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If the order of a, b, etc. matters:

sage: IntegerVectors(8, 5)
Integer vectors of length 5 that sum to 8
sage: list(IntegerVectors(8, 5))
[[8, 0, 0, 0, 0],
 [7, 1, 0, 0, 0],
 [7, 0, 1, 0, 0],
 [7, 0, 0, 1, 0],
 [7, 0, 0, 0, 1],
 [6, 2, 0, 0, 0],
 [6, 1, 1, 0, 0],
 [6, 1, 0, 1, 0],
 [6, 1, 0, 0, 1],
 ...

If you don't care about the order:

sage: Partitions(8, max_length=5)
Partitions of the integer 8 satisfying constraints max_length=5
sage: list(Partitions(8, max_length=5))
[[8],
 [7, 1],
 [6, 2],
 [6, 1, 1],
 [5, 3],
 [5, 2, 1],
 [5, 1, 1, 1],
 [4, 4],
 [4, 3, 1],
 [4, 2, 2],
 [4, 2, 1, 1],
 [4, 1, 1, 1, 1],
 [3, 3, 2],
 [3, 3, 1, 1],
 [3, 2, 2, 1],
 [3, 2, 1, 1, 1],
 [2, 2, 2, 2],
 [2, 2, 2, 1, 1]]