Using a recursive approach -

from random import randint

def sequence(n):
   
   a = []
   
   def f(n):
      m = randint(1, n)
      if n > m:
         a.append(str(m))
         return f(n - m)
      else:
         a.append(str(n))
         return n

   f(n)
   return ' + '.join(a) + ' = %d' % n

Test code:

for i in xrange(10):
   print sequence(20)

Output:

7 + 11 + 2 = 20
14 + 4 + 1 + 1 = 20
5 + 10 + 3 + 2 = 20
1 + 17 + 1 + 1 = 20
1 + 5 + 13 + 1 = 20
3 + 12 + 2 + 3 = 20
17 + 2 + 1 = 20
18 + 2 = 20
3 + 12 + 3 + 2 = 20
18 + 1 + 1 = 20

Using iterative approach -

from random import randint

def sequence(n):

   a, m, c = [], randint(1, n), n   
   while n > m > 0:
      a.append(str(m))
      n -= m
      m = randint(0, n)
   if n: a += [str(n)]
   return ' + '.join(a) + ' = %d' % c

Test code:

for i in xrange(10):
   print sequence(20)

Ouput:

19 + 1 = 20
2 + 15 + 3 = 20
2 + 3 + 12 + 3 = 20
19 + 1 = 20
2 + 10 + 2 + 6 = 20
6 + 3 + 9 + 1 + 1 = 20
14 + 2 + 3 + 1 = 20
3 + 7 + 4 + 4 + 2 = 20
9 + 7 + 3 + 1 = 20
3 + 17 = 20