从列表变量开始：
number=[1,2,3,None,4,5]
我可以采用两种不同的方法得到相同的结果：
方法1：
result=[number for number in numbers if number]
方法2：
result=[]
for number in numbers:
if not number: continue
result.append(number)
除了将更多代码放在一起之外，方法2的执行速度似乎要慢30％。由于方法1在速度和清晰度部门都获胜，我想更频繁地使用它。
两个问题：
问题1：
我可以使用[i for i in m if i]方法来构建字典变量吗？所以，如果我从列表变量开始：
number=[1,2,3,None,4,5]
如何进入字典，其中每个列表变量号都成为字典键：例如：
result={1:None, 2:None, 3:None, 4:None, 5:None}
甚至：
result={1:{}, 2:{}, 3:{}, 4:{}, 5:{}}
问题2：
如果满足某个条件，我break可以[i for i in m if i]迭代吗？例如：
result=[]
for number in numbers:
if not number: continue
if number==5:
result.append(number)
break
显然我不能这样说：
result=[number for number in numbers if number and number==5 break]
第三个问题：[i for i in m if i]迭代的输出 总是列表吗？如果没有，请发一些例子。
问题1：你可以这样做：
result = {i: None for i in number if i}
要么：
result = {i: {} for i in number if i}
问题2：您可以使用itertools.takewhile此：
result = [i for i in itertools.takewhile(lambda n: bool(n), number)]
问题3：如果你这样写，那么是的。但如果你写，(i for i in number if i)那么它不是一个列表。在这种情况下，它是一个生成器表达式：
>>> t = (i for i in number if i)
>>> t
<generator object <genexpr> at 0x106039dc0>
这意味着您可以将其用于iteration(for item in t:)而无需构建列表。您也可以通过调用将其转换回列表list(t)。
Starting with a list variable:
number=[1,2,3,None,4,5]
I can get the same result taking two different approaches:
Approach 1:
result=[number for number in numbers if number]
Approach 2:
result=[]
for number in numbers:
if not number: continue
result.append(number)
Approach 2 aside from taking more code to put together appears to be 30% slower to execute. Since Approach 1 wins in both the speed and clarity department I would like to utilize it more often.
Two questions:
Question 1:
Can I use [i for i in m if i] approach to build a dictionary variable?
So if I start with a list variable :
number=[1,2,3,None,4,5]
how can I get to a dictionary where every list variable number becomes a dictionary key: such as:
result={1:None, 2:None, 3:None, 4:None, 5:None}
or even:
result={1:{}, 2:{}, 3:{}, 4:{}, 5:{}}
Question 2:
Can I break from [i for i in m if i] iteration if a certain condition met? For example:
result=[]
for number in numbers:
if not number: continue
if number==5:
result.append(number)
break
Apparently I can't put it like this:
result=[number for number in numbers if number and number==5 break]
And third bonus question: Is the output of [i for i in m if i] iteration always a list? If not please post some examples.
Question 1: you can do:
result = {i: None for i in number if i}
Or:
result = {i: {} for i in number if i}
Question 2: you can use itertools.takewhile for this:
result = [i for i in itertools.takewhile(lambda n: bool(n), number)]
Question 3: if you write it like that then yes. But if you write (i for i in number if i) then it's not a list. In that case it's a generator expression:
>>> t = (i for i in number if i)
>>> t
<generator object <genexpr> at 0x106039dc0>
This means you can use it for iteration (for item in t:) without constructing the list. You can also convert it back to a list by calling list(t).