#26555: makemigrations serializer isinstance() check fails for subclasses of
Decimal (and DateTime etc)
----------------------------+--------------------
Reporter: oTree-org | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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The migrations serializer uses `isinstance(value, decimal.Decimal)` to
check if a value is a decimal:
https://github.com/django/django/blob/ecb59cc6579402b68ddfd4499bf30edacf5963be/django/db/migrations/serializer.py#L363
If so, it adds `from decimal import Decimal` to the beginning of the
migrations file:
https://github.com/django/django/blob/ecb59cc6579402b68ddfd4499bf30edacf5963be/django/db/migrations/serializer.py#L87
The problem with `isinstance()` is that it results in false positives for
subclasses. I am using a subclass of `Decimal` called `Money` as the
default value for many of my model fields (subclass of `DecimalField`
called `MoneyField`), and the migrations writer adds an incorrect `from
decimal import Decimal` to my migrations files, which still results in a
`NameError` for `Money` when I run `migrate`.
This problem also prevents me from using the `deconstruct()` method as
recommended in the documentation to serialize custom classes:
https://docs.djangoproject.com/ja/1.9/topics/migrations/#adding-a
-deconstruct-method
(Because the check for `Decimal` happens before the check for
`.deconstruct()`.)
I think instead of `isinstance(value, decimal.Decimal)`, the code should
be `if type(value) == decimal.Decimal`.
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Ticket URL: <https://code.djangoproject.com/ticket/26555>
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