Traitlets Python configuration system
Project description
Traitlets
| home | https://github.com/ipython/traitlets |
| pypi-repo | https://pypi.org/project/traitlets/ |
| docs | https://traitlets.readthedocs.io/ |
| license | Modified BSD License |
Traitlets is a pure Python library enabling:
- the enforcement of strong typing for attributes of Python objects (typed attributes are called "traits");
- dynamically calculated default values;
- automatic validation and coercion of trait attributes when attempting a change;
- registering for receiving notifications when trait values change;
- reading configuring values from files or from command line arguments - a distinct layer on top of traitlets, so you may use traitlets without the configuration machinery.
Its implementation relies on the descriptor pattern, and it is a lightweight pure-python alternative of the traits library.
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 and run:
pip install traitlets
For a development installation, clone this repository, change into the
traitlets root directory, and run pip:
git clone https://github.com/ipython/traitlets.git
cd traitlets
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 their 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. In this example, the _username_default
method is decorated with @default('username'):
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().
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`):
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:
{
'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:
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.
Release build:
$ pip install pep517
$ python -m pep517.build .
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