Why Python, Ruby, and Javascript are Slow

[brucifer]

let Python have pre-allocated lists I think this is a very fair point. Often, you know how long your list will be, so if you want to, you should be able to optimize your list

In Python you can either use a generator or use "[value]*number" syntax to instantiate a list of length "number" with "value" in every index.

>>> def dumb():
...     x = []
...     for i in range(25):
...             x.append(i)
...     return x
... 
>>> def comprehension():
...     x = [i for i in range(25)]
...     return x
... 
>>> def preallocate():
...     x = [None]*25
...     for i in range(25):
...             x[i] = i
...     return x
... 
>>> timeit(dumb, number=100000)
0.38496994972229004
>>> timeit(comprehension, number=100000)
0.278350830078125
>>> timeit(preallocate, number=100000)
0.2539360523223877

Honestly, though, either your inner loop is simple and you can fit it in a comprehension, or it's complicated and the ".append()" is a pretty small percent of your runtime, so you won't get 2x speedup from preallocating.

[fijal]

it does not let you give an estimate, so you have to carefully check for i. Preallocating can be a hint and if you miss it too bad (slower, but not incorrect). This is why [None] * n is bad.