python - Dict/Set Parsing Order Consistency -

containers take hashable objects (such dict keys or set items). such, dictionary can have 1 key value 1, 1.0 or true etc. (note: simplified - hash collisions permitted, these values considered equal)

my question is: parsing order well-defined , resulting object predictable across implementations? example, osx python 2.7.11 , 3.5.1 interprets dict so:

>>> { true: 'a', 1: 'b', 1.0: 'c', (1+0j): 'd' } {true: 'd'} 

in case, appears first key , last value preserved.

similar, in case of set:

>>> { true, 1, 1.0, (1+0j) } set([(1+0j)]) 

here appears last item preserved.

but (as mentioned in comments):

>>> set([true, 1, 1.0]) set([true]) 

now first in iterable preserved.

the documentation notes order of items (for example in dict.items) undefined, question refers result of constructing dict or set objects.

• the bug fixed in recent versions of python explained in @jsf's answer

dictionary-displays

if comma-separated sequence of key/datum pairs given, evaluated left right define entries of dictionary: each key object used key dictionary store corresponding datum. means can specify same key multiple times in key/datum list, , final dictionary’s value key last 1 given.

a dict comprehension, in contrast list , set comprehensions, needs 2 expressions separated colon followed usual “for” , “if” clauses. when comprehension run, resulting key , value elements inserted in new dictionary in order produced.

set displays

a set display yields new mutable set object, contents being specified either sequence of expressions or comprehension. when comma-separated list of expressions supplied, elements evaluated left right , added set object. when comprehension supplied, set constructed elements resulting comprehension.

there difference in calling set constructor or using comprehension , plain literal.

def f1():     return {x x in [true, 1]}  def f2():     return set([true, 1]) def f3():     return {true, 1} print(f1()) print(f2()) print(f3()) import dis  print("f1") dis.dis(f1)  print("f2")  dis.dis(f2)  print("f3") dis.dis(f3) 

output:

{true} {true} {1} 

how created influences outcome:

    605           0 load_const               1 (<code object <setcomp> @ 0x7fd17dc9a270, file "/home/padraic/dropbox/python/test.py", line 605>)               3 load_const               2 ('f1.<locals>.<setcomp>')               6 make_function            0               9 load_const               3 (true)              12 load_const               4 (1)              15 build_list               2              18 get_iter              19 call_function            1 (1 positional, 0 keyword pair)              22 return_value f2 608           0 load_global              0 (set)               3 load_const               1 (true)               6 load_const               2 (1)               9 build_list               2              12 call_function            1 (1 positional, 0 keyword pair)              15 return_value f3 611           0 load_const               1 (true)               3 load_const               2 (1)               6 build_set                2               9 return_value 

python runs build_set bytecode when pass pure literal separated commas per:

when comma-separated list of expressions supplied, elements evaluated left right , added set object.

the line comprehension:

when comprehension supplied, set constructed elements resulting comprehension.

so hamish filing bug report indeed come down build_set opcode per raymond hettinger's comment in link the culprit build_set opcode in python/ceval.c unnecessarily loops backwards, implementation of below:

 target(build_set) {             pyobject *set = pyset_new(null);             int err = 0;             if (set == null)                 goto error;             while (--oparg >= 0) {                 pyobject *item = pop();                 if (err == 0)                     err = pyset_add(set, item);                 py_decref(item);             }             if (err != 0) {                 py_decref(set);                 goto error;             }             push(set);             dispatch();         }