Python - Fastest way to create a tuple from a list with computations

Question

I need to create in the fastest way possible a Python tuple from a list, but with intermediate computations. To be clearer I put a code snippet:

a=[1,2,3,4]
b=tuple([id(x) for x in a])

This is what I have by now. Is there something better than this? This code creates a list and then it converts it to a tuple, is there a way to directly create the tuple?

Thank you!

[EDIT] As suggested I tried with

b=tuple(id(x) for x in a)

However it seems slower than before. Test script:

import time

a=[1,2,3,4]

t1=time.time()
for i in range(1000000):
    b=tuple([id(x) for x in a])
print(time.time()-t1)

t2=time.time()
for i in range(1000000):
    b=tuple(id(x) for x in a)
print(time.time()-t2)

It was very unexpected to me.

`tuple(id(x) for x in a)` does not create intermediate lists — vaultah, Aug 10 '16 at 13:09
Thank, I did not know this possibility. But It seems slower, I edit my question. — stefano1, Aug 10 '16 at 13:15
Not directly related to your question, but consider using the `timeit` module rather than `time.time` when comparing the run-time of code. `timeit` gives more reliable results because it runs each section multiple times, which lessens the impact of outside interference (such as Python going to sleep for a millisecond so the CPU can concentrate on a different application on your computer) — Kevin, Aug 10 '16 at 13:19

Antti Haapala -- Слава Україні · Answer 1 · 2021-03-16T06:04:48.090

5

Using the list comprehension is faster. This is because the size of the tuple is known beforehand (memory can be allocated for len(list) items).

With generator, the generator needs to be converted to a sequence first so that a tuple can be allocated. This will be slower because of the inherent overhead in generators.

The fastest option is naturally to not use a tuple at all and use a list instead. In my opinion, a tuple is not the same as "an immutable list"; one should really use tuple only when one needs to use a sequence as a dictionary key, or as a set element, or when you could name each element (as in namedtuple).

edited Mar 16 '21 at 06:04

answered Aug 10 '16 at 13:22

Antti Haapala -- Слава Україні

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Ok, thanks. List does not fit for me since I need an immutable collection. Are there alternatives faster to tuples in this example? – stefano1 Aug 10 '16 at 13:25
@stefano1 Tuple is an immutable sequence of arbitrary elements. Used especially if you need it as a dictionary key or as an element in a set; for other cases list is as good, just say that it is not to be modified. After all, there is no such thing as a "immutable dictionary" for example. – Antti Haapala -- Слава Україні Aug 10 '16 at 13:27
I need it exactly as a dictionary key :) But thanks for your explanations in any case! – stefano1 Aug 10 '16 at 13:31
Could you expound why we shouldn't use tuples unnecessarily? I read in places such as [here](https://stackoverflow.com/a/48936610/13312697) that tuples of literals compute more quickly than lists. – Lyndon Gingerich Mar 16 '21 at 03:56
@themadpsychologist yes but this is not a tuple of literals. – Antti Haapala -- Слава Україні Mar 16 '21 at 06:02
True. But why should we not use tuples unnecessarily? What advantages does a list have? – Lyndon Gingerich Mar 16 '21 at 18:27

Python - Fastest way to create a tuple from a list with computations

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