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# -*- coding: utf-8 -*-
"""
flask.helpers
~~~~~~~~~~~~~

Implements various helpers.

:copyright: (c) 2011 by Armin Ronacher.
:license: BSD, see LICENSE for more details.
"""

from __future__ import with_statement

import os
import sys
import pkgutil
import posixpath
import mimetypes
from time import time
from zlib import adler32
from threading import RLock
from werkzeug.routing import BuildError
from werkzeug.urls import url_quote
from functools import update_wrapper


from werkzeug.datastructures import Headers
from werkzeug.exceptions import NotFound

# this was moved in 0.7
try:
    from werkzeug.wsgi import wrap_file
except ImportError:
    from werkzeug.utils import wrap_file

from jinja2 import FileSystemLoader

from .globals import session, _request_ctx_stack, _app_ctx_stack, \
     current_app, request


# sentinel
_missing = object()


# what separators does this operating system provide that are not a slash?
# this is used by the send_from_directory function to ensure that nobody is
# able to access files from outside the filesystem.
_os_alt_seps = list(sep for sep in [os.path.sep, os.path.altsep]
                    if sep not in (None, '/'))


def _endpoint_from_view_func(view_func):
    """Internal helper that returns the default endpoint for a given
function. This always is the function name.
"""
    assert view_func is not None, 'expected view func if endpoint ' \
                                  'is not provided.'
    return view_func.__name__


def stream_with_context(generator_or_function):
    """Request contexts disappear when the response is started on the server.
This is done for efficiency reasons and to make it less likely to encounter
memory leaks with badly written WSGI middlewares. The downside is that if
you are using streamed responses, the generator cannot access request bound
information any more.

This function however can help you keep the context around for longer::

from flask import stream_with_context, request, Response

@app.route('/stream')
def streamed_response():
@stream_with_context
def generate():
yield 'Hello '
yield request.args['name']
yield '!'
return Response(generate())

Alternatively it can also be used around a specific generator::

from flask import stream_with_context, request, Response

@app.route('/stream')
def streamed_response():
def generate():
yield 'Hello '
yield request.args['name']
yield '!'
return Response(stream_with_context(generate()))

.. versionadded:: 0.9
"""
    try:
        gen = iter(generator_or_function)
    except TypeError:
        def decorator(*args, **kwargs):
            gen = generator_or_function()
            return stream_with_context(gen)
        return update_wrapper(decorator, generator_or_function)

    def generator():
        ctx = _request_ctx_stack.top
        if ctx is None:
            raise RuntimeError('Attempted to stream with context but '
                'there was no context in the first place to keep around.')
        with ctx:
            # Dummy sentinel. Has to be inside the context block or we're
            # not actually keeping the context around.
            yield None

            # The try/finally is here so that if someone passes a WSGI level
            # iterator in we're still running the cleanup logic. Generators
            # don't need that because they are closed on their destruction
            # automatically.
            try:
                for item in gen:
                    yield item
            finally:
                if hasattr(gen, 'close'):
                    gen.close()

    # The trick is to start the generator. Then the code execution runs until
    # the first dummy None is yielded at which point the context was already
    # pushed. This item is discarded. Then when the iteration continues the
    # real generator is executed.
    wrapped_g = generator()
    wrapped_g.next()
    return wrapped_g


def make_response(*args):
    """Sometimes it is necessary to set additional headers in a view. Because
views do not have to return response objects but can return a value that
is converted into a response object by Flask itself, it becomes tricky to
add headers to it. This function can be called instead of using a return
and you will get a response object which you can use to attach headers.

If view looked like this and you want to add a new header::

def index():
return render_template('index.html', foo=42)

You can now do something like this::

def index():
response = make_response(render_template('index.html', foo=42))
response.headers['X-Parachutes'] = 'parachutes are cool'
return response

This function accepts the very same arguments you can return from a
view function. This for example creates a response with a 404 error
code::

response = make_response(render_template('not_found.html'), 404)

The other use case of this function is to force the return value of a
view function into a response which is helpful with view
decorators::

response = make_response(view_function())
response.headers['X-Parachutes'] = 'parachutes are cool'

Internally this function does the following things:

- if no arguments are passed, it creates a new response argument
- if one argument is passed, :meth:`flask.Flask.make_response`
is invoked with it.
- if more than one argument is passed, the arguments are passed
to the :meth:`flask.Flask.make_response` function as tuple.

.. versionadded:: 0.6
"""
    if not args:
        return current_app.response_class()
    if len(args) == 1:
        args = args[0]
    return current_app.make_response(args)


def url_for(endpoint, **values):
    """Generates a URL to the given endpoint with the method provided.

Variable arguments that are unknown to the target endpoint are appended
to the generated URL as query arguments. If the value of a query argument
is `None`, the whole pair is skipped. In case blueprints are active
you can shortcut references to the same blueprint by prefixing the
local endpoint with a dot (``.``).

This will reference the index function local to the current blueprint::

url_for('.index')

For more information, head over to the :ref:`Quickstart <url-building>`.

To integrate applications, :class:`Flask` has a hook to intercept URL build
errors through :attr:`Flask.build_error_handler`. The `url_for` function
results in a :exc:`~werkzeug.routing.BuildError` when the current app does
not have a URL for the given endpoint and values. When it does, the
:data:`~flask.current_app` calls its :attr:`~Flask.build_error_handler` if
it is not `None`, which can return a string to use as the result of
`url_for` (instead of `url_for`'s default to raise the
:exc:`~werkzeug.routing.BuildError` exception) or re-raise the exception.
An example::

def external_url_handler(error, endpoint, **values):
"Looks up an external URL when `url_for` cannot build a URL."
# This is an example of hooking the build_error_handler.
# Here, lookup_url is some utility function you've built
# which looks up the endpoint in some external URL registry.
url = lookup_url(endpoint, **values)
if url is None:
# External lookup did not have a URL.
# Re-raise the BuildError, in context of original traceback.
exc_type, exc_value, tb = sys.exc_info()
if exc_value is error:
raise exc_type, exc_value, tb
else:
raise error
# url_for will use this result, instead of raising BuildError.
return url

app.build_error_handler = external_url_handler

Here, `error` is the instance of :exc:`~werkzeug.routing.BuildError`, and
`endpoint` and `**values` are the arguments passed into `url_for`. Note
that this is for building URLs outside the current application, and not for
handling 404 NotFound errors.

.. versionadded:: 0.9
The `_anchor` and `_method` parameters were added.

.. versionadded:: 0.9
Calls :meth:`Flask.handle_build_error` on
:exc:`~werkzeug.routing.BuildError`.

:param endpoint: the endpoint of the URL (name of the function)
:param values: the variable arguments of the URL rule
:param _external: if set to `True`, an absolute URL is generated. Server
address can be changed via `SERVER_NAME` configuration variable which
defaults to `localhost`.
:param _anchor: if provided this is added as anchor to the URL.
:param _method: if provided this explicitly specifies an HTTP method.
"""
    appctx = _app_ctx_stack.top
    reqctx = _request_ctx_stack.top
    if appctx is None:
        raise RuntimeError('Attempted to generate a URL with the application '
                           'context being pushed. This has to be executed ')

    # If request specific information is available we have some extra
    # features that support "relative" urls.
    if reqctx is not None:
        url_adapter = reqctx.url_adapter
        blueprint_name = request.blueprint
        if not reqctx.request._is_old_module:
            if endpoint[:1] == '.':
                if blueprint_name is not None:
                    endpoint = blueprint_name + endpoint
                else:
                    endpoint = endpoint[1:]
        else:
            # TODO: get rid of this deprecated functionality in 1.0
            if '.' not in endpoint:
                if blueprint_name is not None:
                    endpoint = blueprint_name + '.' + endpoint
            elif endpoint.startswith('.'):
                endpoint = endpoint[1:]
        external = values.pop('_external', False)

    # Otherwise go with the url adapter from the appctx and make
    # the urls external by default.
    else:
        url_adapter = appctx.url_adapter
        if url_adapter is None:
            raise RuntimeError('Application was not able to create a URL '
                               'adapter for request independent URL generation. '
                               'You might be able to fix this by setting '
                               'the SERVER_NAME config variable.')
        external = values.pop('_external', True)

    anchor = values.pop('_anchor', None)
    method = values.pop('_method', None)
    appctx.app.inject_url_defaults(endpoint, values)
    try:
        rv = url_adapter.build(endpoint, values, method=method,
                               force_external=external)
    except BuildError, error:
        # We need to inject the values again so that the app callback can
        # deal with that sort of stuff.
        values['_external'] = external
        values['_anchor'] = anchor
        values['_method'] = method
        return appctx.app.handle_url_build_error(error, endpoint, values)

    rv = url_adapter.build(endpoint, values, method=method,
                           force_external=external)
    if anchor is not None:
        rv += '#' + url_quote(anchor)
    return rv


def get_template_attribute(template_name, attribute):
    """Loads a macro (or variable) a template exports. This can be used to
invoke a macro from within Python code. If you for example have a
template named `_cider.html` with the following contents:

.. sourcecode:: html+jinja

{% macro hello(name) %}Hello {{ name }}!{% endmacro %}

You can access this from Python code like this::

hello = get_template_attribute('_cider.html', 'hello')
return hello('World')

.. versionadded:: 0.2

:param template_name: the name of the template
:param attribute: the name of the variable of macro to acccess
"""
    return getattr(current_app.jinja_env.get_template(template_name).module,
                   attribute)


def flash(message, category='message'):
    """Flashes a message to the next request. In order to remove the
flashed message from the session and to display it to the user,
the template has to call :func:`get_flashed_messages`.

.. versionchanged:: 0.3
`category` parameter added.

:param message: the message to be flashed.
:param category: the category for the message. The following values
are recommended: ``'message'`` for any kind of message,
``'error'`` for errors, ``'info'`` for information
messages and ``'warning'`` for warnings. However any
kind of string can be used as category.
"""
    # Original implementation:
    #
    # session.setdefault('_flashes', []).append((category, message))
    #
    # This assumed that changes made to mutable structures in the session are
    # are always in sync with the sess on object, which is not true for session
    # implementations that use external storage for keeping their keys/values.
    flashes = session.get('_flashes', [])
    flashes.append((category, message))
    session['_flashes'] = flashes


def get_flashed_messages(with_categories=False, category_filter=[]):
    """Pulls all flashed messages from the session and returns them.
Further calls in the same request to the function will return
the same messages. By default just the messages are returned,
but when `with_categories` is set to `True`, the return value will
be a list of tuples in the form ``(category, message)`` instead.

Filter the flashed messages to one or more categories by providing those
categories in `category_filter`. This allows rendering categories in
separate html blocks. The `with_categories` and `category_filter`
arguments are distinct:

* `with_categories` controls whether categories are returned with message
text (`True` gives a tuple, where `False` gives just the message text).
* `category_filter` filters the messages down to only those matching the
provided categories.

See :ref:`message-flashing-pattern` for examples.

.. versionchanged:: 0.3
`with_categories` parameter added.

.. versionchanged:: 0.9
`category_filter` parameter added.

:param with_categories: set to `True` to also receive categories.
:param category_filter: whitelist of categories to limit return values
"""
    flashes = _request_ctx_stack.top.flashes
    if flashes is None:
        _request_ctx_stack.top.flashes = flashes = session.pop('_flashes') \
            if '_flashes' in session else []
    if category_filter:
        flashes = filter(lambda f: f[0] in category_filter, flashes)
    if not with_categories:
        return [x[1] for x in flashes]
    return flashes


def send_file(filename_or_fp, mimetype=None, as_attachment=False,
              attachment_filename=None, add_etags=True,
              cache_timeout=None, conditional=False):
    """Sends the contents of a file to the client. This will use the
most efficient method available and configured. By default it will
try to use the WSGI server's file_wrapper support. Alternatively
you can set the application's :attr:`~Flask.use_x_sendfile` attribute
to ``True`` to directly emit an `X-Sendfile` header. This however
requires support of the underlying webserver for `X-Sendfile`.

By default it will try to guess the mimetype for you, but you can
also explicitly provide one. For extra security you probably want
to send certain files as attachment (HTML for instance). The mimetype
guessing requires a `filename` or an `attachment_filename` to be
provided.

Please never pass filenames to this function from user sources without
checking them first. Something like this is usually sufficient to
avoid security problems::

if '..' in filename or filename.startswith('/'):
abort(404)

.. versionadded:: 0.2

.. versionadded:: 0.5
The `add_etags`, `cache_timeout` and `conditional` parameters were
added. The default behavior is now to attach etags.

.. versionchanged:: 0.7
mimetype guessing and etag support for file objects was
deprecated because it was unreliable. Pass a filename if you are
able to, otherwise attach an etag yourself. This functionality
will be removed in Flask 1.0

.. versionchanged:: 0.9
cache_timeout pulls its default from application config, when None.

:param filename_or_fp: the filename of the file to send. This is
relative to the :attr:`~Flask.root_path` if a
relative path is specified.
Alternatively a file object might be provided
in which case `X-Sendfile` might not work and
fall back to the traditional method. Make sure
that the file pointer is positioned at the start
of data to send before calling :func:`send_file`.
:param mimetype: the mimetype of the file if provided, otherwise
auto detection happens.
:param as_attachment: set to `True` if you want to send this file with
a ``Content-Disposition: attachment`` header.
:param attachment_filename: the filename for the attachment if it
differs from the file's filename.
:param add_etags: set to `False` to disable attaching of etags.
:param conditional: set to `True` to enable conditional responses.

:param cache_timeout: the timeout in seconds for the headers. When `None`
(default), this value is set by
:meth:`~Flask.get_send_file_max_age` of
:data:`~flask.current_app`.
"""
    mtime = None
    if isinstance(filename_or_fp, basestring):
        filename = filename_or_fp
        file = None
    else:
        from warnings import warn
        file = filename_or_fp
        filename = getattr(file, 'name', None)

        # XXX: this behavior is now deprecated because it was unreliable.
        # removed in Flask 1.0
        if not attachment_filename and not mimetype \
           and isinstance(filename, basestring):
            warn(DeprecationWarning('The filename support for file objects '
                'passed to send_file is now deprecated. Pass an '
                'attach_filename if you want mimetypes to be guessed.'),
                stacklevel=2)
        if add_etags:
            warn(DeprecationWarning('In future flask releases etags will no '
                'longer be generated for file objects passed to the send_file '
                'function because this behavior was unreliable. Pass '
                'filenames instead if possible, otherwise attach an etag '
                'yourself based on another value'), stacklevel=2)

    if filename is not None:
        if not os.path.isabs(filename):
            filename = os.path.join(current_app.root_path, filename)
    if mimetype is None and (filename or attachment_filename):
        mimetype = mimetypes.guess_type(filename or attachment_filename)[0]
    if mimetype is None:
        mimetype = 'application/octet-stream'

    headers = Headers()
    if as_attachment:
        if attachment_filename is None:
            if filename is None:
                raise TypeError('filename unavailable, required for '
                                'sending as attachment')
            attachment_filename = os.path.basename(filename)
        headers.add('Content-Disposition', 'attachment',
                    filename=attachment_filename)

    if current_app.use_x_sendfile and filename:
        if file is not None:
            file.close()
        headers['X-Sendfile'] = filename
        headers['Content-Length'] = os.path.getsize(filename)
        data = None
    else:
        if file is None:
            file = open(filename, 'rb')
            mtime = os.path.getmtime(filename)
        data = wrap_file(request.environ, file)

    rv = current_app.response_class(data, mimetype=mimetype, headers=headers,
                                    direct_passthrough=True)

    # if we know the file modification date, we can store it as the
    # the time of the last modification.
    if mtime is not None:
        rv.last_modified = int(mtime)

    rv.cache_control.public = True
    if cache_timeout is None:
        cache_timeout = current_app.get_send_file_max_age(filename)
    if cache_timeout is not None:
        rv.cache_control.max_age = cache_timeout
        rv.expires = int(time() + cache_timeout)

    if add_etags and filename is not None:
        rv.set_etag('flask-%s-%s-%s' % (
            os.path.getmtime(filename),
            os.path.getsize(filename),
            adler32(
                filename.encode('utf-8') if isinstance(filename, unicode)
                else filename
            ) & 0xffffffff
        ))
        if conditional:
            rv = rv.make_conditional(request)
            # make sure we don't send x-sendfile for servers that
            # ignore the 304 status code for x-sendfile.
            if rv.status_code == 304:
                rv.headers.pop('x-sendfile', None)
    return rv


def safe_join(directory, filename):
    """Safely join `directory` and `filename`.

Example usage::

@app.route('/wiki/<path:filename>')
def wiki_page(filename):
filename = safe_join(app.config['WIKI_FOLDER'], filename)
with open(filename, 'rb') as fd:
content = fd.read() # Read and process the file content...

:param directory: the base directory.
:param filename: the untrusted filename relative to that directory.
:raises: :class:`~werkzeug.exceptions.NotFound` if the resulting path
would fall out of `directory`.
"""
    filename = posixpath.normpath(filename)
    for sep in _os_alt_seps:
        if sep in filename:
            raise NotFound()
    if os.path.isabs(filename) or \
       filename == '..' or \
       filename.startswith('../'):
        raise NotFound()
    return os.path.join(directory, filename)


def send_from_directory(directory, filename, **options):
    """Send a file from a given directory with :func:`send_file`. This
is a secure way to quickly expose static files from an upload folder
or something similar.

Example usage::

@app.route('/uploads/<path:filename>')
def download_file(filename):
return send_from_directory(app.config['UPLOAD_FOLDER'],
filename, as_attachment=True)

.. admonition:: Sending files and Performance

It is strongly recommended to activate either `X-Sendfile` support in
your webserver or (if no authentication happens) to tell the webserver
to serve files for the given path on its own without calling into the
web application for improved performance.

.. versionadded:: 0.5

:param directory: the directory where all the files are stored.
:param filename: the filename relative to that directory to
download.
:param options: optional keyword arguments that are directly
forwarded to :func:`send_file`.
"""
    filename = safe_join(directory, filename)
    if not os.path.isfile(filename):
        raise NotFound()
    options.setdefault('conditional', True)
    return send_file(filename, **options)


def get_root_path(import_name):
    """Returns the path to a package or cwd if that cannot be found. This
returns the path of a package or the folder that contains a module.

Not to be confused with the package path returned by :func:`find_package`.
"""
    # Module already imported and has a file attribute. Use that first.
    mod = sys.modules.get(import_name)
    if mod is not None and hasattr(mod, '__file__'):
        return os.path.dirname(os.path.abspath(mod.__file__))

    # Next attempt: check the loader.
    loader = pkgutil.get_loader(import_name)

    # Loader does not exist or we're referring to an unloaded main module
    # or a main module without path (interactive sessions), go with the
    # current working directory.
    if loader is None or import_name == '__main__':
        return os.getcwd()

    # For .egg, zipimporter does not have get_filename until Python 2.7.
    # Some other loaders might exhibit the same behavior.
    if hasattr(loader, 'get_filename'):
        filepath = loader.get_filename(import_name)
    else:
        # Fall back to imports.
        __import__(import_name)
        filepath = sys.modules[import_name].__file__

    # filepath is import_name.py for a module, or __init__.py for a package.
    return os.path.dirname(os.path.abspath(filepath))


def find_package(import_name):
    """Finds a package and returns the prefix (or None if the package is
not installed) as well as the folder that contains the package or
module as a tuple. The package path returned is the module that would
have to be added to the pythonpath in order to make it possible to
import the module. The prefix is the path below which a UNIX like
folder structure exists (lib, share etc.).
"""
    root_mod_name = import_name.split('.')[0]
    loader = pkgutil.get_loader(root_mod_name)
    if loader is None or import_name == '__main__':
        # import name is not found, or interactive/main module
        package_path = os.getcwd()
    else:
        # For .egg, zipimporter does not have get_filename until Python 2.7.
        if hasattr(loader, 'get_filename'):
            filename = loader.get_filename(root_mod_name)
        elif hasattr(loader, 'archive'):
            # zipimporter's loader.archive points to the .egg or .zip
            # archive filename is dropped in call to dirname below.
            filename = loader.archive
        else:
            # At least one loader is missing both get_filename and archive:
            # Google App Engine's HardenedModulesHook
            #
            # Fall back to imports.
            __import__(import_name)
            filename = sys.modules[import_name].__file__
        package_path = os.path.abspath(os.path.dirname(filename))
        # package_path ends with __init__.py for a package
        if loader.is_package(root_mod_name):
            package_path = os.path.dirname(package_path)

    site_parent, site_folder = os.path.split(package_path)
    py_prefix = os.path.abspath(sys.prefix)
    if package_path.startswith(py_prefix):
        return py_prefix, package_path
    elif site_folder.lower() == 'site-packages':
        parent, folder = os.path.split(site_parent)
        # Windows like installations
        if folder.lower() == 'lib':
            base_dir = parent
        # UNIX like installations
        elif os.path.basename(parent).lower() == 'lib':
            base_dir = os.path.dirname(parent)
        else:
            base_dir = site_parent
        return base_dir, package_path
    return None, package_path


class locked_cached_property(object):
    """A decorator that converts a function into a lazy property. The
function wrapped is called the first time to retrieve the result
and then that calculated result is used the next time you access
the value. Works like the one in Werkzeug but has a lock for
thread safety.
"""

    def __init__(self, func, name=None, doc=None):
        self.__name__ = name or func.__name__
        self.__module__ = func.__module__
        self.__doc__ = doc or func.__doc__
        self.func = func
        self.lock = RLock()

    def __get__(self, obj, type=None):
        if obj is None:
            return self
        with self.lock:
            value = obj.__dict__.get(self.__name__, _missing)
            if value is _missing:
                value = self.func(obj)
                obj.__dict__[self.__name__] = value
            return value


class _PackageBoundObject(object):

    def __init__(self, import_name, template_folder=None):
        #: The name of the package or module. Do not change this once
        #: it was set by the constructor.
        self.import_name = import_name

        #: location of the templates. `None` if templates should not be
        #: exposed.
        self.template_folder = template_folder

        #: Where is the app root located?
        self.root_path = get_root_path(self.import_name)

        self._static_folder = None
        self._static_url_path = None

    def _get_static_folder(self):
        if self._static_folder is not None:
            return os.path.join(self.root_path, self._static_folder)
    def _set_static_folder(self, value):
        self._static_folder = value
    static_folder = property(_get_static_folder, _set_static_folder)
    del _get_static_folder, _set_static_folder

    def _get_static_url_path(self):
        if self._static_url_path is None:
            if self.static_folder is None:
                return None
            return '/' + os.path.basename(self.static_folder)
        return self._static_url_path
    def _set_static_url_path(self, value):
        self._static_url_path = value
    static_url_path = property(_get_static_url_path, _set_static_url_path)
    del _get_static_url_path, _set_static_url_path

    @property
    def has_static_folder(self):
        """This is `True` if the package bound object's container has a
folder named ``'static'``.

.. versionadded:: 0.5
"""
        return self.static_folder is not None

    @locked_cached_property
    def jinja_loader(self):
        """The Jinja loader for this package bound object.

.. versionadded:: 0.5
"""
        if self.template_folder is not None:
            return FileSystemLoader(os.path.join(self.root_path,
                                                 self.template_folder))

    def get_send_file_max_age(self, filename):
        """Provides default cache_timeout for the :func:`send_file` functions.

By default, this function returns ``SEND_FILE_MAX_AGE_DEFAULT`` from
the configuration of :data:`~flask.current_app`.

Static file functions such as :func:`send_from_directory` use this
function, and :func:`send_file` calls this function on
:data:`~flask.current_app` when the given cache_timeout is `None`. If a
cache_timeout is given in :func:`send_file`, that timeout is used;
otherwise, this method is called.

This allows subclasses to change the behavior when sending files based
on the filename. For example, to set the cache timeout for .js files
to 60 seconds::

class MyFlask(flask.Flask):
def get_send_file_max_age(self, name):
if name.lower().endswith('.js'):
return 60
return flask.Flask.get_send_file_max_age(self, name)

.. versionadded:: 0.9
"""
        return current_app.config['SEND_FILE_MAX_AGE_DEFAULT']

    def send_static_file(self, filename):
        """Function used internally to send static files from the static
folder to the browser.

.. versionadded:: 0.5
"""
        if not self.has_static_folder:
            raise RuntimeError('No static folder for this object')
        # Ensure get_send_file_max_age is called in all cases.
        # Here, we ensure get_send_file_max_age is called for Blueprints.
        cache_timeout = self.get_send_file_max_age(filename)
        return send_from_directory(self.static_folder, filename,
                                   cache_timeout=cache_timeout)

    def open_resource(self, resource, mode='rb'):
        """Opens a resource from the application's resource folder. To see
how this works, consider the following folder structure::

/myapplication.py
/schema.sql
/static
/style.css
/templates
/layout.html
/index.html

If you want to open the `schema.sql` file you would do the
following::

with app.open_resource('schema.sql') as f:
contents = f.read()
do_something_with(contents)

:param resource: the name of the resource. To access resources within
subfolders use forward slashes as separator.
"""
        if mode not in ('r', 'rb'):
            raise ValueError('Resources can only be opened for reading')
        return open(os.path.join(self.root_path, resource), mode)
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