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# Traitlets
[](https://travis-ci.org/ipython/traitlets)
[](http://traitlets.readthedocs.org/en/latest/?badge=latest)
Traitlets is 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.
Its implementation relies on the [descriptor](https://docs.python.org/howto/descriptor.html)
pattern.
Traitlets 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](https://pip.pypa.io/en/stable/installing/) and run:
```bash
pip install traitlets
```
For a **development installation**, clone this repository, change into the
`traitlets` root directory, and run pip:
```bash
git clone https://github.com/ipython/traitlets.git
cd traitlets
pip install -e .
```
## Running the tests
```bash
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 their properties, see the
[Trait Types](https://traitlets.readthedocs.io/en/latest/trait_types.html)
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. In this example, the `_username_default`
method is decorated with `@default('username')`:
```Python
import getpass
from traitlets import HasTraits, Unicode, default
class Identity(HasTraits):
username = Unicode()
@default('username')
def _username_default(self):
return getpass.getuser()
```
### Callbacks when a trait attribute changes
When a trait changes, an application can follow this trait change with
additional actions.
To do something when a trait attribute is changed, decorate a method with
[`traitlets.observe()`](https://traitlets.readthedocs.io/en/latest/api.html?highlight=observe#traitlets.observe).
The method will be called with a single argument, a dictionary which contains
an owner, new value, old value, name of the changed trait, and the event type.
In this example, the `_num_changed` method is decorated with ``@observe(`num`)``:
```Python
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))
```
and is passed the following dictionary when called:
```Python
{
'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'
}
```
### Validation and coercion
Each trait type (`Int`, `Unicode`, `Dict` etc.) may have its own validation or
coercion logic. In addition, we can register custom cross-validators
that may depend on the state of other attributes. For example:
```Python
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.
|
