On 06/07/2017 06:19 PM, Greg Silverman wrote:
Thanks, it worked!
I tried something similar using the @hybrid_property decorator, but
could not get it to work. What exactly is the difference between that
and the @property decorator?
@property is super normal Python stuff that's in all the Python
tutorials and is something everyone should use all the time.
@hybrid_property is more of a magic trick that's specific to the kind of
query interface that SQLAlchemy has - in that in an ORM like SQLAlchemy
(and lots of others), you access attributes off of classes, as well as
instances of those classes, as part of regular use of the API. This is
not a standard object-oriented thing, it's kind of odd. The
@hybrid_property allows you to build functions that act a lot like
@property, but they accommodate for accessing an attribute both at the
"instance" (e.g. 'self') level and at the "class" level. Which
translated to SQLAlchemy means, at the "row I've loaded into memory"
level and the "SQL expression I want to execute on the database" level.
The kinds of functions we can use in these two very different contexts
are quite different.
Greg--
On Wed, Jun 7, 2017 at 5:08 PM, mike bayer <mike...@zzzcomputing.com
<mailto:mike...@zzzcomputing.com>> wrote:
On 06/07/2017 02:31 PM, GMS wrote:
I am sure this is easier than I am making it, but I just want to
add a property to a class so that decimal representations get
truncated at 3 decimal digits.
My class is this:
class Measures(Model):
__tablename__= 'xcelera_measures'
id= Column(Numeric,primary_key=True)
studyidk= Column(Numeric, ForeignKey('xcel.studyidk'))
xceleragroup= relationship("Xcelera")
explainstring= Column(String(255))
mrn= Column(String(255))
value= Column(Numeric)
__mapper_args__= {
"order_by": [mrn]
}
and 'value' is the property/attribute that I want to also have a
truncated version available in the class.
I tried adding this to the class value_new =
column_property("%.3f" % value())
but got an error that
value_new = column_property("%.3f" % value)
"%" is a Python function, that doesn't execute on the database.
This works as a normal Python descriptor:
class MyClass(Base):
# ...
@property
def value_new(self):
return "%.3f" % self.value
TypeError: float argument required, not Column
I also tried this as a hybrid_property, but I don't think I was
using it correctly.
All I want to do is have the truncated version of my attribute
available in my class.
Thanks!
Greg--
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