heh, this is actually some dumb numpy thing, check this out: >>> from numpy.ma.core import exp >>> x = exp(1.0) >>> a = x == x >>> a True >>> a is True False >>>
there’s your problem, the recipe fixes if you just say this:
class InexactFloat(TypeDecorator):
impl = Float
def compare_values(self, x, y):
return bool(x == y)
On Nov 19, 2013, at 8:30 PM, Michael Bayer <mike...@zzzcomputing.com> wrote:
>
> On Nov 19, 2013, at 8:10 PM, Seth P <spadow...@gmail.com> wrote:
>
>> I have an issue where, I believe due to floating-point representation
>> issues, reassigning the same value to a floating-point field causes
>> SQLAlchemy to think the value has been modified, and therefore emits a
>> gratuitous UPDATE. (This is particularly problematic when using the
>> versioning mixin,
>> http://docs.sqlalchemy.org/en/rel_0_8/orm/examples.html?highlight=versioned.)
>> This doesn't happen all the time, e.g. in the example below the problem
>> disappears if exp is imported from math rather than from numpy.ma.core. Am I
>> doing something wrong? If not, is there a way to tell SQLAlchemy that a
>> particular Float column (or even all Float columns) should not be considered
>> modified if the new and old values differ by less than some threshold? Is
>> there a way to provide a custom comparison function used for this testing?
>> Apologies if I missed something about this in the docs.
>
> well floating points are an inexact representation, so you are hitting that
> issue. If you used Numeric() instead, the floats would be coerced to Decimal
> objects on the Python side, these don’t perform as well as floats but are an
> exact, immutable representation - but you still can have variability in how
> these come back from SQLite.
>
> You can in fact change the comparison function used by the unit of work to
> compare for a change, just use a TypeDecorator:
>
> class InexactFloat(TypeDecorator):
> impl = Float
>
> def compare_values(self, x, y):
> return round(x) == round(y) # or whatever
>
> class A(Base):
> __tablename__ = 'a'
> id = Column(Integer, primary_key=True)
> value = Column(InexactFloat)
>
>
>
>
>
>>
>> from sqlalchemy import Column, Integer, Float, create_engine
>> from sqlalchemy.ext.declarative import declarative_base
>> from sqlalchemy.orm import create_session
>> from numpy.ma.core import exp
>>
>> sqlite = 'sqlite:///C:\\temp1.db'
>> engine = create_engine(sqlite, echo=True)
>> Base = declarative_base(bind=engine)
>>
>> class A(Base):
>> __tablename__ = 'a'
>> id = Column(Integer, primary_key=True)
>> value = Column(Float)
>>
>> if __name__ == '__main__':
>> x = exp(1.0)
>> print " x = %0.60f" % x
>> Base.metadata.drop_all()
>> Base.metadata.create_all()
>> session = create_session(bind=engine, autocommit=False)
>> a = A(id=1, value=x)
>> session.add(a)
>> session.commit()
>> print "a.value = %0.60f" % a.value
>> print "session.is_modified(a) = %s" % session.is_modified(a) # False
>> a.value = x
>> print "a.value = %0.60f" % a.value
>> print "session.is_modified(a) = %s" % session.is_modified(a) # True
>> session.commit() # UPDATE
>> session.close()
>>
>> Thanks,
>>
>> Seth
>>
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