python list comprehension基于字母的返回名称

Question

我有DataFrame：

df = pd.DataFrame(['A','B','C'], columns = ['Letters'])

我有一个names的列表：

names = ['George All', 'George Ball','George Ago','George Call']

如何在DataFrame中创建一个新列，其中包含姓氏以Letters列开头的名称列表。

例如：

Letters  Names
A        ['George All','George Ago']
B        George Ball
C        George Call

这就是我现在所拥有的：

df['Names'] = [name for name in names if (name.split()[1][0] == df['Letters'])]

Answer 1

>>> df['Names'] = [[n for n in names if n.split()[1][0] == x] for x in df['Letters']]
>>> df
  Letters                     Names
0       A  [George All, George Ago]
1       B             [George Ball]
2       C             [George Call]

[3 rows x 2 columns]

您可以通过事先将所有名称按首字母分组来更有效地执行此操作（如果names是大型列表，则很重要）。

>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> for item in names:
...     d[item.split()[1][0]].append(item)
... 
>>> df['Names'] = [d[x] for x in df['Letters']]
>>> df
  Letters                     Names
0       A  [George All, George Ago]
1       B             [George Ball]
2       C             [George Call]