inp = [
    {'n': 'name1', 'a': '1'},
    {'n': 'name2', 'a': '1'},
    {'n': 'name3', 'a': '2'},
    {'n': 'name4', 'a': '2'},
    {'n': 'name5', 'a': '3'},
    {'n': 'name6', 'a': '4'},
    {'n': 'name7', 'a': ''},
    {'n': 'name8', 'a': ''},
]

vals = sorted(d['a'] for d in inp)
for i, val in enumerate(vals):
    inp[i]['a'] = val

print(inp)