Types and Property Classes

The App Engine datastore supports a fixed set of value types for properties on data entities. Property classes can define new types that are converted to and from the underlying value types, and the value types can be used directly with Expando dynamic properties and ListProperty aggregate property models.

The following table describes the Property classes whose values correspond directly with the underlying data types. Any of these value types can be used in an Expando dynamic property or ListProperty aggregate type.

Datastore Value Types

Datastore entity property values can be of one of the following types. See above for a list of corresponding Property classes to use with Model definitions.

Other than the Python standard types and users.User, all classes described in this section are provided by the google.appengine.ext.db module.

str or unicode

A short string value, less than 500 bytes in length.

A str value is assumed to be text encoded with the ascii codec, and is converted to a unicode value before being stored. The value is returned by the datastore as a unicode value. For short strings using other codecs, use a unicode value.

Short strings are indexed by the datastore, and can be used in filters and sort orders. For text strings longer than 500 bytes (which are not indexed), use a Text instance. For unencoded byte strings longer than 500 bytes (also not indexed), use a Blob instance.

Model property: StringProperty

bool

A Boolean value, True or False.

Model property: BooleanProperty

int or long

An integer value.

Python int values are converted to Python long values prior to storage. A value stored as an int will be returned as a long.

Model property: IntegerProperty

float

A floating point value.

Model property: FloatProperty

datetime.datetime

A date and time. See the datetime module documentation.

If the datetime value has a tzinfo attribute, it will be converted to the UTC time zone for storage. Values come back from the datastore as UTC, with a tzinfo of None. An application that needs date and time values to be in a particular time zone must set tzinfo correctly when updating the value, and convert values to the timezone when accessing the value.

Some libraries use the TZ environment variable to control the time zone applied to date-time values. App Engine sets this environment variable to "UTC". Note that changing this variable in an application will not change the behavior of some datetime functions, because changes to environment variables are not visible outside of the Python code.

If you only convert values to and from a particular time zone, you can implement a custom datetime.tzinfo to convert values from the datastore:

class Pacific_tzinfo(datetime_module.tzinfo):
 """Implementation of the Pacific timezone."""
 def utcoffset(self, dt):
   return datetime_module.timedelta(hours=-8) + self.dst(dt)

 def _FirstSunday(self, dt):
   """First Sunday on or after dt."""
   return dt + datetime_module.timedelta(days=(6-dt.weekday()))

 def dst(self, dt):
   # 2 am on the second Sunday in March
   dst_start = self._FirstSunday(datetime_module.datetime(dt.year, 3, 8, 2))
   # 1 am on the first Sunday in November
   dst_end = self._FirstSunday(datetime_module.datetime(dt.year, 11, 1, 1))

   if dst_start <= dt.replace(tzinfo=None) < dst_end:
     return datetime_module.timedelta(hours=1)
   else:
     return datetime_module.timedelta(hours=0)

 def tzname(self, dt):
   if self.dst(dt) == datetime_module.timedelta(hours=0):
     return "PST"
   else:
     return "PDT"

pacific_time = utc_time.astimezone(Pacific_tzinfo())

See the datetime module documentation (including datetime.tzinfo). See also the third-party module pytz, though note that the pytz distribution has many files.

The DateTimeProperty model property class includes features such as the ability to automatically use the date and time a model instance is stored. These are features of the model, and are not available on the raw datastore value (such as in an Expando dynamic property).

Model properties: DateTimeProperty, DateProperty, TimeProperty

list

A list of values, each of which is of one of the supported data types. See Entities and Models: Lists.

When a list is used as the value of an Expando dynamic property, it cannot be an empty list. This is due to how list values are stored: When a list property has no items, it has no representation in the datastore. You can use a static property and the ListProperty class to represent an empty list value for a property.

Model property: ListProperty

db.Key

The key for another datastore entity.

m = Employee(name="Susan", key_name="susan5")
m.put()
e = Employee(name="Bob", manager=m.key())
e.put()

m_key = Key.from_path("Employee", "susan5")
e = Employee(name="Jennifer", manager=m_key)

Model properties: ReferenceProperty, SelfReferenceProperty

users.User

A user with a Google account.

Model property: UserProperty

class Blob(arg=None)

Binary data, as a byte string. This is a subclass of the built-in str type.

Blob properties are not indexed, and cannot be used in filters or sort orders.

Blob is for binary data, such as images. It takes a str value, but this value is stored as a byte string and is not encoded as text. Use a Text instance for large text data.

Model property: BlobProperty

class MyModel(db.Model):
  blob = db.BlobProperty()

m = MyModel()
m.blob = db.Blob(open("image.png").read())

class Text(arg=None, encoding=None)

A long string. This is a subclass of the built-in unicode type.

arg a unicode or str value. If arg is a str, then it is parsed with the encoding specified by encoding, or ascii if no encoding is specified. See the list of standard encodings for possible values for encoding.

Unlike an entity property whose value is a simple str or unicode, a Text property can be more than 500 bytes long. However, Text properties are not indexed, and cannot be used in filters or sort orders.

Model property: TextProperty

class MyModel(db.Model):
  text = db.TextProperty()

m = MyModel()
m.text = db.Text(u"kittens")

m.text = db.Text("kittens", encoding="latin-1")

class Category(tag)

A category or "tag". This is a subclass of the built-in unicode type.

Model property: CategoryProperty

class MyModel(db.Model):
  category = db.CategoryProperty()

m = MyModel()
m.category = db.Category("kittens")

In XML, this is an Atom category element. See the Atom specification.

class Email(email)

An email address. This is a subclass of the built-in unicode type.

Neither the property class nor the value class perform validation of email addresses, they just store the value.

Model property: EmailProperty

class MyModel(db.Model):
  email_address = db.EmailProperty()

m = MyModel()
m.email_address = db.Email("larry@example.com")

In XML, this is a gd:email element. See the GData API reference.

class GeoPt(lat, lon=None)

A geographical point represented by floating-point latitude and longitude coordinates.

Model property: GeoPtProperty

In XML, this is a georss:point element. See georss.org.

class IM(protocol, address=None)

An instant messaging handle.

protocol is the canonical URL of the instant messaging service. Some possible values:

Protocol	Description
sip	SIP/SIMPLE
xmpp	XMPP/Jabber
http://aim.com/	AIM
http://icq.com/	ICQ
http://talk.google.com/	Google Talk
http://messenger.msn.com/	MSN Messenger
http://messenger.yahoo.com/	Yahoo Messenger
http://sametime.com/	Lotus Sametime
http://gadu-gadu.pl/	Gadu-Gadu
unknown	Unknown or unspecified

address is the handle's address.

Model property: IMProperty

class MyModel(db.Model):
  im = db.IMProperty()

m = MyModel()
m.im = db.IM("http://example.com/", "Larry97")

In XML, this is a gd:im element. See the GData API reference.

class Link(link)

A fully qualified URL. This is a subclass of the built-in unicode type.

Model property: LinkProperty

class MyModel(db.Model):
  link = db.LinkProperty()

m = MyModel()
m.link = db.Link("http://www.google.com/")

In XML, this is an Atom link element. See the Atom specification.

class PhoneNumber(phone)

A human-readable telephone number. This is a subclass of the built-in unicode type.

Model property: PhoneNumberProperty

class MyModel(db.Model):
  phone = db.PhoneNumberProperty()

m = MyModel()
m.phone = db.PhoneNumber("1 (206) 555-1212")

In XML, this is a gd.phoneNumber element. See the GData API reference.

class PostalAddress(address)

A postal address. This is a subclass of the built-in unicode type.

Model property: PostalAddressProperty

class MyModel(db.Model):
  address = db.PostalAddressProperty()

m = MyModel()
m.address = db.PostalAddress("1600 Ampitheater Pkwy., Mountain View, CA")

In XML, this is a gd:postalAddress element. See the GData API reference.

class Rating(rating)

A user-provided rating for a piece of content, as an integer between 0 and 100. This is a subclass of the built-in long type. The class validates that the value is an integer between 0 and 100, and raises a BadValueError if the value is invalid.

Model property: RatingProperty

class MyModel(db.Model):
  rating = db.RatingProperty()

m = MyModel()
m.rating = db.Rating(97)

In XML, this is a gd:rating element. See the GData API reference.

Property Classes

All model property classes provided by google.appengine.ext.db are subclasses of the Property base class, and support all of the base constructor's arguments. See the base class documentation for information about those arguments.

The google.appengine.ext.db package provides the following model property classes:

class BlobProperty(...)

A binary data property.

Blob data is a byte string. For text data, which may involve encoding, use TextProperty.

Value type: Blob

class BooleanProperty(...)

A Boolean property.

Value type: bool

class CategoryProperty(...)

A category or "tag," a descriptive word or phrase.

Value type: Category

class DateProperty(verbose_name=None, auto_now=False, auto_now_add=False, ...)

A date property, without a time of day. See DateTimeProperty for more information.

Value type: datetime.date. This is converted to a datetime.datetime internally.

class DateTimeProperty(verbose_name=None, auto_now=False, auto_now_add=False, ...)

A date and time property.

If auto_now is True, the property value is set to the current time whenever the model instance is stored in the datastore, overwriting the property's previous value. This is useful for tracking a "last modified" date and time for a model instance.

If auto_now_add is True, the property value is set to the current time the first time the model instance is stored in the datastore, unless the property has already been assigned a value. This is useful for storing a "created" date and time for a model instance.

Date-time values are stored as and returned using the UTC time zone. See datetime.datetime for a discussion of how to manage time zones.

Value type: datetime.datetime

class EmailProperty(...)

An email address.

Neither the property class nor the value class perform validation of email addresses, they just store the value.

Value type: Email

class FloatProperty(...)

A floating point number property.

Value type: float

class GeoPtProperty(...)

A geographical point represented by floating-point latitude and longitude coordinates.

Value type: GeoPt

class IMProperty(...)

An instant messaging handle.

Value type: IM

class IntegerProperty(...)

An integer property.

Python int values are converted to Python long values prior to storage. A value stored as an int will be returned as a long.

Value type: int or long

class LinkProperty(...)

A fully qualified URL.

Value type: Link

class ListProperty(item_type, verbose_name=None, default=None, ...)

A list of values of the type given as item_type.

In a query, comparing a list property to a value performs the test against the list members: list_property = value tests if the value appears anywhere in the list, list_property < value tests if any of the members of the list are less than the given value, and so forth.

A query cannot compare two list values. There is no way to test two lists for equality without testing each element for membership separately.

item_type is the type of the items in the list, as a Python type or class. All items in the list value must be of the given type. item_type must be one of the datastore value types, and cannot be list. See Datastore Value Types, above.

The value of a ListProperty static property cannot be None. It can, however, be an empty list.

Tip: Because ListProperty aggregate types do not use the Property classes, Property class features such as automatic values and validation are not applied automatically to members of the list value. If you want to validate a member value using a Property class, you can instantiate the class and call its validate() method on the value.

default is the default value for the list property. If None, the default is an empty list. A list property can define a custom validator to disallow the empty list.

See Entities and Models for more information on ListProperty and list values.

Value type: a Python list of zero or more values, where each value is of the configured type

class PhoneNumberProperty(...)

A human-readable telephone number.

Value type: PhoneNumber

class PostalAddressProperty(...)

A postal address.

Value type: PostalAddress

class RatingProperty()

A user-provided rating for a piece of content, as an integer between 0 and 100.

Value type: Rating

class ReferenceProperty(reference_class=None, verbose_name=None, collection_name=None, ...)

A reference to another model instance. For example, a reference may indicate a many-to-one relationship between the model with the property and the model referenced by the property.

reference_class is the model class of the model instance being referenced. If specified, only model instances of the class can be assigned to this property. If None, any model instance can be the value of this property.

collection_name is the name of the property to give to the referenced model class whose value is a Query for all entities that reference the entity. If no collection_name is set, then modelname_set (with the name of the model in lowercase letters and "_set" added) is used.

Note: collection_name must be set if there are multiple properties within the same model referencing the same model class. Otherwise, a DuplicatePropertyError will be raised when the default names are generated.

ReferenceProperty automatically references and dereferences model instances as property values: A model instance can be assigned to a ReferenceProperty directly, and its key will be used. The ReferenceProperty value can be used as if it were a model instance, and the datastore entity will be fetched and the model instance created when it is first used in this way. Untouched reference properties do not query for unneeded data.

class Author(db.Model):
  name = db.StringProperty()

class Story(db.Model):
  author = db.ReferenceProperty(Author)

story = db.get(story_key)
author_name = story.author.name

As with a Key value, it is possible for a reference property value to refer to a data entity that does not exist. If a referenced entity is deleted from the datastore, references to the entity are not updated. An application can explicitly db.get() the value of a ReferenceProperty (which is a Key) to test whether the referenced entity exists.

Deleting an entity does not delete entities referred to by a ReferenceProperty.

See also this introduction to reference properties.

Value type: db.Key (see above)

class SelfReferenceProperty(verbose_name=None, collection_name=None, ...)

A reference to another model instance of the same class. See ReferenceProperty.

Value type: db.Key (see above)

class StringListProperty(verbose_name=None, default=None, ...)

A ListProperty of Python str or unicode (basestring) values. See ListProperty.

Value type: a Python list of str or unicode values

class StringProperty(verbose_name=None, multiline=False, ...)

A short string property. Takes a Python str or unicode (basestring) value of 500 bytes or less.

StringProperty property values are indexed, and can be used in filters and sort orders.

If multiline is False, then the value cannot include linefeed characters. The djangoforms library uses this to enforce a difference between text fields and textarea fields in the data model, and others can use it for a similar purpose.

Value type: str or unicode

class TextProperty()

A long string.

Unlike StringProperty, a TextProperty value can be more than 500 bytes long. However, TextProperty values are not indexed, and cannot be used in filters or sort orders.

TextProperty values store text with a text encoding. For binary data, use BlobProperty.

Value type: Text

class TimeProperty(verbose_name=None, auto_now=False, auto_now_add=False, ...)

A time property, without a date. Takes a Python standard library datetime.time value. See DateTimeProperty for more information.

Value type: datetime.time. This is converted to a datetime.datetime internally.

class UserProperty()

A user with a Google account.

The UserProperty constructor does not accept a default value.

Value type: users.User (see above)