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Why an InitVar pseudo field in dataclasses cannot have a default_factory?


Hi all,

I was surprised by an error when trying to set a default_factory for an
InitVar pseudo-field in a dataclass. Inspecting the code in dataclasses.py
led me to this:

# Special restrictions for ClassVar and InitVar.
    if f._field_type in (_FIELD_CLASSVAR, _FIELD_INITVAR):
        if f.default_factory is not MISSING:
            raise TypeError(f'field {f.name} cannot have a '
                            'default factory')
        # Should I check for other field settings? default_factory
        # seems the most serious to check for.  Maybe add others.  For
        # example, how about init=False (or really,
        # init=<not-the-default-init-value>)?  It makes no sense for
        # ClassVar and InitVar to specify init=<anything>.

So this case is very explicitly prevented but I could not see why. Does
anybody know what problem this is trying to prevent or what is the
rationale behind this restriction?

I asked in stackoverflow[1] and I was suggested to ask here.

[1] https://stackoverflow.com/questions/57056029

Thanks,
Jacobo de Vera
@jovianjake