替代方法，与@jamylak的解决方案一样快，并且使用更多的pythonic：
from itertools import imap
max(imap(len, d))
查看比较：
$ python -m timeit -s "d = {'group 1': 1, 'group 1000': 0}" "len(max(d,key=len))"
1000000 loops, best of 3: 0.538 usec per loop
$ python -m timeit -s "d = {'group 1': 1, 'group 1000': 0}" "max(len(x) for x in d)"
1000000 loops, best of 3: 0.7 usec per loop
$ python -m timeit -s "d = {'group 1': 1, 'group 1000': 0}; from itertools import imap" \
"max(imap(len, d))"
1000000 loops, best of 3: 0.557 usec per loop
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