3 ms·
actually, only dict(d1, **d2) has that problem. it works fine if you unpack into a dict literal: >>> d1 = {1: 'a'} >>> d2 = {2: 'b'} >>> {**d1, **d2}
by uryga 7y ago
actually, only
dict(d1, **d2)
has that problem. it works fine if you unpack into a dict literal:
>>> d1 = {1: 'a'}
>>> d2 = {2: 'b'}
>>> {**d1, **d2}
{1: 'a', 2: 'b'}
iirc the pep mostly just says that it's suboptimal because it's syntactically heavy/noisy, non-obvious and can't be overloaded in dict subclasses
---
i was curious why the two double-stars behave differently despite syntactic similarity. so i went and checked the bytecode, and it turns out they compile down to different opcodes! `{××d1, ××d2}` yields a BUILD_MAP_UNPACK, while `dict(d1, ××d2)` yields a CALL_FUNCTION_EX/CALL_FUNCTION_KW (depending on the CPython version)
- dahfizz 7y agoThis seems pretty straightforward. When doing `dict(d1, d2)` You are calling the dict function, and using the normal syntax for unpacking a dictionary into kwargs. In this case, the name for kwargs must be strings.
- uryga 7y agoyeah, that side of the (in)equation was pretty obvious, i was mostly interested in the `{...}` one. i admit that a bytecode listing probably isn't the best exposition, i just like digging into VM stuff :)