IPython/core/component.py

#!/usr/bin/env python
# encoding: utf-8
"""
A lightweight component system for IPython.

Authors:

* Brian Granger
* Fernando Perez
"""

#-----------------------------------------------------------------------------
#  Copyright (C) 2008-2009  The IPython Development Team
#
#  Distributed under the terms of the BSD License.  The full license is in
#  the file COPYING, distributed as part of this software.
#-----------------------------------------------------------------------------

#-----------------------------------------------------------------------------
# Imports
#-----------------------------------------------------------------------------

from copy import deepcopy
import datetime
from weakref import WeakValueDictionary

from IPython.utils.ipstruct import Struct
from IPython.utils.traitlets import (
    HasTraitlets, TraitletError, MetaHasTraitlets, Instance, This
)


#-----------------------------------------------------------------------------
# Helper classes for Components
#-----------------------------------------------------------------------------


class ComponentError(Exception):
    pass

class MetaComponentTracker(type):
    """A metaclass that tracks instances of Components and its subclasses."""

    def __init__(cls, name, bases, d):
        super(MetaComponentTracker, cls).__init__(name, bases, d)
        cls.__instance_refs = WeakValueDictionary()
        cls.__numcreated = 0

    def __call__(cls, *args, **kw):
        """Called when *class* is called (instantiated)!!!
        
        When a Component or subclass is instantiated, this is called and
        the instance is saved in a WeakValueDictionary for tracking.
        """
        instance = cls.__new__(cls, *args, **kw)
        # Do this before __init__ is called so get_instances works inside
        # __init__ methods!
        for c in cls.__mro__:
            if issubclass(cls, c) and issubclass(c, Component):
                c.__numcreated += 1
                c.__instance_refs[c.__numcreated] = instance
        if isinstance(instance, cls):
            cls.__init__(instance, *args, **kw)

        return instance

    def get_instances(cls, name=None, root=None, classname=None):
        """Get all instances of cls and its subclasses.

        Parameters
        ----------
        name : str
            Limit to components with this name.
        root : Component or subclass
            Limit to components having this root.
        classname : str
            The string name of a class to match exactly.
        """
        instances = cls.__instance_refs.values()
        if name is not None:
            instances = [i for i in instances if i.name == name]
        if classname is not None:
            instances = [i for i in instances if i.__class__.__name__ == classname]
        if root is not None:
            instances = [i for i in instances if i.root == root]
        return instances

    def get_instances_by_condition(cls, call, name=None, root=None,
                                   classname=None):
        """Get all instances of cls, i such that call(i)==True.

        This also takes the ``name`` and ``root`` and ``classname`` 
        arguments of :meth:`get_instance`
        """
        return [i for i in cls.get_instances(name, root, classname) if call(i)]


class ComponentNameGenerator(object):
    """A Singleton to generate unique component names."""

    def __init__(self, prefix):
        self.prefix = prefix
        self.i = 0

    def __call__(self):
        count = self.i
        self.i += 1
        return "%s%s" % (self.prefix, count)


ComponentNameGenerator = ComponentNameGenerator('ipython.component')


class MetaComponent(MetaHasTraitlets, MetaComponentTracker):
    pass


#-----------------------------------------------------------------------------
# Component implementation
#-----------------------------------------------------------------------------


class Component(HasTraitlets):

    __metaclass__ = MetaComponent

    # Traitlets are fun!
    config = Instance(Struct,(),{})
    parent = This()
    root = This()
    created = None

    def __init__(self, parent, name=None, config=None):
        """Create a component given a parent and possibly and name and config.

        Parameters
        ----------
        parent : Component subclass
            The parent in the component graph.  The parent is used
            to get the root of the component graph.
        name : str
            The unique name of the component.  If empty, then a unique
            one will be autogenerated.
        config : Struct
            If this is empty, self.config = parent.config, otherwise
            self.config = config and root.config is ignored.  This argument
            should only be used to *override* the automatic inheritance of 
            parent.config.  If a caller wants to modify parent.config 
            (not override), the caller should make a copy and change 
            attributes and then pass the copy to this argument.
        
        Notes
        -----
        Subclasses of Component must call the :meth:`__init__` method of
        :class:`Component` *before* doing anything else and using 
        :func:`super`::
        
            class MyComponent(Component):
                def __init__(self, parent, name=None, config=None):
                    super(MyComponent, self).__init__(parent, name, config)
                    # Then any other code you need to finish initialization.

        This ensures that the :attr:`parent`, :attr:`name` and :attr:`config`
        attributes are handled properly.
        """
        super(Component, self).__init__()
        self._children = []
        if name is None:
            self.name = ComponentNameGenerator()
        else:
            self.name = name
        self.root = self # This is the default, it is set when parent is set
        self.parent = parent     
        if config is not None:
            self.config = deepcopy(config)
        else:
            if self.parent is not None:
                self.config = deepcopy(self.parent.config)

        self.created = datetime.datetime.now()

    #-------------------------------------------------------------------------
    # Static traitlet notifiations
    #-------------------------------------------------------------------------

    def _parent_changed(self, name, old, new):
        if old is not None:
            old._remove_child(self)
        if new is not None:
            new._add_child(self)

        if new is None:
            self.root = self
        else:
            self.root = new.root

    def _root_changed(self, name, old, new):
        if self.parent is None:
            if not (new is self):
                raise ComponentError("Root not self, but parent is None.")
        else:
            if not self.parent.root is new:
                raise ComponentError("Error in setting the root attribute: "
                                     "root != parent.root")

    def _config_changed(self, name, old, new):
        """Update all the class traits having a config_key with the config.

        For any class traitlet with a ``config_key`` metadata attribute, we
        update the traitlet with the value of the corresponding config entry.

        In the future, we might want to do a pop here so stale config info
        is not passed onto children.
        """
        # Get all traitlets with a config_key metadata entry
        traitlets = self.traitlets('config_key')
        for k, v in traitlets.items():
            try:
                config_value = new[v.get_metadata('config_key')]
            except KeyError:
                pass
            else:
                setattr(self, k, config_value)

    @property
    def children(self):
        """A list of all my child components."""
        return self._children

    def _remove_child(self, child):
        """A private method for removing children components."""
        if child in self._children:
            index = self._children.index(child)
            del self._children[index]

    def _add_child(self, child):
        """A private method for adding children components."""
        if child not in self._children:
            self._children.append(child)

    def __repr__(self):
        return "<Component('%s')>" % self.name