In [361]: l = list(range(20))

In [362]: l
Out[362]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

In [363]: l[:] = l[:1] + l[m:m+n] + [x for i,x in enumerate(l[1:]) if not m <= i <= m+n]

In [364]: l
Out[364]: [0, 5, 6, 7, 8, 9, 10, 11, 1, 2, 3, 4, 5, 14, 15, 16, 17, 18, 19]