Solution to the key problem

key_solver.py

#!/usr/bin/env python

"""
Problem statement: Find the three digit number
147 - One digit is right but in the wrong place
189 - One digit is right and in its place
964 - Two digits are correct but both are in the wrong place
523 - All digits are wrong
286 - One digit is right but in the wrong place
"""

def n(a,i): a = a[:]; del a[i]; return a # Exclude element at I from A
def q(a,b): return sum([ a[i] in n(b,i) for i,_ in enumerate(a) ]) # Total right but wrong   place
def p(a,b): return sum([ a[i] == b[i]   for i,_ in enumerate(a) ]) # Total right and correct place
def r(a,b,n): return q(a,b) == n and p(a,b) == 0 # N right but wrong   place && 0 right and correct place
def s(a,b,n): return p(a,b) == n and q(a,b) == 0 # N right and correct place && 0 right but wrong place

for i in range(10): 
    for j in range(10): 
        for k in range(10): 
            if (r([i,j,k],[1,4,7], 1) and
                s([i,j,k],[1,8,9], 1) and
                r([i,j,k],[9,6,4], 2) and
                s([i,j,k],[5,2,3], 0) and
                r([i,j,k],[2,8,6], 1)): print(i,j,k)