如何将元组元组转换为一行列表(pythonic)？

Question

query = 'select mydata from mytable'
cursor.execute(query)
myoutput = cursor.fetchall() 
print myoutput

(('aa',), ('bb',), ('cc',))

为什么它(cursor.fetchall)返回一个元组元组而不是一个元组,因为我的查询只询问一列数据？

将其转换为最佳方式是什么['aa', 'bb', 'cc']？

我可以这样做:

mylist = []
myoutput = list(myoutput)
for each in myoutput:
   mylist.append(each[0])

我相信这不是最好的方法.请赐教!

Answer 1

这也有效:

>>> tu = (('aa',), ('bb',), ('cc',))
>>> import itertools
>>> list(itertools.chain(*tu))
['aa', 'bb', 'cc']

编辑 Could you please comment on the cost tradeoff? (for loop and itertools)

Itertools明显更快:

>>> t = timeit.Timer(stmt="itertools.chain(*(('aa',), ('bb',), ('cc',)))")
>>> print t.timeit()
0.341422080994
>>> t = timeit.Timer(stmt="[a[0] for a in (('aa',), ('bb',), ('cc',))]")
>>> print t.timeit()
0.575773954391

编辑2 Could you pl explain itertools.chain(*)

该*解包序列插入位置参数,在此情况下的元组的嵌套元组.

例:

>>> def f(*args):
...    print "len args:",len(args)
...    for a in args:
...       print a
... 
>>> tu = (('aa',), ('bb',), ('cc',))
>>> f(tu)
len args: 1
(('aa',), ('bb',), ('cc',))
>>> f(*tu)
len args: 3
('aa',)
('bb',)
('cc',)

另一个例子:

>>> f('abcde')
len args: 1
abcde
>>> f(*'abcde')
len args: 5
a
b
c
d
e

请参阅拆包文件.