Generating Kaprekar's constants in Python using map, reduce, and filter .... I know ...

Kaprekar.py

#!/usr/bin/python

#1. Take any four-digit number, using at least two different digits. (Leading zeros are allowed.)
#2. Arrange the digits in ascending and then in descending order to get two four-digit numbers, adding leading zeros if necessary.
#3. Subtract the smaller number from the bigger number.
#4. Go back to step 2.

#The above process, known as Kaprekar's routine, will always reach 6174 in at most 7 iterations.[4] Once 6174 is reached, 
#the process will continue yielding 7641 – 1467 = 6174. For example, choose 3524:
#5432 – 2345 = 3087
#8730 – 0378 = 8352
#8532 – 2358 = 6174

# This program emits the number of steps it takes to reach either 0 or 6174.

DIGITS = map(lambda x: map(lambda x: x, str(x).zfill(4)), range(10000))

def kaprekar(x, y=0):
  if int(''.join(x)) == 6174:
    print y
  elif int(''.join(x)) == 0:
    print y
  else:
    x = map(lambda x: x, str(int(''.join(sorted(x, reverse=True))) - int(''.join(sorted(x)))))
    y += 1
    kaprekar(x,y)


if __name__ == '__main__':
  
  for i in DIGITS:
    kaprekar(i)