New submission from Eric V. Smith <[email protected]>:
Consider:
class FtpHelper(ftplib.FTP):
host: str
baz: str = 'baz default'
>>> FtpHelper.host
''
>>> FtpHelper.baz
'baz default'
>>> getattr(FtpHelper, "host")
''
>>> getattr(FtpHelper, "baz")
'baz default'
But:
>>> FtpHelper.__dict__['host']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'host'
>>> FtpHelper.__dict__['baz']
'baz default'
Now make this a dataclass, without a default value for baz:
@dataclass
class FtpHelper(ftplib.FTP):
host: str
baz: str
This gives an error:
TypeError: non-default argument 'baz' follows default argument
And this is because it's picking up a default value for host from the base
class ftplib.FTP, and generating an __init__ like:
def __init__(self, host='', baz):
@dataclass uses getattr(cls, field_name, MISSING) to find the default value for
the field, but in this case that's wrong. I think what should happen is that it
should use getattr(cls.__dict__, field_name, MISSING). This would be consistent
with other features where dataclasses does not look in base classes for various
things, but only in the class itself (like __hash__).
----------
assignee: eric.smith
components: Library (Lib)
messages: 396000
nosy: eric.smith
priority: normal
severity: normal
status: open
title: dataclass looks up default on the class, not the classes __dict__
versions: Python 3.10, Python 3.11, Python 3.9
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Python tracker <[email protected]>
<https://bugs.python.org/issue44443>
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