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Writing Property Subclasses | App Engine standard environment for Python 2

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The Property class is designed to be subclassed. However, it is normally easier to subclass an existing Property subclass.

All special Property attributes, even those considered 'public', have names starting with an underscore. This is because StructuredProperty uses the non-underscore attribute namespace to refer to nested Property names; this is essential for specifying queries on subproperties.

The Property class and its predefined subclasses allow subclassing using composable (or stackable) validation and conversion APIs. These require some terminology definitions:

• A user value is a value such as would be set and accessed by the application code using standard attributes on the entity.
• A base value is a value such as would be serialized to and deserialized from the Datastore.

A Property subclass that implements a specific transformation between user values and serializable values should implement two methods, _to_base_type() and _from_base_type(). These should not call their super() method. This is what is meant by composable (or stackable) APIs.

The API supports stacking classes with ever more sophisticated user-base conversions: the user-to-base conversion goes from more sophisticated to less sophisticated, while the base-to-user conversion goes from less sophisticated to more sophisticated. For example, see the relationship between BlobProperty, TextProperty, and StringProperty. For example, TextProperty inherits from BlobProperty; its code is pretty simple because it inherits most of the behavior it needs.

In addition to _to_base_type() and _from_base_type(), the _validate() method is also a composable API.

The validation API distinguishes between lax and strict user values. The set of lax values is a superset of the set of strict values. The _validate() method takes a lax value and if necessary converts it to a strict value. This means that when setting the property value, lax values are accepted, while when getting the property value, only strict values will be returned. If no conversion is needed, _validate() may return None. If the argument is outside the set of accepted lax values, _validate() should raise an exception, preferably TypeError or datastore_errors.BadValueError.

The _validate(), _to_base_type(), and _from_base_type() do not need to handle:

• None: They will not be called with None (and if they return None, this means that the value does not need conversion).
• Repeated values: The infrastructure takes care of calling _from_base_type() or _to_base_type() for each list item in a repeated value.
• Distinguishing user values from base values: The infrastructure handles this by calling the composable APIs.
• Comparisons: The comparison operations call _to_base_type() on their operand.
• Distinguishing between user and base values: the infrastructure guarantees that _from_base_type() will be called with an (unwrapped) base value, and that _to_base_type() will be called with a user value.

For example, suppose you need to store really long integers. The standard IntegerProperty only supports (signed) 64-bit integers. Your property might store a longer integer as a string; it would be good to have the property class handle the conversion. An application using your property class might look something like

This looks simple and straightforward. It also demonstrates the use of some standard property options (default, repeated); as the author of LongIntegerProperty, you will be glad to hear you don't have to write any "boilerplate" to get those working. It's easier to define a subclass of another property, for example:

When you set a property value on an entity, e.g. ent.abc = 42, your _validate() method is called, and (if it doesn't raise an exception) the value is stored on the entity. When you write the entity to the Datastore, your _to_base_type() method is called, converting the value to the string. Then that value is serialized by the base class, StringProperty. The inverse chain of events happens when the entity is read back from the Datastore. The StringProperty and Property classes together take care of the other details, such as serializing the and deserializing the string, setting the default, and handling repeated property values.

In this example, supporting inequalities (i.e. queries using <, <=, >, >=) requires more work. The following example implementation imposes a maximum size of integer and stores values as fixed-length strings:

This can be used in the same way as LongIntegerProperty except that you must pass the number of bits to the property constructor, e.g. BoundedLongIntegerProperty(1024).

You can subclass other property types in similar ways.

This approach also works for storing structured data. Suppose you have a FuzzyDate Python class that represents a date range; it uses fields first and last to store the date range's beginning and end:

You can create a FuzzyDateProperty that derives from StructuredProperty. Unfortunately, the latter doesn't work with plain old Python classes; it needs a Model subclass. So define a Model subclass as an intermediate representation;

Next, construct a subclass of StructuredProperty that hardcodes the modelclass argument to be FuzzyDateModel, and defines _to_base_type() and _from_base_type() methods to convert between FuzzyDate and FuzzyDateModel:

An application might use this class like so:

Suppose you want to accept plain date objects in addition to FuzzyDate objects as the values for FuzzyDateProperty. To do this, modify the _validate() method as follows:

You could instead subclass FuzzyDateProperty as follows (assuming FuzzyDateProperty._validate() is as shown above).

When you assign a value to a MaybeFuzzyDateProperty field, both MaybeFuzzyDateProperty._validate() and FuzzyDateProperty._validate() are invoked, in that order. The same applies to _to_base_type() and _from_base_type(): the methods in in superclass and subclass are implicitly combined. (Don't use super to control inherited behavior for this. For these three methods, the interaction is subtle and super doesn't do what you want.)

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2025-08-07 UTC.

[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-07 UTC."],[[["This API is designed for use exclusively within first-generation runtimes in the App Engine standard environment, with specific migration guidance available for those updating to the App Engine Python 3 runtime."],["The `Property` class, while designed for subclassing, is typically easier to extend by subclassing an existing `Property` subclass rather than the base class."],["Composable APIs (`_validate`, `_to_base_type`, and `_from_base_type`) facilitate the creation of custom `Property` subclasses, allowing for stackable validation and value conversion between user values and base values."],["The validation process distinguishes between lax and strict user values, accepting lax values during property value setting and returning only strict values upon property value retrieval."],["Subclassing `Property` allows for the handling of complex data types and structures, such as long integers or date ranges, with custom validation and conversion logic."]]],[]]

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