Source code for soma.factory

# -*- coding: utf-8 -*-

from __future__ import absolute_import
from __future__ import print_function

import six
from importlib import import_module
from pkgutil import iter_modules


def find_subclasses_in_module(module_name, parent_class):
    '''
    Finds all the classes defined in a Python module that derive from a
    given class or class name. If the module is a package, it also look
    into submodules.
    '''
    if isinstance(parent_class, six.string_types):
        check = lambda item: (isinstance(item, type) and 
                              item.__module__ == module_name and 
                              parent_class in (i.__name__ 
                                               for i in item.__mro__))
    else:
        check = lambda item: (isinstance(item, type) and 
                              item.__module__ == module_name and 
                              issubclass(item, parent_class))
    for i in find_items_in_module(module_name, check):
        yield i


def find_items_in_module(module_name, check):
    '''
    Finds all the items defined in a Python module that where *check(cls)* is
    *True*. If the module is a package, it also look recursively into
    submodules.
    '''
    try:
        module = import_module(module_name)
    except ImportError:
        return
    
    for i in six.itervalues(module.__dict__):
        if check(i):
            yield i
    path = getattr(module, '__path__', None)
    if path:
        for importer, submodule_name, ispkg in iter_modules(path):
            for j in find_items_in_module('%s.%s' % 
                    (module.__name__, submodule_name), check):
                yield j


class ClassFactory(object):
    '''
    *ClassFactory* is the base class for creating factories that can look
    for classes in Python modules and create instances.
    '''
    
    def __init__(self, class_types={}):
        # List of Python modules where classes are looked for
        self.module_path = []
        # Instance-level association between a class_type (which is a string)
        # and the corresponding class. There can also be class-level
        # class_types (see get_class method).
        self.class_types = {}
        # Cache of instances returned by get method. This is a dictionary
        # whose keys are (class_type, factory_id) and values are instances.
        self.instances = {}
    
    def find_class(self, class_type, factory_id):
        '''
        Finds a class deriving of the class corresponding to *class_type* and
        whose *factory_id* attribute has a given value. Look for all
        subclasses of parent class declared in the modules of
        :attr:`self.module_path`
        and returns the first one having ``cls.factory_id == factory_id``. If
        none is found, returns *None*.
        '''
        base_class = self.get_class(class_type)
        for module_name in self.module_path:
            for cls in find_subclasses_in_module(module_name, base_class):
                if cls.factory_id == factory_id:
                    return cls
        return None
    
    def get_class(self, class_type):
        '''
        Returns the class corresponding to a given class type. First look
        for a dictionary in self.class_types, then look into parent
        classes.
        '''
        class_types = getattr(self, 'class_types', None)
        if class_types:
            cls = class_types.get(class_type)
            if cls is not None:
                return cls
        for parent_class in self.__class__.__mro__:
            class_types = getattr(parent_class, 'class_types', None)
            if class_types:
                cls = class_types.get(class_type)
                if cls is not None:
                    return cls
        raise ValueError('Unknown class type: %s' % class_type)



    def get(self, class_type, factory_id):
        '''
        Returns an instance of the class identified by *class_type* and
        *factory_id*. There can be only one instance per *class_type* and
        *factory_id*. Once created with the first call of this method, it
        is stored in :attr:`self.instances` and simply returned in subsequent
        calls.

        If *get_class* is *True*, returns the class instead of an instance
        '''
        instance = self.instances.get((class_type, factory_id))
        if instance is None:
            cls = self.find_class(class_type, factory_id)
            if cls is not None:
                if hasattr(cls.__init__, '__code__') \
                        and cls.__init__.__code__.co_argcount != 1:
                    # The class constructor takes arguments: we cannot
                    # instantiate it: register the class itself
                    instance = cls
                else:
                    instance = cls()
                self.instances[(class_type, factory_id)] = instance
            else:
                raise ValueError('Cannot find a class for class type "%s" '
                                 'and factory id "%s"' % (class_type, 
                                                          factory_id))
        return instance