Dataclasses can raise RecursionError in __repr__

[ericvsmith]

BPO	33947
Nosy	@rhettinger, @ericvsmith, @remilapeyre, @tirkarthi
PRs	Closes bpo-33947: Dataclasses can raise RecursionError in __repr__ #9916 [3.7] bpo-33947: dataclasses no longer can raise RecursionError in repr (GF9916) #9970

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

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GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2018-10-19.17:29:46.158>
created_at = <Date 2018-06-23.12:58:46.131>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'Dataclasses can raise RecursionError in __repr__'
updated_at = <Date 2018-10-19.17:29:46.157>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2018-10-19.17:29:46.157>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2018-10-19.17:29:46.158>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2018-06-23.12:58:46.131>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 33947
keywords = ['patch']
message_count = 5.0
messages = ['320305', '320322', '320327', '320331', '328055']
nosy_count = 4.0
nosy_names = ['rhettinger', 'eric.smith', 'remi.lapeyre', 'xtreak']
pr_nums = ['9916', '9970']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33947'
versions = ['Python 3.7', 'Python 3.8']

[ericvsmith]

>>> @dataclass
... class C:
...   f: "C"
...
>>> c = C(None)
>>> c.f = c
>>> c
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 2, in __repr__
  File "<string>", line 2, in __repr__
  File "<string>", line 2, in __repr__
  [Previous line repeated 328 more times]
RecursionError: maximum recursion depth exceeded
>>>

It would be better to produce "C(f=...)".

[remilapeyre]

This seems like a difficult problem to tackle in all cases, if two dataclasses reference each other the cycle could be complex to identify and introduce complexity.

The way repr is defined is part of PEP-557 and actually force this behavior.

Should the repr parameter default to False or not include the repr of each field?

[rhettinger]

We have a tool designed for this problem. See: https://docs.python.org/3/library/reprlib.html#reprlib.recursive_repr

If you want to avoid a dependency, it is easy to inline the logic. For an example, see the Python 2.7 version of collections.OrderedDict.__repr__.

[ericvsmith]

Thanks, Raymond. I'm working on a patch.

[ericvsmith]

New changeset b9182aa by Eric V. Smith (Miss Islington (bot)) in branch '3.7':
bpo-33947: dataclasses no longer can raise RecursionError in repr (GF9916) (bpo-9970)
b9182aa