# Copyright (c) 2012-2013 Mitch Garnaat http://garnaat.org/
# Copyright 2012-2014 Amazon.com, Inc. or its affiliates. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License"). You
# may not use this file except in compliance with the License. A copy of
# the License is located at
#
# http://aws.amazon.com/apache2.0/
#
# or in the "license" file accompanying this file. This file is
# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
# ANY KIND, either express or implied. See the License for the specific
# language governing permissions and limitations under the License.
"""
This module contains the main interface to the botocore package, the
Session object.
"""

import copy
import logging
import os
import platform
import shlex

from . import __version__
from . import config as bc_config
from . import credentials as bc_credentials
from . import client as bc_client
from .endpoint import EndpointCreator
from .exceptions import EventNotFound, ConfigNotFound, ProfileNotFound
from . import handlers
from .hooks import HierarchicalEmitter, first_non_none_response
from .loaders import Loader
from .provider import get_provider
from . import regions
from .model import ServiceModel
from . import service as bc_service
from . import waiter


class Session(object):
    """
    The Session object collects together useful functionality
    from `botocore` as well as important data such as configuration
    information and credentials into a single, easy-to-use object.

    :ivar available_profiles: A list of profiles defined in the config
        file associated with this session.
    :ivar profile: The current profile.
    """

    AllEvents = {
        'after-call': '.%s.%s',
        'after-parsed': '.%s.%s.%s.%s',
        'before-parameter-build': '.%s.%s',
        'before-call': '.%s.%s',
        'service-created': '',
        'service-data-loaded': '.%s',
        'creating-endpoint': '.%s',
        'before-auth': '.%s',
        'needs-retry': '.%s.%s',
    }
    """
    A dictionary where each key is an event name and the value
    is the formatting string used to construct a new event.
    """

    SessionVariables = {
        # logical:  config_file, env_var,        default_value
        'profile': (None, 'BOTO_DEFAULT_PROFILE', None),
        'region': ('region', 'BOTO_DEFAULT_REGION', None),
        'data_path': ('data_path', 'BOTO_DATA_PATH', None),
        'config_file': (None, 'AWS_CONFIG_FILE', '~/.aws/config'),
        'provider': ('provider', 'BOTO_PROVIDER_NAME', 'aws'),

        # These variables are intended for internal use so don't have any
        # user settable values.
        # This is the shared credentials file amongst sdks.
        'credentials_file': (None, None, '~/.aws/credentials'),

        # These variables only exist in the config file.

        # This is the number of seconds until we time out a request to
        # the instance metadata service.
        'metadata_service_timeout': ('metadata_service_timeout', None, 1),
        # This is the number of request attempts we make until we give
        # up trying to retrieve data from the instance metadata service.
        'metadata_service_num_attempts': ('metadata_service_num_attempts',
                                          None, 1),
        }
    """
    A default dictionary that maps the logical names for session variables
    to the specific environment variables and configuration file names
    that contain the values for these variables.

    When creating a new Session object, you can pass in your own dictionary to
    remap the logical names or to add new logical names.  You can then get the
    current value for these variables by using the ``get_config_variable``
    method of the :class:`botocore.session.Session` class.
    The default set of logical variable names are:

    * profile - Default profile name you want to use.
    * region - Default region name to use, if not otherwise specified.
    * data_path - Additional directories to search for data files.
    * config_file - Location of a Boto config file.
    * provider - The name of the service provider (e.g. aws)

    These form the keys of the dictionary.  The values in the dictionary
    are tuples of (<config_name>, <environment variable>, <default value).
    The ``profile`` and ``config_file`` variables should always have a
    None value for the first entry in the tuple because it doesn't make
    sense to look inside the config file for the location of the config
    file or for the default profile to use.

    The ``config_name`` is the name to look for in the configuration file,
    the ``env var`` is the OS environment variable (``os.environ``) to
    use, and ``default_value`` is the value to use if no value is otherwise
    found.
    """

    FmtString = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'

    def __init__(self, session_vars=None, event_hooks=None,
                 include_builtin_handlers=True, loader=None):
        """
        Create a new Session object.

        :type session_vars: dict
        :param session_vars: A dictionary that is used to override some or all
            of the environment variables associated with this session.  The
            key/value pairs defined in this dictionary will override the
            corresponding variables defined in ``SessionVariables``.

        :type event_hooks: BaseEventHooks
        :param event_hooks: The event hooks object to use. If one is not
            provided, an event hooks object will be automatically created
            for you.

        :type include_builtin_handlers: bool
        :param include_builtin_handlers: Indicates whether or not to
            automatically register builtin handlers.
        """
        self.session_var_map = copy.copy(self.SessionVariables)
        if session_vars:
            self.session_var_map.update(session_vars)
        if event_hooks is None:
            self._events = HierarchicalEmitter()
        else:
            self._events = event_hooks
        if include_builtin_handlers:
            self._register_builtin_handlers(self._events)
        self.user_agent_name = 'Botocore'
        self.user_agent_version = __version__
        self.user_agent_extra = ''
        self._profile = None
        self._config = None
        self._credentials = None
        self._profile_map = None
        self._provider = None
        # This is a dict that stores per session specific config variable
        # overrides via set_config_variable().
        self._session_instance_vars = {}
        if loader is None:
            loader = Loader()
        self._loader = loader
        # _data_paths_added is used to track whether or not we added
        # extra paths to the loader.  We will do this lazily
        # only when we ask for the loader.
        self._data_paths_added = False
        self._components = ComponentLocator()
        self._register_components()

    def _register_components(self):
        self._register_credential_provider()
        self._register_data_loader()
        self._register_endpoint_resolver()
        self._register_event_emitter()

    def _register_event_emitter(self):
        self._components.register_component('event_emitter', self._events)

    def _register_credential_provider(self):
        self._components.lazy_register_component(
            'credential_provider',
            lambda:  bc_credentials.create_credential_resolver(self))

    def _register_data_loader(self):
        self._components.lazy_register_component(
            'data_loader',
            lambda:  Loader(self.get_config_variable('data_path') or ''))

    def _register_endpoint_resolver(self):
        self._components.lazy_register_component(
            'endpoint_resolver',
            lambda:  regions.EndpointResolver(self.get_data('aws/_endpoints')))

    def _reset_components(self):
        self._register_components()

    def _register_builtin_handlers(self, events):
        for spec in handlers.BUILTIN_HANDLERS:
            if len(spec) == 2:
                event_name, handler = spec
                self.register(event_name, handler)
            else:
                event_name, handler, register_type = spec
                if register_type is handlers.REGISTER_FIRST:
                    self._events.register_first(event_name, handler)
                elif register_first is handlers.REGISTER_LAST:
                    self._events.register_last(event_name, handler)

    @property
    def provider(self):
        if self._provider is None:
            self._provider = get_provider(
                self, self.get_config_variable('provider'))
        return self._provider

    @property
    def available_profiles(self):
        return list(self._build_profile_map().keys())

    def _build_profile_map(self):
        # This will build the profile map if it has not been created,
        # otherwise it will return the cached value.  The profile map
        # is a list of profile names, to the config values for the profile.
        if self._profile_map is None:
            self._profile_map = self.full_config['profiles']
        return self._profile_map

    @property
    def profile(self):
        return self._profile

    @profile.setter
    def profile(self, profile):
        # Since provider can be specified in profile, changing the
        # profile should reset the provider.
        self._provider = None
        self._profile = profile
        # Need to potentially reload the config file/creds.
        self._reset_components()

    def get_config_variable(self, logical_name,
                            methods=('instance', 'env', 'config'),
                            default=None):
        """
        Retrieve the value associated with the specified logical_name
        from the environment or the config file.  Values found in the
        environment variable take precedence of values found in the
        config file.  If no value can be found, a None will be returned.

        :type logical_name: str
        :param logical_name: The logical name of the session variable
            you want to retrieve.  This name will be mapped to the
            appropriate environment variable name for this session as
            well as the appropriate config file entry.

        :type method: tuple
        :param method: Defines which methods will be used to find
            the variable value.  By default, all available methods
            are tried but you can limit which methods are used
            by supplying a different value to this parameter.
            Valid choices are: instance|env|config

        :param default: The default value to return if there is no
            value associated with the config file.  This value will
            override any default value specified in ``SessionVariables``.

        :returns: str value of variable of None if not defined.

        """
        value = None
        # There's two types of defaults here.  One if the
        # default value specified in the SessionVariables.
        # The second is an explicit default value passed into this
        # function (the default parameter).
        # config_default is tracking the default value specified
        # in the SessionVariables.
        config_default = None
        if logical_name in self.session_var_map:
            # Short circuit case, check if the var has been explicitly
            # overriden via set_config_variable.
            if 'instance' in methods and \
                    logical_name in self._session_instance_vars:
                return self._session_instance_vars[logical_name]
            config_name, envvar_name, config_default = self.session_var_map[
                logical_name]
            if logical_name in ('config_file', 'profile'):
                config_name = None
            if logical_name == 'profile' and self._profile:
                value = self._profile
            elif 'env' in methods and envvar_name and envvar_name in os.environ:
                value = os.environ[envvar_name]
            elif 'config' in methods:
                if config_name:
                    config = self.get_scoped_config()
                    value = config.get(config_name)
        # If we don't have a value at this point, we need to try to assign
        # a default value.  An explicit default argument will win over the
        # default value from SessionVariables.
        if value is None and default is not None:
            value = default
        if value is None and config_default is not None:
            value = config_default
        return value

    def set_config_variable(self, logical_name, value):
        """Set a configuration variable to a specific value.

        By using this method, you can override the normal lookup
        process used in ``get_config_variable`` by explicitly setting
        a value.  Subsequent calls to ``get_config_variable`` will
        use the ``value``.  This gives you per-session specific
        configuration values.

        ::
            >>> # Assume logical name 'foo' maps to env var 'FOO'
            >>> os.environ['FOO'] = 'myvalue'
            >>> s.get_config_variable('foo')
            'myvalue'
            >>> s.set_config_variable('foo', 'othervalue')
            >>> s.get_config_variable('foo')
            'othervalue'

        :type logical_name: str
        :param logical_name: The logical name of the session variable
            you want to set.  These are the keys in ``SessionVariables``.
        :param value: The value to associate with the config variable.

        """
        self._session_instance_vars[logical_name] = value


    def get_scoped_config(self):
        """
        Returns the config values from the config file scoped to the current
        profile.

        The configuration data is loaded **only** from the config file.
        It does not resolve variables based on different locations
        (e.g. first from the session instance, then from environment
        variables, then from the config file).  If you want this lookup
        behavior, use the ``get_config_variable`` method instead.

        Note that this configuration is specific to a single profile (the
        ``profile`` session variable).

        If the ``profile`` session variable is set and the profile does
        not exist in the config file, a ``ProfileNotFound`` exception
        will be raised.

        :raises: ConfigNotFound, ConfigParseError, ProfileNotFound
        :rtype: dict

        """
        profile_name = self.get_config_variable('profile')
        profile_map = self._build_profile_map()
        # If a profile is not explicitly set return the default
        # profile config or an empty config dict if we don't have
        # a default profile.
        if profile_name is None:
            return profile_map.get('default', {})
        elif profile_name not in profile_map:
            # Otherwise if they specified a profile, it has to
            # exist (even if it's the default profile) otherwise
            # we complain.
            raise ProfileNotFound(profile=profile_name)
        else:
            return profile_map[profile_name]

    @property
    def full_config(self):
        """Return the parsed config file.

        The ``get_config`` method returns the config associated with the
        specified profile.  This property returns the contents of the
        **entire** config file.

        :rtype: dict
        """
        if self._config is None:
            try:
                config_file = self.get_config_variable('config_file')
                self._config = bc_config.load_config(config_file)
            except ConfigNotFound:
                self._config = {'profiles': {}}
            try:
                # Now we need to inject the profiles from the
                # credentials file.  We don't actually need the values
                # in the creds file, only the profile names so that we
                # can validate the user is not referring to a nonexistent
                # profile.
                cred_file = self.get_config_variable('credentials_file')
                cred_profiles = bc_config.raw_config_parse(cred_file)
                for profile in cred_profiles:
                    cred_vars = cred_profiles[profile]
                    if profile not in self._config['profiles']:
                        self._config['profiles'][profile] = cred_vars
                    else:
                        self._config['profiles'][profile].update(cred_vars)
            except ConfigNotFound:
                pass
        return self._config

    def set_credentials(self, access_key, secret_key, token=None):
        """
        Manually create credentials for this session.  If you would
        prefer to use botocore without a config file, environment variables,
        or IAM roles, you can pass explicit credentials into this
        method to establish credentials for this session.

        :type access_key: str
        :param access_key: The access key part of the credentials.

        :type secret_key: str
        :param secret_key: The secret key part of the credentials.

        :type token: str
        :param token: An option session token used by STS session
            credentials.
        """
        self._credentials = bc_credentials.Credentials(access_key,
                                                             secret_key,
                                                             token)

    def get_credentials(self):
        """
        Return the :class:`botocore.credential.Credential` object
        associated with this session.  If the credentials have not
        yet been loaded, this will attempt to load them.  If they
        have already been loaded, this will return the cached
        credentials.

        """
        if self._credentials is None:
            self._credentials = self._components.get_component(
                'credential_provider').load_credentials()
        return self._credentials

    def user_agent(self):
        """
        Return a string suitable for use as a User-Agent header.
        The string will be of the form:

        <agent_name>/<agent_version> Python/<py_ver> <plat_name>/<plat_ver>

        Where:

         - agent_name is the value of the `user_agent_name` attribute
           of the session object (`Boto` by default).
         - agent_version is the value of the `user_agent_version`
           attribute of the session object (the botocore version by default).
           by default.
         - py_ver is the version of the Python interpreter beng used.
         - plat_name is the name of the platform (e.g. Darwin)
         - plat_ver is the version of the platform

        If ``user_agent_extra`` is not empty, then this value will be
        appended to the end of the user agent string.

        """
        base = '%s/%s Python/%s %s/%s' % (self.user_agent_name,
                                          self.user_agent_version,
                                          platform.python_version(),
                                          platform.system(),
                                          platform.release())
        if self.user_agent_extra:
            base += ' %s' % self.user_agent_extra
        return base

    def get_data(self, data_path):
        """
        Retrieve the data associated with `data_path`.

        :type data_path: str
        :param data_path: The path to the data you wish to retrieve.
        """
        return self.get_component('data_loader').load_data(data_path)

    def get_service_model(self, service_name, api_version=None):
        """Get the service model object.

        :type service_name: string
        :param service_name: The service name

        :type api_version: string
        :param api_version: The API version of the service.  If none is
            provided, then the latest API version will be used.

        :rtype: L{botocore.model.ServiceModel}
        :return: The botocore service model for the service.

        """
        service_description = self.get_service_data(service_name, api_version)
        return ServiceModel(service_description)

    def get_waiter_model(self, service_name, api_version=None):
        loader = self.get_component('data_loader')
        latest = loader.determine_latest('%s/%s' % (
            self.provider.name, service_name), api_version)
        waiter_path = latest.replace('.api', '.waiters')
        waiter_config = loader.load_data(waiter_path)
        return waiter.WaiterModel(waiter_config)

    def get_service_data(self, service_name, api_version=None):
        """
        Retrieve the fully merged data associated with a service.
        """
        data_path = '%s/%s' % (self.provider.name, service_name)
        service_data = self.get_component('data_loader').load_service_model(
            data_path,
            api_version=api_version
        )
        event_name = self.create_event('service-data-loaded', service_name)
        self._events.emit(event_name, service_data=service_data,
                          service_name=service_name, session=self)
        return service_data

    def get_available_services(self):
        """
        Return a list of names of available services.
        """
        data_path = '%s' % self.provider.name
        return self.get_component('data_loader')\
                .list_available_services(data_path)

    def get_service(self, service_name, api_version=None):
        """
        Get information about a service.

        :type service_name: str
        :param service_name: The name of the service (e.g. 'ec2')

        :returns: :class:`botocore.service.Service`
        """
        service = bc_service.get_service(self, service_name,
                                               self.provider,
                                               api_version=api_version)
        event = self.create_event('service-created')
        self._events.emit(event, service=service)
        return service

    def set_debug_logger(self, logger_name='botocore'):
        """
        Convenience function to quickly configure full debug output
        to go to the console.
        """
        self.set_stream_logger(logger_name, logging.DEBUG)

    def set_stream_logger(self, logger_name, log_level, stream=None,
                          format_string=None):
        """
        Convenience method to configure a stream logger.

        :type logger_name: str
        :param logger_name: The name of the logger to configure

        :type log_level: str
        :param log_level: The log level to set for the logger.  This
            is any param supported by the ``.setLevel()`` method of
            a ``Log`` object.

        :type stream: file
        :param stream: A file like object to log to.  If none is provided
            then sys.stderr will be used.

        :type format_string: str
        :param format_string: The format string to use for the log
            formatter.  If none is provided this will default to
            ``self.FmtString``.

        """
        log = logging.getLogger(logger_name)
        log.setLevel(logging.DEBUG)

        ch = logging.StreamHandler(stream)
        ch.setLevel(log_level)

        # create formatter
        if format_string is None:
            format_string = self.FmtString
        formatter = logging.Formatter(format_string)

        # add formatter to ch
        ch.setFormatter(formatter)

        # add ch to logger
        log.addHandler(ch)

    def set_file_logger(self, log_level, path, logger_name='botocore'):
        """
        Convenience function to quickly configure any level of logging
        to a file.

        :type log_level: int
        :param log_level: A log level as specified in the `logging` module

        :type path: string
        :param path: Path to the log file.  The file will be created
            if it doesn't already exist.
        """
        log = logging.getLogger(logger_name)
        log.setLevel(logging.DEBUG)

        # create console handler and set level to debug
        ch = logging.FileHandler(path)
        ch.setLevel(log_level)

        # create formatter
        formatter = logging.Formatter(self.FmtString)

        # add formatter to ch
        ch.setFormatter(formatter)

        # add ch to logger
        log.addHandler(ch)

    def register(self, event_name, handler, unique_id=None,
                 unique_id_uses_count=False):
        """Register a handler with an event.

        :type event_name: str
        :param event_name: The name of the event.

        :type handler: callable
        :param handler: The callback to invoke when the event
            is emitted.  This object must be callable, and must
            accept ``**kwargs``.  If either of these preconditions are
            not met, a ``ValueError`` will be raised.

        :type unique_id: str
        :param unique_id: An optional identifier to associate with the
            registration.  A unique_id can only be used once for
            the entire session registration (unless it is unregistered).
            This can be used to prevent an event handler from being
            registered twice.

        :param unique_id_uses_count: boolean
        :param unique_id_uses_count: Specifies if the event should maintain
            a count when a ``unique_id`` is registered and unregisted. The
            event can only be completely unregistered once every register call
            using the unique id has been matched by an ``unregister`` call.
            If ``unique_id`` is specified, subsequent ``register``
            calls must use the same value for  ``unique_id_uses_count``
            as the ``register`` call that first registered the event.

        :raises ValueError: If the call to ``register`` uses ``unique_id``
            but the value for ``unique_id_uses_count`` differs from the
            ``unique_id_uses_count`` value declared by the very first
            ``register`` call for that ``unique_id``.
        """
        self._events.register(event_name, handler, unique_id,
                              unique_id_uses_count=unique_id_uses_count)

    def unregister(self, event_name, handler=None, unique_id=None,
                   unique_id_uses_count=False):
        """Unregister a handler with an event.

        :type event_name: str
        :param event_name: The name of the event.

        :type handler: callable
        :param handler: The callback to unregister.

        :type unique_id: str
        :param unique_id: A unique identifier identifying the callback
            to unregister.  You can provide either the handler or the
            unique_id, you do not have to provide both.

        :param unique_id_uses_count: boolean
        :param unique_id_uses_count: Specifies if the event should maintain
            a count when a ``unique_id`` is registered and unregisted. The
            event can only be completely unregistered once every ``register``
            call using the ``unique_id`` has been matched by an ``unregister``
            call. If the ``unique_id`` is specified, subsequent
            ``unregister`` calls must use the same value for
            ``unique_id_uses_count`` as the ``register`` call that first
            registered the event.

        :raises ValueError: If the call to ``unregister`` uses ``unique_id``
            but the value for ``unique_id_uses_count`` differs from the
            ``unique_id_uses_count`` value declared by the very first
            ``register`` call for that ``unique_id``.
        """
        self._events.unregister(event_name, handler=handler,
                                unique_id=unique_id,
                                unique_id_uses_count=unique_id_uses_count)

    def register_event(self, event_name, fmtstr):
        """
        Register a new event.  The event will be added to ``AllEvents``
        and will then be able to be created using ``create_event``.

        :type event_name: str
        :param event_name: The base name of the event.

        :type fmtstr: str
        :param fmtstr: The formatting string for the event.
        """
        if event_name not in self.AllEvents:
            self.AllEvents[event_name] = fmtstr

    def create_event(self, event_name, *fmtargs):
        """
        Creates a new event string that can then be emitted.
        You could just create it manually, since it's just
        a string but this helps to define the range of known events.

        :type event_name: str
        :param event_name: The base name of the new event.

        :type fmtargs: tuple
        :param fmtargs: A tuple of values that will be used as the
            arguments pass to the string formatting operation.  The
            actual values passed depend on the type of event you
            are creating.
        """
        if event_name in self.AllEvents:
            fmt_string = self.AllEvents[event_name]
            if fmt_string:
                event = event_name + (fmt_string % fmtargs)
            else:
                event = event_name
            return event
        raise EventNotFound(event_name=event_name)

    def emit(self, event_name, **kwargs):
        return self._events.emit(event_name, **kwargs)

    def emit_first_non_none_response(self, event_name, **kwargs):
        responses = self._events.emit(event_name, **kwargs)
        return first_non_none_response(responses)

    def get_component(self, name):
        return self._components.get_component(name)

    def register_component(self, name, component):
        self._components.register_component(name, component)

    def lazy_register_component(self, name, component):
        self._components.lazy_register_component(name, component)

    def create_client(self, service_name, region_name=None, api_version=None,
                      use_ssl=True, verify=None, endpoint_url=None,
                      aws_access_key_id=None, aws_secret_access_key=None,
                      aws_session_token=None):
        """Create a botocore client.

        :type service_name: string
        :param service_name: The name of the service for which a client will
            be created.  You can use the ``Sesssion.get_available_services()``
            method to get a list of all available service names.

        :type region_name: string
        :param region_name: The name of the region associated with the client.
            A client is associated with a single region.

        :type api_version: string
        :param api_version: The API version to use.  By default, botocore will
            use the latest API version when creating a client.  You only need
            to specify this parameter if you want to use a previous API version
            of the client.

        :type use_ssl: boolean
        :param use_ssl: Whether or not to use SSL.  By default, SSL is used.  Note that
            not all services support non-ssl connections.

        :type verify: boolean/string
        :param verify: Whether or not to verify SSL certificates.  By default SSL certificates
            are verified.  You can provide the following values:

            * False - do not validate SSL certificates.  SSL will still be
              used (unless use_ssl is False), but SSL certificates
              will not be verified.
            * path/to/cert/bundle.pem - A filename of the CA cert bundle to
              uses.  You can specify this argument if you want to use a different
              CA cert bundle than the one used by botocore.

        :type endpoint_url: string
        :param endpoint_url: The complete URL to use for the constructed client.
            Normally, botocore will automatically construct the appropriate URL
            to use when communicating with a service.  You can specify a
            complete URL (including the "http/https" scheme) to override this
            behavior.  If this value is provided, then ``use_ssl`` is ignored.

        :type aws_access_key_id: string
        :param aws_access_key_id: The access key to use when creating
            the client.  This is entirely optional, and if not provided,
            the credentials configured for the session will automatically
            be used.  You only need to provide this argument if you want
            to override the credentials used for this specific client.

        :type aws_secret_access_key: string
        :param aws_secret_access_key: The secret key to use when creating
            the client.  Same semantics as aws_access_key_id above.

        :type aws_session_token: string
        :param aws_session_token: The session token to use when creating
            the client.  Same semantics as aws_access_key_id above.

        :rtype: botocore.client.BaseClient
        :return: A botocore client instance

        """
        loader = self.get_component('data_loader')
        endpoint_creator = self._create_endpoint_creator(aws_access_key_id,
                                                         aws_secret_access_key,
                                                         aws_session_token)
        event_emitter = self.get_component('event_emitter')
        client_creator = bc_client.ClientCreator(loader, endpoint_creator,
                                                       event_emitter)
        client = client_creator.create_client(service_name, region_name, use_ssl,
                                              endpoint_url, verify,
                                              aws_access_key_id,
                                              aws_secret_access_key,
                                              aws_session_token)
        return client

    def _create_endpoint_creator(self, aws_access_key_id, aws_secret_access_key,
                                 aws_session_token):
        resolver = self.get_component('endpoint_resolver')
        region = self.get_config_variable('region')
        event_emitter = self.get_component('event_emitter')
        if aws_secret_access_key is None:
            credentials = self.get_credentials()
        else:
            credentials = None
        user_agent= self.user_agent()
        endpoint_creator = EndpointCreator(resolver, region, event_emitter,
                                           credentials, user_agent)
        return endpoint_creator


class ComponentLocator(object):
    """Service locator for session components."""
    def __init__(self):
        self._components = {}
        self._deferred = {}

    def get_component(self, name):
        if name in self._deferred:
            factory = self._deferred.pop(name)
            self._components[name] = factory()
        try:
            return self._components[name]
        except KeyError:
            raise ValueError("Unknown component: %s" % name)

    def register_component(self, name, component):
        self._components[name] = component
        try:
            del self._deferred[name]
        except KeyError:
            pass

    def lazy_register_component(self, name, no_arg_factory):
        self._deferred[name] = no_arg_factory
        try:
            del self._components[name]
        except KeyError:
            pass


def get_session(env_vars=None):
    """
    Return a new session object.
    """
    return Session(env_vars)
