CODEWITHSUNDEEP
python
Ed.
Ed.

Reputation: 4577

Create list of object attributes in python

I have a list of objects:

[Object_1, Object_2, Object_3]

Each object has an attribute: time:

Object_1.time = 20
Object_2.time = 30
Object_3.time = 40

I want to create a list of the time attributes:

[20, 30, 40]

What is the most efficient way to get this output? It can't be to iterate over the object list, right?:

items = []
for item in objects:
    items.append(item.time)

Upvotes: 15

Views: 16915

Answers (4)

the wolf
the wolf

Reputation: 35522

The fastest (and easiest to understand) is with a list comprehension.

See the timing:

import timeit
import random
c=10000

class SomeObj:
    def __init__(self, i):
        self.attr=i

def loopCR():
    l=[]
    for i in range(c):
        l.append(SomeObj(random.random()))

    return l 

def compCR():
    return [SomeObj(random.random()) for i in range(c)]   

def loopAc():
    lAttr=[]
    for e in l:
        lAttr.append(e.attr)

    return lAttr

def compAc():
    return [e.attr for e in l]             

t1=timeit.Timer(loopCR).timeit(10)
t2=timeit.Timer(compCR).timeit(10)
print "loop create:", t1,"secs"   
print "comprehension create:", t2,"secs"   
print 'Faster of those is', 100.0*abs(t1-t2) / max(t1,t2), '% faster'
print 

l=compCR()

t1=timeit.Timer(loopAc).timeit(10)
t2=timeit.Timer(compAc).timeit(10)
print "loop access:", t1,"secs"   
print "comprehension access:", t2,"secs"   
print 'Faster of those is', 100.0*abs(t1-t2) / max(t1,t2), '% faster'

Prints:

loop create: 0.103852987289 secs
comprehension create: 0.0848100185394 secs
Faster of those is 18.3364670069 % faster

loop access: 0.0206878185272 secs
comprehension access: 0.00913000106812 secs
Faster of those is 55.8677438315 % faster

So list comprehension is both faster to write and faster to execute.

Upvotes: 2

John La Rooy
John La Rooy

Reputation: 304137

from operator import attrgetter
items = map(attrgetter('time'), objects)

Upvotes: 4

user648852
user648852

Reputation:

How about:

items=[item.time for item in objects]

Upvotes: 2

mhawke
mhawke

Reputation: 87064

List comprehension is what you're after:

list_of_objects = [Object_1, Object_2, Object_3]
[x.time for x in list_of_objects]

Upvotes: 37

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