcd(10, 4)))
    print('gcd(4, 10): {}'.format(gcd(4, 10)))

    print('gcd(3, 4): {}'.format(gcd(3, 4)))
    print('gcd(4, 3): {}'.format(gcd(4, 3)))


if __name__ == '__main__':
    main()
