python / python/cpython

dataclass copy constructor doesn't recognize default_factory

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#92,052 15 comments 2 reactions 1 assignee View on GitHub

@ericvsmith is already working on this.

Since Apr 29, 2022.

stdlib topic-dataclasses type-bug
Dominant language
Python
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Description

Bug report
The dataclass copy constructor doesn't recognize the default_factory.

@dataclass
class Orig2:
    a: int
    b: int = 1
    c: List[int] = field(default_factory=list)

Copy2 = dataclass(Orig2)

Causes the following error`

Traceback (most recent call last):
  File "/Users/hsolbrig/git/hsolbrig/dataclasstest/src/dataclasscopytest.py", line 53, in <module>
    Copy2 = dataclass(Orig2)
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/dataclasses.py", line 1185, in dataclass
    return wrap(cls)
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/dataclasses.py", line 1176, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/dataclasses.py", line 1025, in _process_class
    _init_fn(all_init_fields,
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/dataclasses.py", line 546, in _init_fn
    raise TypeError(f'non-default argument {f.name!r} '
TypeError: non-default argument 'c' follows default argument

Your environment
This bug has been shown to exist in versions 3.9 and 3.10. The test above is python 3.10.4.

Using the line numbers in python 3.10.4, this can be fixed with the following change to dataclasses.py line 958:

    for f in cls_fields:
        fields[f.name] = f

        # If the class attribute (which is the default value for this
        # field) exists and is of type 'Field', replace it with the
        # real default.  This is so that normal class introspection
        # sees a real default value, not a Field.
        if isinstance(getattr(cls, f.name, None), Field):
            if f.default is MISSING and f.default_factory is MISSING:      # <-- Add second test
                # If there's no default, delete the class attribute.
                # This happens if we specify field(repr=False), for
                # example (that is, we specified a field object, but
                # no default value).  Also if we're using a default
                # factory.  The class attribute should not be set at
                # all in the post-processed class.
                delattr(cls, f.name)
            else:
                setattr(cls, f.name, f.default_factory if f.default is MISSING else f.default)   # <-- add conditional
Linked PRs
  • gh-92406

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