python - 计算 python 字典的特定键中值的数量

Question

有人可以向我解释一下，和/或指导我以正确的/pythonic 方式做这件事吗？

Python 2.7。

最终，我试图遍历字典 countsD:

countsD = {"aa": None, "bb": None, "cc": None, "dd": None} 

并在相应的字典 d 中进行匹配:

d = {"aa": (5689, 34, 44, 77, 88, 321), "bb": (33, 6742, 89744), "cc": (45, 98), "dd": (1, 33)}

将项目的计数作为值添加到相应的匹配键中，最终创建这个countsD

{"aa": 6, "bb": 3, "cc": 2, "dd": 2}

如果我用上面的方法来做

> d = {"aa": (5689, 34, 44, 77, 88, 321), "bb": (33, 6742, 89744), "cc": (45, 98), "dd": (1, 33)}

> for key in d:

>>     print(key)
>>     print(len(d[key]))

返回的是这个，这就是我想要的

aa
6
cc
2
dd
2
bb
3

但是，如果某个键的值之一仅包含 1 个值(完全可能)，例如(参见“cc”):

d = {"aa": (5689, 34, 44, 77, 88, 321), "bb": (33, 6742, 89744), "cc": (45), "dd": (1, 33)}

然后运行相同的 for 循环，我在“cc”键上得到一个错误:

aa
6
cc
Traceback (most recent call last):
  File "<interactive input>", line 3, in <module>
TypeError: object of type 'int' has no len()

然而，如果我让那个“cc”键有一个空值()，那么一切都很好。

d = {"aa": (5689, 34, 44, 77, 88, 321), "bb": (33, 6742, 89744), "cc": (), "dd": (1, 33)}

>>> d = {"aa": (5689, 34, 44, 77, 88, 321), "bb": (33, 6742, 89744), "cc": (), "dd": (1, 33)}
>>> for key in d:
...     print(key)
...     print(len(d[key]))
... 
aa
6
cc
0
dd
2
bb
3

刚才在输入这篇文章的标题时，我提到了 Count number of values in dictionary for each key一个答案。太棒了，一行!但是同样，对于只有一个值的键，它会失败。这个好:

>>> d = {"aa": (5689, 34, 44, 77, 88, 321), "bb": (33, 6742, 89744), "cc": (), "dd": (1, 33)}
>>> new_countsD = {k: len(v) for k,v in d.items()}
>>> new_countsD
{'aa': 6, 'bb': 3, 'cc': 0, 'dd': 2}

这不是，看键“cc”

>>> d = {"aa": (5689, 34, 44, 77, 88, 321), "bb": (33, 6742, 89744), "cc": (111), "dd": (1, 33)}
>>> new_countsD = {k: len(v) for k,v in d.items()}
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
  File "<interactive input>", line 1, in <dictcomp>
TypeError: object of type 'int' has no len()

那么，是什么给了？？我觉得我错过了一些愚蠢的东西......

感谢您的帮助!

Answer 1

在python中，一个单项元组(称为单例)写成(value,)，所以你输入的(111)应该写成(111,) 代替；否则它将被视为 111 的整数。

来自 tuple's documentation :

A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective.

>>> empty = ()
>>> singleton = 'hello',    # <-- note trailing comma
>>> len(empty)
0
>>> len(singleton)
1
>>> singleton
('hello',)