gh-98169 dataclasses.astuple support DefaultDict

[kwsp]

Basically applying the same fix for asdict from #32056 to astuple. These two should've been the same PR, but I didn't know about astuple previously.

Also modified the handling of DefaultDict in asdict a little so that we only call isinstance(obj, dict) once. This results in a ~3% speed up on my machine on this example:

# benchmark
from dataclasses import dataclass, asdict
from collections import defaultdict
from typing import DefaultDict, Dict, List, NamedTuple
import time


class NT(NamedTuple):
    x: int
    y: float

@dataclass
class C:
    dd: DefaultDict[str, List]
    nt: NT
    d1: Dict[str, List]
    d2: Dict[str, int]

def get_instance():
    instance = C(dd=defaultdict(list), nt=NT(1, 1.), d1={}, d2={})
    instance.dd["x"].append(12)
    instance.dd["x"].append(13)
    instance.d1["y"] = [1, 2, 3]
    instance.d2["z"] = 1
    return instance

instance = get_instance()

def timeit(callable, n_runs = 100_000, name=""):
    t1 = time.perf_counter_ns()
    for _ in range(n_runs):
        callable()

    elapsed_ms = round(1e-6 * (time.perf_counter_ns() - t1), 2)
    print(f"{name}\t{n_runs} runs: {elapsed_ms} ms")

timeit(lambda: asdict(instance), name="asdict")
timeit(lambda: astuple(instance), name="astuple")

• Issue: dataclasses.astuple breaks on DefaultDict #98169

[eendebakpt]

@kwsp Can you address the review comments?

[kwsp]

@eendebakpt sorry I missed the notification. Addressed your change.

Could you please comment on my response to if instanceof(obj, defaultdict)?

[eendebakpt]

@eendebakpt sorry I missed the notification. Addressed your change.

Could you please comment on my response to if instanceof(obj, defaultdict)?

@kwsp I cannot see the comment

[kwsp]

#98170 (comment)

I responded:

Good suggestion. I did this originally to be consistent with the existing code that checks for named tuples, and to avoid importing the colelctions module.

cpython/Lib/dataclasses.py

Line 1302 in f39e3d8

elif isinstance(obj, tuple) and hasattr(obj, '_fields'):

I originally assumed that for the namedtuple case, they could've done a is instance(obj, namedtuple) but didn't to avoid an import from from collections import namedtuple, but I just realised is instance(obj, namedtuple) wouldn't work because namedtuple is a factory function...

So now the question is should we import collections in this module? I assume it's imported elsewhere during Python startup so adding the import here should be free right?

[eendebakpt]

I do not now the origins of the hasattr check, but I agree the import time of collections should not be an issue. But let's keep the code consistent for now and keep the hasattr just as in the asdict code.

[carljm]

IMO it would probably be better to just do isinstance(obj, defaultdict) in both of these cases, but I also think that's a bit out of scope for this PR. The code here LGTM, seems like a straightforward extension of the feature that was already merged for asdict.