On Tue, Aug 13, 2019 at 06:01:27PM -0400, Eric V. Smith wrote:
> dataclasses does something similar: it wants to know if something is a
> typing.ClassVar, but it doesn't want to import typing to find out.
Why not? Is importing typing especially expensive or a bottleneck?
Wouldn't it be a once-off cost?
> So it has:
>
> typing = sys.modules.get('typing')
> if typing:
> if (_is_classvar(a_type, typing)
> or (isinstance(f.type, str)
> and _is_type(f.type, cls, typing, typing.ClassVar,
> _is_classvar))):
>
> If typing hasn't been imported, it knows that a_type can't be
> typing.ClassVar.
That's very clever, but alas it's not clever enough because its false.
Or rather, it's false in general. ClassVar is magic, it can't be used
with isinstance() or issubclass(), so I won't argue about the above
snippet. I don't understand it well enough to argue its correctness.
But for regular classes like Decimal, this clever trick doesn't always
work. Here's an example:
py> import decimal
py> x = decimal.Decimal()
py> del decimal
py> del sys.modules['decimal']
At this point, we have a Decimal instance, but no "decimal" in the
module cache. Your test for whether x is a Decimal will wrongly deny x
is a Decimal:
# the wrong way to do it
py> _decimal = sys.modules.get('decimal')
py> isinstance(x, _decimal.Decimal) if _decimal else False
False
but if we're a bit less clever, we get the truth:
py> import decimal
py> isinstance(x, decimal.Decimal)
True
You can't rely on the presence of 'decimal' in the module cache as a
proxy for whether or not x might be a Decimal instance.
--
Steven
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