Only index needed: enumerate or (x)range?

Question

If I want to use only the index within a loop, should I better use the range/xrange function in combination with len()

a = [1,2,3]         


        

Answer 1

I would use enumerate as it's more generic - eg it will work on iterables and sequences, and the overhead for just returning a reference to an object isn't that big a deal - while xrange(len(something)) although (to me) more easily readable as your intent - will break on objects with no support for len...

Answer 2

xrange should be a little faster, but enumerate will mean you don't need to change it when you realise that you need p afterall

Answer 3

That's a rare requirement – the only information used from the container is its length! In this case, I'd indeed make this fact explicit and use the first version.

Answer 4

Based on your sample code,

res = [[profiel.attr[i].x for i,p in enumerate(profiel.attr)] for profiel in prof_obj]

I would replace it with

res = [[p.x for p in profiel.attr] for profiel in prof_obj]

Answer 5

I ran a time test and found out range is about 2x faster than enumerate. (on python 3.6 for Win32)

best of 3, for len(a) = 1M

• enumerate(a): 0.125s
• range(len(a)): 0.058s

Hope it helps.

FYI: I initialy started this test to compare python vs vba's speed...and found out vba is actually 7x faster than range method...is it because of my poor python skills?

surely python can do better than vba somehow

script for enumerate

import time
a = [0]
a = a * 1000000
time.perf_counter()

for i,j in enumerate(a):
    pass

print(time.perf_counter())

script for range

import time
a = [0]
a = a * 1000000
time.perf_counter()

for i in range(len(a)):
    pass

print(time.perf_counter())

script for vba (0.008s)

Sub timetest_for()
Dim a(1000000) As Byte
Dim i As Long
tproc = Timer
For i = 1 To UBound(a)
Next i
Debug.Print Timer - tproc
End Sub

Answer 6

Just use range(). If you're going to use all the indexes anyway, xrange() provides no real benefit (unless len(a) is really large). And enumerate() creates a richer datastructure that you're going to throw away immediately.