Traitlets

Traitlets are a pure Python library enabling

• the enforcement of strong typing for attributes of Python objects (typed attributes are called "traits"),
• notifications on changes of trait attributes,
• automatic validation and coercion of trait attributes when attempting a change.

The implementation relies on the descriptor pattern. This package powers the configuration system of IPython and Jupyter, and the declarative API of IPython interactive widgets.

Installation

For a local installation, make sure you have pip installed and run:

pip install traitlets

For a development installation:

• Clone this repository and cd into it
• pip install -e .

Running the tests

pip install "traitlets[test]"
py.test traitlets

Usage

Any class with trait attributes must inherit from HasTraits. For the list of available trait types and ther properties, see the Trait Types section of the documentation.

Dynamic default values

To calculate a default value dynamically, decorate a method of your class with @default({traitname})`. This method will be called on the instance, and should return the default value. For example:

import getpass
from traitlets import HasTraits, Unicode, default

class Identity(HasTraits):
    username = Unicode()

    @default('username')
    def _username_default(self):
        return getpass.getuser()

Callbacks when trait attributes change

To do something when a trait attribute is changed, decorate a method with traitlets.observe(). The method will be called with a single argument, a dictionary of the form:

{
  'owner': object, # The HasTraits instance
  'new': 6, # The new value
  'old': 5, # The old value
  'name': "foo", # The name of the changed trait
  'type': 'change', # The event type of the notification, usually 'change'
}

For example:

from traitlets import HasTraits, Integer, observe

class TraitletsExample(HasTraits):
    num = Integer(5, help="a number").tag(config=True)

    @observe('num')
    def _num_changed(self, change):
        print("{name} changed from {old} to {new}".format(**change))

Validation and coercion

Each trait type (Int, Unicode, Dict etc.) may have its own validation or coercion logic, in addition to which we can register custom cross-validators that may depend on the state of other attributes.

from traitlets import HasTraits, TraitError, Int, Bool, validate

class Parity(HasTraits):
    value = Int()
    parity = Int()

    @validate('value')
    def _valid_value(self, proposal):
        if proposal['value'] % 2 != self.parity:
            raise TraitError('value and parity should be consistent')
        return proposal['value']

    @validate('parity')
    def _valid_parity(self, proposal):
        parity = proposal['value']
        if parity not in [0, 1]:
            raise TraitError('parity should be 0 or 1')
        if self.value % 2 != parity:
            raise TraitError('value and parity should be consistent')
        return proposal['value']

parity_check = Parity(value=2)

# Changing required parity and value together while holding cross validation
with parity_check.hold_trait_notifications():
    parity_check.value = 1
    parity_check.parity = 1

However, we recommend that custom cross-validators don't modify the state of the HasTraits instance.