google.appengine.ext.ndb.ComputedProperty

A Property whose value is determined by a user-supplied function.

Inherits From: GenericProperty, Property, ModelAttribute, expected_type

google.appengine.ext.ndb.ComputedProperty(
    func, name=None, indexed=None, repeated=None, verbose_name=None
)

Computed properties cannot be set directly, but are instead generated by a function when required. They are useful to provide fields in Cloud Datastore that can be used for filtering or sorting without having to manually set the value in code — for example, sorting on the length of a BlobProperty, or using an equality filter to check if another field is not empty.

ComputedProperty can be declared as a regular property, passing a function as the first argument, or it can be used as a decorator for the function that does the calculation.

Example:

>>> class DatastoreFile(Model):
...   name = StringProperty()
...   name_lower = ComputedProperty(lambda self: self.name.lower())
...
...   data = BlobProperty()
...
...   @ComputedProperty
...   def size(self):
...     return len(self.data)
...
...   def _compute_hash(self):
...     return hashlib.sha1(self.data).hexdigest()
...   hash = ComputedProperty(_compute_hash, name='sha1')

Args
func	A function that takes one argument, the model instance, and returns a calculated value.

Methods

IN

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IN(
    value
)

Comparison operator for the IN comparison operator.

The Python IN operator cannot be overloaded in the way we want to, so we define a method. For example:

Employee.query(Employee.rank.IN([4, 5, 6]))

Note that the method is called ._IN() but may normally be invoked as .IN(); ._IN() is provided for the case you have a StructuredProperty with a model that has a Property named IN.

__eq__

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__eq__(
    value
)

Return a FilterNode instance representing the = comparison.

__ge__

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__ge__(
    value
)

Return a FilterNode instance representing the >= comparison.

__gt__

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__gt__(
    value
)

Return a FilterNode instance representing the > comparison.

__le__

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__le__(
    value
)

Return a FilterNode instance representing the <= comparison.

__lt__

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__lt__(
    value
)

Return a FilterNode instance representing the < comparison.

__ne__

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__ne__(
    value
)

Return a FilterNode instance representing the != comparison.

__neg__

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__neg__()

Return a descending sort order on this Property.

For example:

Employee.query().order(-Employee.rank)

__pos__

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__pos__()

Returns an ascending sort order on this property.

Note that this is redundant but provided for consistency with __neg__. For example, the following two are equivalent:

Employee.query().order(+Employee.rank)
Employee.query().order(Employee.rank)