bokeh.core.property.descriptor_factory

Provide a Base class for all Bokeh properties.

Bokeh properties work by contributing Python descriptor objects to HasProps classes. These descriptors then delegate attribute access back to the Bokeh property class, which handles validation, serialization, and documentation needs.

The PropertyDescriptorFactory class provides two methods autocreate and make_descriptors that are used by the metaclass MetaHasProps during class creation to create and install the necessary descriptors corresponding to the declared properties.

This machinery helps to make Bokeh much more user friendly. For example, the DataSpec properties mediate between fixed values and references to column data source columns. A user can use a very simple syntax, and the property will correctly serialize and validate automatically:

from bokeh.models import Circle

c = Circle()

c.x = 10      # serializes to {'value': 10}

c.x = 'foo'   # serializes to {'field': 'foo'}

c.x = [1,2,3] # raises a ValueError validation exception

There are many other examples like this throughout Bokeh. In this way users may operate simply and naturally, and not be concerned with the low-level details around validation, serialization, and documentation.

Note

These classes form part of the very low-level machinery that implements the Bokeh model and property system. It is unlikely that any of these classes or their methods will be applicable to any standard usage or to anyone who is not directly developing on Bokeh’s own infrastructure.

class PropertyDescriptorFactory

Base class for all Bokeh properties.

A Bokeh property really consist of two parts: the familiar “property” portion, such as Int, String, etc., as well as an associated Python descriptor that delegates attribute access (e.g. range.start) to the property instance.

Consider the following class definition:

from bokeh.model import Model
from bokeh.core.properties import Int

class SomeModel(Model):
    foo = Int(default=10)

Then we can observe the following:

>>> m = SomeModel()

# The class itself has had a descriptor for 'foo' installed
>>> getattr(SomeModel, 'foo')
<bokeh.core.property.descriptors.BasicPropertyDescriptor at 0x1065ffb38>

# which is used when 'foo' is accessed on instances
>>> m.foo
10

classmethod autocreate()

Called by the MetaHasProps metaclass to create a new instance of this descriptor when the property is assigned using only the property type. For example:

class Foo(Model):

    bar = String   # no parens used here

make_descriptors(name)

Return a list of PropertyDescriptor instances to install on a class, in order to delegate attribute access to this property.

Parameters

name (str) – the name of the property these descriptors are for

Returns

list[PropertyDescriptor]

The descriptors returned are collected by the MetaHasProps metaclass and added to HasProps subclasses during class creation.

Subclasses of PropertyDescriptorFactory are responsible for implementing this function to return descriptors specific to their needs.