参见英文答案 > itertools.groupby() not grouping correctly                                    3个
我已经检查了一些关于groupby()的主题,但我没有得到我的例子的错误：

students = [{'name': 'Paul',    'mail': '@gmail.com'},
            {'name': 'Tom',     'mail': '@yahoo.com'},
            {'name': 'Jim',     'mail': 'gmail.com'},
            {'name': 'Jules',   'mail': '@something.com'},
            {'name': 'Gregory', 'mail': '@gmail.com'},
            {'name': 'Kathrin', 'mail': '@something.com'}]

key_func = lambda student: student['mail']

for key, group in itertools.groupby(students, key=key_func):
    print(key)
    print(list(group))

这将分别打印每个学生.为什么我不能只获得3组：@ gmail.com,@ yahoo.com和@ something.com？

解决方法:

对于初学者来说,一些邮件是gmail.com,有些是@ gmail.com,这就是为什么他们被视为单独的组.

groupby还希望数据能够通过相同的键功能进行预先排序,这就解释了为什么你会两次获得@ something.com.

从docs：

… Generally, the iterable needs to already be sorted on the same key function. …

students = [{'name': 'Paul', 'mail': '@gmail.com'}, {'name': 'Tom', 'mail': '@yahoo.com'},
            {'name': 'Jim', 'mail': 'gmail.com'}, {'name': 'Jules', 'mail': '@something.com'},
            {'name': 'Gregory', 'mail': '@gmail.com'}, {'name': 'Kathrin', 'mail': '@something.com'}]

key_func = lambda student: student['mail']

students.sort(key=key_func)
# sorting by same key function we later use with groupby

for key, group in itertools.groupby(students, key=key_func):
    print(key)
    print(list(group))

#  @gmail.com
#  [{'name': 'Paul', 'mail': '@gmail.com'}, {'name': 'Gregory', 'mail': '@gmail.com'}]
#  @something.com
#  [{'name': 'Jules', 'mail': '@something.com'}, {'name': 'Kathrin', 'mail': '@something.com'}]
#  @yahoo.com
#  [{'name': 'Tom', 'mail': '@yahoo.com'}]
#  gmail.com
#  [{'name': 'Jim', 'mail': 'gmail.com'}]

修复了sort和gmail.com/@gmail.com后,我们得到了预期的输出：

import itertools

students = [{'name': 'Paul', 'mail': '@gmail.com'}, {'name': 'Tom', 'mail': '@yahoo.com'},
            {'name': 'Jim', 'mail': '@gmail.com'}, {'name': 'Jules', 'mail': '@something.com'},
            {'name': 'Gregory', 'mail': '@gmail.com'}, {'name': 'Kathrin', 'mail': '@something.com'}]

key_func = lambda student: student['mail']

students.sort(key=key_func)

for key, group in itertools.groupby(students, key=key_func):
    print(key)
    print(list(group))

#  @gmail.com
#  [{'mail': '@gmail.com', 'name': 'Paul'},
#   {'mail': '@gmail.com', 'name': 'Jim'},
#   {'mail': '@gmail.com', 'name': 'Gregory'}]
#  @something.com
#  [{'mail': '@something.com', 'name': 'Jules'},
#   {'mail': '@something.com', 'name': 'Kathrin'}]
#  @yahoo.com
#  [{'mail': '@yahoo.com', 'name': 'Tom'}]