BUILD_MAP_UNPACK doesn't function as expected for dict subclasses

[mr-nfamous]

BPO	35166
Nosy	@stevendaprano, @serhiy-storchaka, @mr-nfamous, @iritkatriel

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-11-05.06:23:01.388>
labels = ['interpreter-core', 'type-bug', '3.7']
title = "BUILD_MAP_UNPACK doesn't function as expected for dict subclasses"
updated_at = <Date 2021-12-30.13:20:40.929>
user = 'https://github.com/mr-nfamous'

bugs.python.org fields:

activity = <Date 2021-12-30.13:20:40.929>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2018-11-05.06:23:01.388>
creator = 'bup'
dependencies = []
files = []
hgrepos = []
issue_num = 35166
keywords = []
message_count = 5.0
messages = ['329280', '329281', '329282', '329283', '409368']
nosy_count = 4.0
nosy_names = ['steven.daprano', 'serhiy.storchaka', 'bup', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35166'
versions = ['Python 3.6', 'Python 3.7']

[mr-nfamous]

>>> class Dict(dict):
       
	def keys(self): assert 0
	def update(*args, **kwds): assert 0
	def __getitem__(self, key): assert 0
        def __iter__(self): assert 0 

	
>>> {**Dict(a=1)}
{'a': 1}

The opcode uses PyDict_Update, which calls the internal dict_merge function which contains the following line:

if (PyDict_Check(b) && (Py_TYPE(b)->tp_iter == (getiterfunc)dict_iter))

Translated to Python, that should be equal to

if type(b).__flags__ & (1<<29) and type.__getattribute__(type(b), '__iter__') is type.__getattribute__(dict, '__iter__')`

Both that and the line in C evaluate to false for me (while a dict subclass that doesn't override __iter__ evaluates to true), so I
apparently can't help narrow down the problem any further assuming people agree that this is even is a problem...

The BUILD_MAP_UNPACK_WITH_CALL, CALL_FUNCTION_EX, and CALL_FUNCTION_KW opcodes are affected as well.

[serhiy-storchaka]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in keys
AssertionError

This works as expected to me.

[stevendaprano]

You say it doesn't work as expected, but you don't say what you expect or why. (Don't make me guess what you mean -- explicit is better than implicit.)

When I try your subclass in 3.6, I get an unexpected TypeError:

py> class Dict(dict):
...     def keys(self): assert 0
...     def update(*args, **kwds): assert 0
...     def __getitem__(self, key): assert 0
...     def __iter__(self): assert 0
...
py> {**Dict(a=1)}
{'a': 1}
py> Dict(a=1).keys()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in keys
TypeError

[stevendaprano]

How weird... after restarting the interpreter, I can't reproduce that TypeError. I get the AssertionError Serhiy showed.

[iritkatriel]

BUILD_MAP_UNPACK and BUILD_MAP_UNPACK_WITH_CALL have been removed.