understanding python code ----- objs = ((v[i], w[i]) for i in idxs[m:])

Question

n = 4
v = [16, 19, 23, 28]
w = [2, 3, 4, 5]

idxs = list(range(n))
idxs.sort(key=lambda i: v[i]/w[i], reverse=True)  

objs = ((v[i], w[i]) for i in idxs[m:]) 
for av, aw in objs:
    print av
    print aw

I came across the above chunk of code. If m=2, it returns 23 4 28 5. I wonder how does the below indicated line of code work in Python, is this a function call? or what?. Would you point me to Python 2.7 manual which explain this feature? I'd properly need a better title for this question, but I don't know how to name it, any suggestion?

objs = ((v[i], w[i]) for i in idxs[m:])

The code is a bit inefficient, as it loops over the integers more than once. — tommy.carstensen, Jun 30 '13 at 14:12

score 4 · Accepted Answer · edited May 23 '17 at 11:56

4

That's a generator expression, also sometimes called a generator comprehension. The last four lines of code are basically equivalent to

for i in idxs[m:]:
    av, aw = v[i], w[i]
    print av
    print aw

edited May 23 '17 at 11:56

Community

1
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answered Jun 30 '13 at 14:01

David Robinson

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awesome, I can understand this chunk of code in 1s! Thanks for the link! – twfx Jun 30 '13 at 14:03
PEP calls them [`generator expressions`](http://www.python.org/dev/peps/pep-0289/) – ovgolovin Jun 30 '13 at 14:04
@ovgolovin: Thanks for the tip! Linked to both that and the SO question – David Robinson Jun 30 '13 at 14:05

understanding python code ----- objs = ((v[i], w[i]) for i in idxs[m:])

1 Answers1