Author eric.smith
Recipients eric.smith
Date 2018-06-08.08:51:39
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Message-id <1528447899.5.0.592728768989.issue33805@psf.upfronthosting.co.za>
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If a dataclass contains an InitVar without a default value, that InitVar must be specified in the call to replace(). This is because replace() works by first creating a new object, and InitVars without defaults, by definition, must be specified when creating the object. There is no other source for the value of the InitVar to use.

However, the exception you get is confusing:

>>> from dataclasses import *
>>> @dataclass
... class C:
...   i: int
...   j: InitVar[int]
...
>>> c = C(1, 2)
>>> replace(c, i=3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\home\eric\local\python\cpython\lib\dataclasses.py", line 1176, in replace
    changes[f.name] = getattr(obj, f.name)
AttributeError: 'C' object has no attribute 'j'
>>>

This message really should say something like "InitVar 'j' must be specified".
History
Date User Action Args
2018-06-08 08:51:39eric.smithsetrecipients: + eric.smith
2018-06-08 08:51:39eric.smithsetmessageid: <1528447899.5.0.592728768989.issue33805@psf.upfronthosting.co.za>
2018-06-08 08:51:39eric.smithlinkissue33805 messages
2018-06-08 08:51:39eric.smithcreate