Initializing a set comprehension

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Here is an example of a set comprehension:

inp    = [1,2,3,4,4,3,5]
s_comp = {element for element in inp}
# {1, 2, 3, 4, 5}

Does python know to convert the below into an immediate set comprehension (skipping over the initial list?) or does this first do a list comprehension and then convert the resulting list into a set (in other words, it's just a waste of resources using this notation to create a set).

s_comp = set([element for element in inp])
# {1, 2, 3, 4, 5}

Note, from cProfile it seems like it first does a list comprehension for the latter. Though each of them have the same amount of steps (I would imagine the first one should have one step less?).

>>> cProfile.run('{element for element in [1,2,3,4,4,3,5]}')
         4 function calls in 0.000 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 <string>:1(<setcomp>)
        1    0.000    0.000    0.000    0.000 {built-in method builtins.exec}

>>> cProfile.run('set([element for element in [1,2,3,4,4,3,5]])')
         4 function calls in 0.000 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:1(<listcomp>) # list not set
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {built-in method builtins.exec}
set python-3.x python
1 Answers

If you have an iterable it is a waste of resources using a list comprehension [] instead of a set comprehension directly {}.

If you look at the bytecode of func A below you'll see that Python is building a List and passing it as a parameter to set() function.

In the bytecode of Func B Python is optimized to build a Set directly.

I have used the following code to do an analysis.

from dis import dis

def a():
    return set([x for x in range(1, 10)])

def b():
    return {x for x in range(1, 10)}

print('Dis Func A\n')
print(dis(a))
print('Dis Func B\n')
print(dis(b))

Dis Func A

  5           0 LOAD_GLOBAL              0 (set)
              2 LOAD_CONST               1 (<code object <listcomp> at 0x00000249F64B2EA0, file "c:\Projects\stack-overflow\asd.py", line 5>)
              4 LOAD_CONST               2 ('a.<locals>.<listcomp>')
              6 MAKE_FUNCTION            0
              8 LOAD_GLOBAL              1 (range)
             10 LOAD_CONST               3 (1)
             12 LOAD_CONST               4 (10)
             14 CALL_FUNCTION            2
             16 GET_ITER
             18 CALL_FUNCTION            1
             20 CALL_FUNCTION            1
             22 RETURN_VALUE

Disassembly of <code object <listcomp> at 0x00000249F64B2EA0, file "c:\Projects\stack-overflow\asd.py", line 5>:
  5           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 8 (to 14)
              6 STORE_FAST               1 (x)
              8 LOAD_FAST                1 (x)
             10 LIST_APPEND              2
             12 JUMP_ABSOLUTE            4
        >>   14 RETURN_VALUE

Dis Func B

  9           0 LOAD_CONST               1 (<code object <setcomp> at 0x00000249F64CE030, file "c:\Projects\stack-overflow\asd.py", line 9>)
              2 LOAD_CONST               2 ('b.<locals>.<setcomp>')
              4 MAKE_FUNCTION            0
              6 LOAD_GLOBAL              0 (range)
              8 LOAD_CONST               3 (1)
             10 LOAD_CONST               4 (10)
             12 CALL_FUNCTION            2
             14 GET_ITER
             16 CALL_FUNCTION            1
             18 RETURN_VALUE

Disassembly of <code object <setcomp> at 0x00000249F64CE030, file "c:\Projects\stack-overflow\asd.py", line 9>:
  9           0 BUILD_SET                0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 8 (to 14)
              6 STORE_FAST               1 (x)
              8 LOAD_FAST                1 (x)
             10 SET_ADD                  2
             12 JUMP_ABSOLUTE            4
        >>   14 RETURN_VALUE
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