结合两个字典，连接字符串值？答案

【问题标题】：Combine two dictionaries, concatenate string values?结合两个字典，连接字符串值？
【发布时间】：2013-07-10 09:37:57
【问题描述】：

我想合并两个 string:string 字典，并连接这些值。上面的帖子推荐使用collections.Counter，但它不处理字符串连接。

>>> from collections import Counter
>>> a = Counter({'foo':'bar', 'baz':'bazbaz'})
>>> b = Counter({'foo':'baz'})
>>> a + b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/collections.py", line 569, in __add__
TypeError: cannot concatenate 'str' and 'int' objects

（我的猜测是 Counter 尝试将 b['baz'] 设置为 0。）

我想得到{'foo':'barbaz', 'baz':'bazbaz'} 的结果。串联顺序对我来说并不重要。什么是干净的 Pythonic 方式来做到这一点？

【问题讨论】：

如果第二个字典看起来像：{'foo':'baz','spam':'eggs'}?
@AshwiniChaudhary {'foo':'barbaz', 'baz':'bazbaz', 'spam':'eggs'}

标签： python string dictionary

【解决方案1】：

字典理解：

>>> d = {'foo': 'bar', 'baz': 'bazbaz'}
>>> d1 = {'foo': 'baz'}
>>> keys = d.viewkeys() | d1.viewkeys()
>>> {k : d.get(k, '') + d1.get(k, '') for k in keys}
{'foo': 'barbaz', 'baz': 'bazbaz'}

对于 Python 2.6 及更早版本：

>>> dict((k, d.get(k, '') + d1.get(k, '')) for k in keys)
{'foo': 'barbaz', 'baz': 'bazbaz'}

这适用于任意数量的字典：

def func(*dicts):
    keys = set().union(*dicts)
    return {k: "".join(dic.get(k, '') for dic in dicts)  for k in keys}
... 
>>> d = {'foo': 'bar', 'baz': 'bazbaz'}
>>> d1 = {'foo': 'baz','spam': 'eggs'}
>>> d2 = {'foo': 'foofoo', 'spam': 'bar'}
>>> func(d, d1, d2)
{'foo': 'barbazfoofoo', 'baz': 'bazbaz', 'spam': 'eggsbar'}

【讨论】：

【解决方案2】：

可以写一个通用的helper，比如：

a = {'foo':'bar', 'baz':'bazbaz'}
b = {'foo':'baz'}

def concatd(*dicts):
    if not dicts:
        return {} # or should this be None or an exception?
    fst = dicts[0]
    return {k: ''.join(d.get(k, '') for d in dicts) for k in fst}

print concatd(a, b)
# {'foo': 'barbaz', 'baz': 'bazbaz'}

c = {'foo': '**not more foo!**'}
print concatd(a, b, c)
# {'foo': 'barbaz**not more foo!**', 'baz': 'bazbaz'}

【讨论】：

【解决方案3】：

可以使用defaultdict 来实现：

from collections import defaultdict

a = {'foo': 'bar', 'baz': 'bazbaz'}
b = {'foo': 'baz'}

new_dict = defaultdict(str)
for key, value in a.items():
    new_dict[key] += value
for key, value in b.items():
    new_dict[key] += value

print(new_dict)
# defaultdict(<class 'str'>, {'foo': 'barbaz', 'baz': 'bazbaz'})
print(dict(new_dict))
# {'foo': 'barbaz', 'baz': 'bazbaz'}

如果要加入的字典很多，我们可以使用itertools.chain.from_iterable:

from collections import defaultdict
from itertools import chain

a = {'foo': 'bar', 'baz': 'bazbaz'}
b = {'foo': 'baz'}
c = {'baz': '123'}
dicts = [a, b, c]

new_dict = defaultdict(str)
for key, value in chain.from_iterable(map(dict.items, dicts)):
    new_dict[key] += value

print(dict(new_dict))
# {'foo': 'barbaz', 'baz': 'bazbaz123'}

【讨论】：