Brackets around multiple outputs in Python function

Question

In matlab, if we have a function that returns multiple variables, we do something like

[output1, output2] = some_func()

In Python, you can simply do

output1, output2 = some_func()

Or you could do

[output1, output2] = some_func()

Or

(output1, output2) = some_func()

The last 2 makes some temporary list and tuple, respectively, but it is not assigned to anything, and you can access the 2 output variables identical to the case without [] or (). Is there anything actually functionally advantageous to using the last 2 syntax besides somewhat looking a little more elegant?

I'm new to Python and I was asking if there is anything actually functionally different with using `[]` and `()` when you have multiple output variables. Maybe I should have asked if there's anything advantageous to using `[]` and `()`. I'll update. — 24n8, Jan 02 '19 at 23:14

DeepSpace · Accepted Answer · 2019-01-02T23:45:34.107

The only difference I can think of between

[output1, output2] = some_func()

and

(output1, output2) = some_func()

is that the latter's memory footprint should be smaller (but I'm sure that this is an implementation detail) since tuples take less memory than lists with the same number of elements, mainly because tuples are immutable (so the interpreter should not need to worry about adding or removing [ie re-allocating memory] elements).

import sys

print(sys.getsizeof([1, 2]))
print(sys.getsizeof((1, 2)))
print(sys.getsizeof([1, 2, 3, 4]))
print(sys.getsizeof((1, 2, 3, 4)))
print(sys.getsizeof(list(range(1000))))
print(sys.getsizeof(tuple(range(1000))))

#  80
#  64
#  96
#  80 
#  9112
#  8048

The generated bytecode is exactly the same for all 3 examples:

from dis import dis


def foo(): return 1, 2


def a():
    output1, output2 = foo()


def b():
    [output1, output2] = foo()


def c():
    (output1, output2) = foo()

dis(a)
print('-----------------------------------------------------')
dis(b)
print('-----------------------------------------------------')
dis(c)

outputs

 81           0 LOAD_GLOBAL              0 (foo)
              2 CALL_FUNCTION            0
              4 UNPACK_SEQUENCE          2
              6 STORE_FAST               0 (output1)
              8 STORE_FAST               1 (output2)
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE
-----------------------------------------------------
 85           0 LOAD_GLOBAL              0 (foo)
              2 CALL_FUNCTION            0
              4 UNPACK_SEQUENCE          2
              6 STORE_FAST               0 (output1)
              8 STORE_FAST               1 (output2)
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE
-----------------------------------------------------
 89           0 LOAD_GLOBAL              0 (foo)
              2 CALL_FUNCTION            0
              4 UNPACK_SEQUENCE          2
              6 STORE_FAST               0 (output1)
              8 STORE_FAST               1 (output2)
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE

@PranavChaudhary you can see that just above the bytecode – DeepSpace Jan 02 '19 at 23:42 — DeepSpace, Jan 02 '19 at 23:42
Okay, Thanks. That was helpful. – Pranav Chaudhary Jan 02 '19 at 23:43 — Pranav Chaudhary, Jan 02 '19 at 23:43

Brackets around multiple outputs in Python function

1 Answers1