environment.py

# -*- coding: utf-8 -*-
"""Classes for managing templates and their runtime and compile time
options.
"""
import os
import sys
import weakref
from functools import partial
from functools import reduce

from markupsafe import Markup

from . import nodes
from ._compat import encode_filename
from ._compat import implements_iterator
from ._compat import implements_to_string
from ._compat import iteritems
from ._compat import PY2
from ._compat import PYPY
from ._compat import reraise
from ._compat import string_types
from ._compat import text_type
from .compiler import CodeGenerator
from .compiler import generate
from .defaults import BLOCK_END_STRING
from .defaults import BLOCK_START_STRING
from .defaults import COMMENT_END_STRING
from .defaults import COMMENT_START_STRING
from .defaults import DEFAULT_FILTERS
from .defaults import DEFAULT_NAMESPACE
from .defaults import DEFAULT_POLICIES
from .defaults import DEFAULT_TESTS
from .defaults import KEEP_TRAILING_NEWLINE
from .defaults import LINE_COMMENT_PREFIX
from .defaults import LINE_STATEMENT_PREFIX
from .defaults import LSTRIP_BLOCKS
from .defaults import NEWLINE_SEQUENCE
from .defaults import TRIM_BLOCKS
from .defaults import VARIABLE_END_STRING
from .defaults import VARIABLE_START_STRING
from .exceptions import TemplateNotFound
from .exceptions import TemplateRuntimeError
from .exceptions import TemplatesNotFound
from .exceptions import TemplateSyntaxError
from .exceptions import UndefinedError
from .lexer import get_lexer
from .lexer import TokenStream
from .nodes import EvalContext
from .parser import Parser
from .runtime import Context
from .runtime import new_context
from .runtime import Undefined
from .utils import concat
from .utils import consume
from .utils import have_async_gen
from .utils import import_string
from .utils import internalcode
from .utils import LRUCache
from .utils import missing

# for direct template usage we have up to ten living environments
_spontaneous_environments = LRUCache(10)


def get_spontaneous_environment(cls, *args):
    """Return a new spontaneous environment. A spontaneous environment
    is used for templates created directly rather than through an
    existing environment.

    :param cls: Environment class to create.
    :param args: Positional arguments passed to environment.
    """
    key = (cls, args)

    try:
        return _spontaneous_environments[key]
    except KeyError:
        _spontaneous_environments[key] = env = cls(*args)
        env.shared = True
        return env


def create_cache(size):
    """Return the cache class for the given size."""
    if size == 0:
        return None
    if size < 0:
        return {}
    return LRUCache(size)


def copy_cache(cache):
    """Create an empty copy of the given cache."""
    if cache is None:
        return None
    elif type(cache) is dict:
        return {}
    return LRUCache(cache.capacity)


def load_extensions(environment, extensions):
    """Load the extensions from the list and bind it to the environment.
    Returns a dict of instantiated environments.
    """
    result = {}
    for extension in extensions:
        if isinstance(extension, string_types):
            extension = import_string(extension)
        result[extension.identifier] = extension(environment)
    return result


def fail_for_missing_callable(string, name):
    msg = string % name
    if isinstance(name, Undefined):
        try:
            name._fail_with_undefined_error()
        except Exception as e:
            msg = "%s (%s; did you forget to quote the callable name?)" % (msg, e)
    raise TemplateRuntimeError(msg)


def _environment_sanity_check(environment):
    """Perform a sanity check on the environment."""
    assert issubclass(
        environment.undefined, Undefined
    ), "undefined must be a subclass of undefined because filters depend on it."
    assert (
        environment.block_start_string
        != environment.variable_start_string
        != environment.comment_start_string
    ), "block, variable and comment start strings must be different"
    assert environment.newline_sequence in (
        "\r",
        "\r\n",
        "\n",
    ), "newline_sequence set to unknown line ending string."
    return environment


class Environment(object):
    r"""The core component of Jinja is the `Environment`.  It contains
    important shared variables like configuration, filters, tests,
    globals and others.  Instances of this class may be modified if
    they are not shared and if no template was loaded so far.
    Modifications on environments after the first template was loaded
    will lead to surprising effects and undefined behavior.

    Here are the possible initialization parameters:

        `block_start_string`
            The string marking the beginning of a block.  Defaults to ``'{%'``.

        `block_end_string`
            The string marking the end of a block.  Defaults to ``'%}'``.

        `variable_start_string`
            The string marking the beginning of a print statement.
            Defaults to ``'{{'``.

        `variable_end_string`
            The string marking the end of a print statement.  Defaults to
            ``'}}'``.

        `comment_start_string`
            The string marking the beginning of a comment.  Defaults to ``'{#'``.

        `comment_end_string`
            The string marking the end of a comment.  Defaults to ``'#}'``.

        `line_statement_prefix`
            If given and a string, this will be used as prefix for line based
            statements.  See also :ref:`line-statements`.

        `line_comment_prefix`
            If given and a string, this will be used as prefix for line based
            comments.  See also :ref:`line-statements`.

            .. versionadded:: 2.2

        `trim_blocks`
            If this is set to ``True`` the first newline after a block is
            removed (block, not variable tag!).  Defaults to `False`.

        `lstrip_blocks`
            If this is set to ``True`` leading spaces and tabs are stripped
            from the start of a line to a block.  Defaults to `False`.

        `newline_sequence`
            The sequence that starts a newline.  Must be one of ``'\r'``,
            ``'\n'`` or ``'\r\n'``.  The default is ``'\n'`` which is a
            useful default for Linux and OS X systems as well as web
            applications.

        `keep_trailing_newline`
            Preserve the trailing newline when rendering templates.
            The default is ``False``, which causes a single newline,
            if present, to be stripped from the end of the template.

            .. versionadded:: 2.7

        `extensions`
            List of Jinja extensions to use.  This can either be import paths
            as strings or extension classes.  For more information have a
            look at :ref:`the extensions documentation <jinja-extensions>`.

        `optimized`
            should the optimizer be enabled?  Default is ``True``.

        `undefined`
            :class:`Undefined` or a subclass of it that is used to represent
            undefined values in the template.

        `finalize`
            A callable that can be used to process the result of a variable
            expression before it is output.  For example one can convert
            ``None`` implicitly into an empty string here.

        `autoescape`
            If set to ``True`` the XML/HTML autoescaping feature is enabled by
            default.  For more details about autoescaping see
            :class:`~markupsafe.Markup`.  As of Jinja 2.4 this can also
            be a callable that is passed the template name and has to
            return ``True`` or ``False`` depending on autoescape should be
            enabled by default.

            .. versionchanged:: 2.4
               `autoescape` can now be a function

        `loader`
            The template loader for this environment.

        `cache_size`
            The size of the cache.  Per default this is ``400`` which means
            that if more than 400 templates are loaded the loader will clean
            out the least recently used template.  If the cache size is set to
            ``0`` templates are recompiled all the time, if the cache size is
            ``-1`` the cache will not be cleaned.

            .. versionchanged:: 2.8
               The cache size was increased to 400 from a low 50.

        `auto_reload`
            Some loaders load templates from locations where the template
            sources may change (ie: file system or database).  If
            ``auto_reload`` is set to ``True`` (default) every time a template is
            requested the loader checks if the source changed and if yes, it
            will reload the template.  For higher performance it's possible to
            disable that.

        `bytecode_cache`
            If set to a bytecode cache object, this object will provide a
            cache for the internal Jinja bytecode so that templates don't
            have to be parsed if they were not changed.

            See :ref:`bytecode-cache` for more information.

        `enable_async`
            If set to true this enables async template execution which allows
            you to take advantage of newer Python features.  This requires
            Python 3.6 or later.
    """

    #: if this environment is sandboxed.  Modifying this variable won't make
    #: the environment sandboxed though.  For a real sandboxed environment
    #: have a look at jinja2.sandbox.  This flag alone controls the code
    #: generation by the compiler.
    sandboxed = False

    #: True if the environment is just an overlay
    overlayed = False

    #: the environment this environment is linked to if it is an overlay
    linked_to = None

    #: shared environments have this set to `True`.  A shared environment
    #: must not be modified
    shared = False

    #: the class that is used for code generation.  See
    #: :class:`~jinja2.compiler.CodeGenerator` for more information.
    code_generator_class = CodeGenerator

    #: the context class thatis used for templates.  See
    #: :class:`~jinja2.runtime.Context` for more information.
    context_class = Context

    def __init__(
        self,
        block_start_string=BLOCK_START_STRING,
        block_end_string=BLOCK_END_STRING,
        variable_start_string=VARIABLE_START_STRING,
        variable_end_string=VARIABLE_END_STRING,
        comment_start_string=COMMENT_START_STRING,
        comment_end_string=COMMENT_END_STRING,
        line_statement_prefix=LINE_STATEMENT_PREFIX,
        line_comment_prefix=LINE_COMMENT_PREFIX,
        trim_blocks=TRIM_BLOCKS,
        lstrip_blocks=LSTRIP_BLOCKS,
        newline_sequence=NEWLINE_SEQUENCE,
        keep_trailing_newline=KEEP_TRAILING_NEWLINE,
        extensions=(),
        optimized=True,
        undefined=Undefined,
        finalize=None,
        autoescape=False,
        loader=None,
        cache_size=400,
        auto_reload=True,
        bytecode_cache=None,
        enable_async=False,
    ):
        # !!Important notice!!
        #   The constructor accepts quite a few arguments that should be
        #   passed by keyword rather than position.  However it's important to
        #   not change the order of arguments because it's used at least
        #   internally in those cases:
        #       -   spontaneous environments (i18n extension and Template)
        #       -   unittests
        #   If parameter changes are required only add parameters at the end
        #   and don't change the arguments (or the defaults!) of the arguments
        #   existing already.

        # lexer / parser information
        self.block_start_string = block_start_string
        self.block_end_string = block_end_string
        self.variable_start_string = variable_start_string
        self.variable_end_string = variable_end_string
        self.comment_start_string = comment_start_string
        self.comment_end_string = comment_end_string
        self.line_statement_prefix = line_statement_prefix
        self.line_comment_prefix = line_comment_prefix
        self.trim_blocks = trim_blocks
        self.lstrip_blocks = lstrip_blocks
        self.newline_sequence = newline_sequence
        self.keep_trailing_newline = keep_trailing_newline

        # runtime information
        self.undefined = undefined
        self.optimized = optimized
        self.finalize = finalize
        self.autoescape = autoescape

        # defaults
        self.filters = DEFAULT_FILTERS.copy()
        self.tests = DEFAULT_TESTS.copy()
        self.globals = DEFAULT_NAMESPACE.copy()

        # set the loader provided
        self.loader = loader
        self.cache = create_cache(cache_size)
        self.bytecode_cache = bytecode_cache
        self.auto_reload = auto_reload

        # configurable policies
        self.policies = DEFAULT_POLICIES.copy()

        # load extensions
        self.extensions = load_extensions(self, extensions)

        self.enable_async = enable_async
        self.is_async = self.enable_async and have_async_gen
        if self.is_async:
            # runs patch_all() to enable async support
            from . import asyncsupport  # noqa: F401

        _environment_sanity_check(self)

    def add_extension(self, extension):
        """Adds an extension after the environment was created.

        .. versionadded:: 2.5
        """
        self.extensions.update(load_extensions(self, [extension]))

    def extend(self, **attributes):
        """Add the items to the instance of the environment if they do not exist
        yet.  This is used by :ref:`extensions <writing-extensions>` to register
        callbacks and configuration values without breaking inheritance.
        """
        for key, value in iteritems(attributes):
            if not hasattr(self, key):
                setattr(self, key, value)

    def overlay(
        self,
        block_start_string=missing,
        block_end_string=missing,
        variable_start_string=missing,
        variable_end_string=missing,
        comment_start_string=missing,
        comment_end_string=missing,
        line_statement_prefix=missing,
        line_comment_prefix=missing,
        trim_blocks=missing,
        lstrip_blocks=missing,
        extensions=missing,
        optimized=missing,
        undefined=missing,
        finalize=missing,
        autoescape=missing,
        loader=missing,
        cache_size=missing,
        auto_reload=missing,
        bytecode_cache=missing,
    ):
        """Create a new overlay environment that shares all the data with the
        current environment except for cache and the overridden attributes.
        Extensions cannot be removed for an overlayed environment.  An overlayed
        environment automatically gets all the extensions of the environment it
        is linked to plus optional extra extensions.

        Creating overlays should happen after the initial environment was set
        up completely.  Not all attributes are truly linked, some are just
        copied over so modifications on the original environment may not shine
        through.
        """
        args = dict(locals())
        del args["self"], args["cache_size"], args["extensions"]

        rv = object.__new__(self.__class__)
        rv.__dict__.update(self.__dict__)
        rv.overlayed = True
        rv.linked_to = self

        for key, value in iteritems(args):
            if value is not missing:
                setattr(rv, key, value)

        if cache_size is not missing:
            rv.cache = create_cache(cache_size)
        else:
            rv.cache = copy_cache(self.cache)

        rv.extensions = {}
        for key, value in iteritems(self.extensions):
            rv.extensions[key] = value.bind(rv)
        if extensions is not missing:
            rv.extensions.update(load_extensions(rv, extensions))

        return _environment_sanity_check(rv)

    lexer = property(get_lexer, doc="The lexer for this environment.")

    def iter_extensions(self):
        """Iterates over the extensions by priority."""
        return iter(sorted(self.extensions.values(), key=lambda x: x.priority))

    def getitem(self, obj, argument):
        """Get an item or attribute of an object but prefer the item."""
        try:
            return obj[argument]
        except (AttributeError, TypeError, LookupError):
            if isinstance(argument, string_types):
                try:
                    attr = str(argument)
                except Exception:
                    pass
                else:
                    try:
                        return getattr(obj, attr)
                    except AttributeError:
                        pass
            return self.undefined(obj=obj, name=argument)

    def getattr(self, obj, attribute):
        """Get an item or attribute of an object but prefer the attribute.
        Unlike :meth:`getitem` the attribute *must* be a bytestring.
        """
        try:
            return getattr(obj, attribute)
        except AttributeError:
            pass
        try:
            return obj[attribute]
        except (TypeError, LookupError, AttributeError):
            return self.undefined(obj=obj, name=attribute)

    def call_filter(
        self, name, value, args=None, kwargs=None, context=None, eval_ctx=None
    ):
        """Invokes a filter on a value the same way the compiler does it.

        Note that on Python 3 this might return a coroutine in case the
        filter is running from an environment in async mode and the filter
        supports async execution.  It's your responsibility to await this
        if needed.

        .. versionadded:: 2.7
        """
        func = self.filters.get(name)
        if func is None:
            fail_for_missing_callable("no filter named %r", name)
        args = [value] + list(args or ())
        if getattr(func, "contextfilter", False) is True:
            if context is None:
                raise TemplateRuntimeError(
                    "Attempted to invoke context filter without context"
                )
            args.insert(0, context)
        elif getattr(func, "evalcontextfilter", False) is True:
            if eval_ctx is None:
                if context is not None:
                    eval_ctx = context.eval_ctx
                else:
                    eval_ctx = EvalContext(self)
            args.insert(0, eval_ctx)
        elif getattr(func, "environmentfilter", False) is True:
            args.insert(0, self)
        return func(*args, **(kwargs or {}))

    def call_test(self, name, value, args=None, kwargs=None):
        """Invokes a test on a value the same way the compiler does it.

        .. versionadded:: 2.7
        """
        func = self.tests.get(name)
        if func is None:
            fail_for_missing_callable("no test named %r", name)
        return func(value, *(args or ()), **(kwargs or {}))

    @internalcode
    def parse(self, source, name=None, filename=None):
        """Parse the sourcecode and return the abstract syntax tree.  This
        tree of nodes is used by the compiler to convert the template into
        executable source- or bytecode.  This is useful for debugging or to
        extract information from templates.

        If you are :ref:`developing Jinja extensions <writing-extensions>`
        this gives you a good overview of the node tree generated.
        """
        try:
            return self._parse(source, name, filename)
        except TemplateSyntaxError:
            self.handle_exception(source=source)

    def _parse(self, source, name, filename):
        """Internal parsing function used by `parse` and `compile`."""
        return Parser(self, source, name, encode_filename(filename)).parse()

    def lex(self, source, name=None, filename=None):
        """Lex the given sourcecode and return a generator that yields
        tokens as tuples in the form ``(lineno, token_type, value)``.
        This can be useful for :ref:`extension development <writing-extensions>`
        and debugging templates.

        This does not perform preprocessing.  If you want the preprocessing
        of the extensions to be applied you have to filter source through
        the :meth:`preprocess` method.
        """
        source = text_type(source)
        try:
            return self.lexer.tokeniter(source, name, filename)
        except TemplateSyntaxError:
            self.handle_exception(source=source)

    def preprocess(self, source, name=None, filename=None):
        """Preprocesses the source with all extensions.  This is automatically
        called for all parsing and compiling methods but *not* for :meth:`lex`
        because there you usually only want the actual source tokenized.
        """
        return reduce(
            lambda s, e: e.preprocess(s, name, filename),
            self.iter_extensions(),
            text_type(source),
        )

    def _tokenize(self, source, name, filename=None, state=None):
        """Called by the parser to do the preprocessing and filtering
        for all the extensions.  Returns a :class:`~jinja2.lexer.TokenStream`.
        """
        source = self.preprocess(source, name, filename)
        stream = self.lexer.tokenize(source, name, filename, state)
        for ext in self.iter_extensions():
            stream = ext.filter_stream(stream)
            if not isinstance(stream, TokenStream):
                stream = TokenStream(stream, name, filename)
        return stream

    def _generate(self, source, name, filename, defer_init=False):
        """Internal hook that can be overridden to hook a different generate
        method in.

        .. versionadded:: 2.5
        """
        return generate(
            source,
            self,
            name,
            filename,
            defer_init=defer_init,
            optimized=self.optimized,
        )

    def _compile(self, source, filename):
        """Internal hook that can be overridden to hook a different compile
        method in.

        .. versionadded:: 2.5
        """
        return compile(source, filename, "exec")

    @internalcode
    def compile(self, source, name=None, filename=None, raw=False, defer_init=False):
        """Compile a node or template source code.  The `name` parameter is
        the load name of the template after it was joined using
        :meth:`join_path` if necessary, not the filename on the file system.
        the `filename` parameter is the estimated filename of the template on
        the file system.  If the template came from a database or memory this
        can be omitted.

        The return value of this method is a python code object.  If the `raw`
        parameter is `True` the return value will be a string with python
        code equivalent to the bytecode returned otherwise.  This method is
        mainly used internally.

        `defer_init` is use internally to aid the module code generator.  This
        causes the generated code to be able to import without the global
        environment variable to be set.

        .. versionadded:: 2.4
           `defer_init` parameter added.
        """
        source_hint = None
        try:
            if isinstance(source, string_types):
                source_hint = source
                source = self._parse(source, name, filename)
            source = self._generate(source, name, filename, defer_init=defer_init)
            if raw:
                return source
            if filename is None:
                filename = "<template>"
            else:
                filename = encode_filename(filename)
            return self._compile(source, filename)
        except TemplateSyntaxError:
            self.handle_exception(source=source_hint)

    def compile_expression(self, source, undefined_to_none=True):
        """A handy helper method that returns a callable that accepts keyword
        arguments that appear as variables in the expression.  If called it
        returns the result of the expression.

        This is useful if applications want to use the same rules as Jinja
        in template "configuration files" or similar situations.

        Example usage:

        >>> env = Environment()
        >>> expr = env.compile_expression('foo == 42')
        >>> expr(foo=23)
        False
        >>> expr(foo=42)
        True

        Per default the return value is converted to `None` if the
        expression returns an undefined value.  This can be changed
        by setting `undefined_to_none` to `False`.

        >>> env.compile_expression('var')() is None
        True
        >>> env.compile_expression('var', undefined_to_none=False)()
        Undefined

        .. versionadded:: 2.1
        """
        parser = Parser(self, source, state="variable")
        try:
            expr = parser.parse_expression()
            if not parser.stream.eos:
                raise TemplateSyntaxError(
                    "chunk after expression", parser.stream.current.lineno, None, None
                )
            expr.set_environment(self)
        except TemplateSyntaxError:
            if sys.exc_info() is not None:
                self.handle_exception(source=source)

        body = [nodes.Assign(nodes.Name("result", "store"), expr, lineno=1)]
        template = self.from_string(nodes.Template(body, lineno=1))
        return TemplateExpression(template, undefined_to_none)

    def compile_templates(
        self,
        target,
        extensions=None,
        filter_func=None,
        zip="deflated",
        log_function=None,
        ignore_errors=True,
        py_compile=False,
    ):
        """Finds all the templates the loader can find, compiles them
        and stores them in `target`.  If `zip` is `None`, instead of in a
        zipfile, the templates will be stored in a directory.
        By default a deflate zip algorithm is used. To switch to
        the stored algorithm, `zip` can be set to ``'stored'``.

        `extensions` and `filter_func` are passed to :meth:`list_templates`.
        Each template returned will be compiled to the target folder or
        zipfile.

        By default template compilation errors are ignored.  In case a
        log function is provided, errors are logged.  If you want template
        syntax errors to abort the compilation you can set `ignore_errors`
        to `False` and you will get an exception on syntax errors.

        If `py_compile` is set to `True` .pyc files will be written to the
        target instead of standard .py files.  This flag does not do anything
        on pypy and Python 3 where pyc files are not picked up by itself and
        don't give much benefit.

        .. versionadded:: 2.4
        """
        from .loaders import ModuleLoader

        if log_function is None:

            def log_function(x):
                pass

        if py_compile:
            if not PY2 or PYPY:
                import warnings

                warnings.warn(
                    "'py_compile=True' has no effect on PyPy or Python"
                    " 3 and will be removed in version 3.0",
                    DeprecationWarning,
                    stacklevel=2,
                )
                py_compile = False
            else:
                import imp
                import marshal

                py_header = imp.get_magic() + u"\xff\xff\xff\xff".encode("iso-8859-15")

                # Python 3.3 added a source filesize to the header
                if sys.version_info >= (3, 3):
                    py_header += u"\x00\x00\x00\x00".encode("iso-8859-15")

        def write_file(filename, data):
            if zip:
                info = ZipInfo(filename)
                info.external_attr = 0o755 << 16
                zip_file.writestr(info, data)
            else:
                if isinstance(data, text_type):
                    data = data.encode("utf8")

                with open(os.path.join(target, filename), "wb") as f:
                    f.write(data)

        if zip is not None:
            from zipfile import ZipFile, ZipInfo, ZIP_DEFLATED, ZIP_STORED

            zip_file = ZipFile(
                target, "w", dict(deflated=ZIP_DEFLATED, stored=ZIP_STORED)[zip]
            )
            log_function('Compiling into Zip archive "%s"' % target)
        else:
            if not os.path.isdir(target):
                os.makedirs(target)
            log_function('Compiling into folder "%s"' % target)

        try:
            for name in self.list_templates(extensions, filter_func):
                source, filename, _ = self.loader.get_source(self, name)
                try:
                    code = self.compile(source, name, filename, True, True)
                except TemplateSyntaxError as e:
                    if not ignore_errors:
                        raise
                    log_function('Could not compile "%s": %s' % (name, e))
                    continue

                filename = ModuleLoader.get_module_filename(name)

                if py_compile:
                    c = self._compile(code, encode_filename(filename))
                    write_file(filename + "c", py_header + marshal.dumps(c))
                    log_function('Byte-compiled "%s" as %s' % (name, filename + "c"))
                else:
                    write_file(filename, code)
                    log_function('Compiled "%s" as %s' % (name, filename))
        finally:
            if zip:
                zip_file.close()

        log_function("Finished compiling templates")

    def list_templates(self, extensions=None, filter_func=None):
        """Returns a list of templates for this environment.  This requires
        that the loader supports the loader's
        :meth:`~BaseLoader.list_templates` method.

        If there are other files in the template folder besides the
        actual templates, the returned list can be filtered.  There are two
        ways: either `extensions` is set to a list of file extensions for
        templates, or a `filter_func` can be provided which is a callable that
        is passed a template name and should return `True` if it should end up
        in the result list.

        If the loader does not support that, a :exc:`TypeError` is raised.

        .. versionadded:: 2.4
        """
        names = self.loader.list_templates()

        if extensions is not None:
            if filter_func is not None:
                raise TypeError(
                    "either extensions or filter_func can be passed, but not both"
                )

            def filter_func(x):
                return "." in x and x.rsplit(".", 1)[1] in extensions

        if filter_func is not None:
            names = [name for name in names if filter_func(name)]

        return names

    def handle_exception(self, source=None):
        """Exception handling helper.  This is used internally to either raise
        rewritten exceptions or return a rendered traceback for the template.
        """
        from .debug import rewrite_traceback_stack

        reraise(*rewrite_traceback_stack(source=source))

    def join_path(self, template, parent):
        """Join a template with the parent.  By default all the lookups are
        relative to the loader root so this method returns the `template`
        parameter unchanged, but if the paths should be relative to the
        parent template, this function can be used to calculate the real
        template name.

        Subclasses may override this method and implement template path
        joining here.
        """
        return template

    @internalcode
    def _load_template(self, name, globals):
        if self.loader is None:
            raise TypeError("no loader for this environment specified")
        cache_key = (weakref.ref(self.loader), name)
        if self.cache is not None:
            template = self.cache.get(cache_key)
            if template is not None and (
                not self.auto_reload or template.is_up_to_date
            ):
                return template
        template = self.loader.load(self, name, globals)
        if self.cache is not None:
            self.cache[cache_key] = template
        return template

    @internalcode
    def get_template(self, name, parent=None, globals=None):
        """Load a template from the loader.  If a loader is configured this
        method asks the loader for the template and returns a :class:`Template`.
        If the `parent` parameter is not `None`, :meth:`join_path` is called
        to get the real template name before loading.

        The `globals` parameter can be used to provide template wide globals.
        These variables are available in the context at render time.

        If the template does not exist a :exc:`TemplateNotFound` exception is
        raised.

        .. versionchanged:: 2.4
           If `name` is a :class:`Template` object it is returned from the
           function unchanged.
        """
        if isinstance(name, Template):
            return name
        if parent is not None:
            name = self.join_path(name, parent)
        return self._load_template(name, self.make_globals(globals))

    @internalcode
    def select_template(self, names, parent=None, globals=None):
        """Works like :meth:`get_template` but tries a number of templates
        before it fails.  If it cannot find any of the templates, it will
        raise a :exc:`TemplatesNotFound` exception.

        .. versionchanged:: 2.11
            If names is :class:`Undefined`, an :exc:`UndefinedError` is
            raised instead. If no templates were found and names
            contains :class:`Undefined`, the message is more helpful.

        .. versionchanged:: 2.4
           If `names` contains a :class:`Template` object it is returned
           from the function unchanged.

        .. versionadded:: 2.3
        """
        if isinstance(names, Undefined):
            names._fail_with_undefined_error()

        if not names:
            raise TemplatesNotFound(
                message=u"Tried to select from an empty list " u"of templates."
            )
        globals = self.make_globals(globals)
        for name in names:
            if isinstance(name, Template):
                return name
            if parent is not None:
                name = self.join_path(name, parent)
            try:
                return self._load_template(name, globals)
            except (TemplateNotFound, UndefinedError):
                pass
        raise TemplatesNotFound(names)

    @internalcode
    def get_or_select_template(self, template_name_or_list, parent=None, globals=None):
        """Does a typecheck and dispatches to :meth:`select_template`
        if an iterable of template names is given, otherwise to
        :meth:`get_template`.

        .. versionadded:: 2.3
        """
        if isinstance(template_name_or_list, (string_types, Undefined)):
            return self.get_template(template_name_or_list, parent, globals)
        elif isinstance(template_name_or_list, Template):
            return template_name_or_list
        return self.select_template(template_name_or_list, parent, globals)

    def from_string(self, source, globals=None, template_class=None):
        """Load a template from a string.  This parses the source given and
        returns a :class:`Template` object.
        """
        globals = self.make_globals(globals)
        cls = template_class or self.template_class
        return cls.from_code(self, self.compile(source), globals, None)

    def make_globals(self, d):
        """Return a dict for the globals."""
        if not d:
            return self.globals
        return dict(self.globals, **d)


class Template(object):
    """The central template object.  This class represents a compiled template
    and is used to evaluate it.

    Normally the template object is generated from an :class:`Environment` but
    it also has a constructor that makes it possible to create a template
    instance directly using the constructor.  It takes the same arguments as
    the environment constructor but it's not possible to specify a loader.

    Every template object has a few methods and members that are guaranteed
    to exist.  However it's important that a template object should be
    considered immutable.  Modifications on the object are not supported.

    Template objects created from the constructor rather than an environment
    do have an `environment` attribute that points to a temporary environment
    that is probably shared with other templates created with the constructor
    and compatible settings.

    >>> template = Template('Hello {{ name }}!')
    >>> template.render(name='John Doe') == u'Hello John Doe!'
    True
    >>> stream = template.stream(name='John Doe')
    >>> next(stream) == u'Hello John Doe!'
    True
    >>> next(stream)
    Traceback (most recent call last):
        ...
    StopIteration
    """

    #: Type of environment to create when creating a template directly
    #: rather than through an existing environment.
    environment_class = Environment

    def __new__(
        cls,
        source,
        block_start_string=BLOCK_START_STRING,
        block_end_string=BLOCK_END_STRING,
        variable_start_string=VARIABLE_START_STRING,
        variable_end_string=VARIABLE_END_STRING,
        comment_start_string=COMMENT_START_STRING,
        comment_end_string=COMMENT_END_STRING,
        line_statement_prefix=LINE_STATEMENT_PREFIX,
        line_comment_prefix=LINE_COMMENT_PREFIX,
        trim_blocks=TRIM_BLOCKS,
        lstrip_blocks=LSTRIP_BLOCKS,
        newline_sequence=NEWLINE_SEQUENCE,
        keep_trailing_newline=KEEP_TRAILING_NEWLINE,
        extensions=(),
        optimized=True,
        undefined=Undefined,
        finalize=None,
        autoescape=False,
        enable_async=False,
    ):
        env = get_spontaneous_environment(
            cls.environment_class,
            block_start_string,
            block_end_string,
            variable_start_string,
            variable_end_string,
            comment_start_string,
            comment_end_string,
            line_statement_prefix,
            line_comment_prefix,
            trim_blocks,
            lstrip_blocks,
            newline_sequence,
            keep_trailing_newline,
            frozenset(extensions),
            optimized,
            undefined,
            finalize,
            autoescape,
            None,
            0,
            False,
            None,
            enable_async,
        )
        return env.from_string(source, template_class=cls)

    @classmethod
    def from_code(cls, environment, code, globals, uptodate=None):
        """Creates a template object from compiled code and the globals.  This
        is used by the loaders and environment to create a template object.
        """
        namespace = {"environment": environment, "__file__": code.co_filename}
        exec(code, namespace)
        rv = cls._from_namespace(environment, namespace, globals)
        rv._uptodate = uptodate
        return rv

    @classmethod
    def from_module_dict(cls, environment, module_dict, globals):
        """Creates a template object from a module.  This is used by the
        module loader to create a template object.

        .. versionadded:: 2.4
        """
        return cls._from_namespace(environment, module_dict, globals)

    @classmethod
    def _from_namespace(cls, environment, namespace, globals):
        t = object.__new__(cls)
        t.environment = environment
        t.globals = globals
        t.name = namespace["name"]
        t.filename = namespace["__file__"]
        t.blocks = namespace["blocks"]

        # render function and module
        t.root_render_func = namespace["root"]
        t._module = None

        # debug and loader helpers
        t._debug_info = namespace["debug_info"]
        t._uptodate = None

        # store the reference
        namespace["environment"] = environment
        namespace["__jinja_template__"] = t

        return t

    def render(self, *args, **kwargs):
        """This method accepts the same arguments as the `dict` constructor:
        A dict, a dict subclass or some keyword arguments.  If no arguments
        are given the context will be empty.  These two calls do the same::

            template.render(knights='that say nih')
            template.render({'knights': 'that say nih'})

        This will return the rendered template as unicode string.
        """
        vars = dict(*args, **kwargs)
        try:
            return concat(self.root_render_func(self.new_context(vars)))
        except Exception:
            self.environment.handle_exception()

    def render_async(self, *args, **kwargs):
        """This works similar to :meth:`render` but returns a coroutine
        that when awaited returns the entire rendered template string.  This
        requires the async feature to be enabled.

        Example usage::

            await template.render_async(knights='that say nih; asynchronously')
        """
        # see asyncsupport for the actual implementation
        raise NotImplementedError(
            "This feature is not available for this version of Python"
        )

    def stream(self, *args, **kwargs):
        """Works exactly like :meth:`generate` but returns a
        :class:`TemplateStream`.
        """
        return TemplateStream(self.generate(*args, **kwargs))

    def generate(self, *args, **kwargs):
        """For very large templates it can be useful to not render the whole
        template at once but evaluate each statement after another and yield
        piece for piece.  This method basically does exactly that and returns
        a generator that yields one item after another as unicode strings.

        It accepts the same arguments as :meth:`render`.
        """
        vars = dict(*args, **kwargs)
        try:
            for event in self.root_render_func(self.new_context(vars)):
                yield event
        except Exception:
            yield self.environment.handle_exception()

    def generate_async(self, *args, **kwargs):
        """An async version of :meth:`generate`.  Works very similarly but
        returns an async iterator instead.
        """
        # see asyncsupport for the actual implementation
        raise NotImplementedError(
            "This feature is not available for this version of Python"
        )

    def new_context(self, vars=None, shared=False, locals=None):
        """Create a new :class:`Context` for this template.  The vars
        provided will be passed to the template.  Per default the globals
        are added to the context.  If shared is set to `True` the data
        is passed as is to the context without adding the globals.

        `locals` can be a dict of local variables for internal usage.
        """
        return new_context(
            self.environment, self.name, self.blocks, vars, shared, self.globals, locals
        )

    def make_module(self, vars=None, shared=False, locals=None):
        """This method works like the :attr:`module` attribute when called
        without arguments but it will evaluate the template on every call
        rather than caching it.  It's also possible to provide
        a dict which is then used as context.  The arguments are the same
        as for the :meth:`new_context` method.
        """
        return TemplateModule(self, self.new_context(vars, shared, locals))

    def make_module_async(self, vars=None, shared=False, locals=None):
        """As template module creation can invoke template code for
        asynchronous executions this method must be used instead of the
        normal :meth:`make_module` one.  Likewise the module attribute
        becomes unavailable in async mode.
        """
        # see asyncsupport for the actual implementation
        raise NotImplementedError(
            "This feature is not available for this version of Python"
        )

    @internalcode
    def _get_default_module(self):
        if self._module is not None:
            return self._module
        self._module = rv = self.make_module()
        return rv

    @property
    def module(self):
        """The template as module.  This is used for imports in the
        template runtime but is also useful if one wants to access
        exported template variables from the Python layer:

        >>> t = Template('{% macro foo() %}42{% endmacro %}23')
        >>> str(t.module)
        '23'
        >>> t.module.foo() == u'42'
        True

        This attribute is not available if async mode is enabled.
        """
        return self._get_default_module()

    def get_corresponding_lineno(self, lineno):
        """Return the source line number of a line number in the
        generated bytecode as they are not in sync.
        """
        for template_line, code_line in reversed(self.debug_info):
            if code_line <= lineno:
                return template_line
        return 1

    @property
    def is_up_to_date(self):
        """If this variable is `False` there is a newer version available."""
        if self._uptodate is None:
            return True
        return self._uptodate()

    @property
    def debug_info(self):
        """The debug info mapping."""
        if self._debug_info:
            return [tuple(map(int, x.split("="))) for x in self._debug_info.split("&")]
        return []

    def __repr__(self):
        if self.name is None:
            name = "memory:%x" % id(self)
        else:
            name = repr(self.name)
        return "<%s %s>" % (self.__class__.__name__, name)


@implements_to_string
class TemplateModule(object):
    """Represents an imported template.  All the exported names of the
    template are available as attributes on this object.  Additionally
    converting it into an unicode- or bytestrings renders the contents.
    """

    def __init__(self, template, context, body_stream=None):
        if body_stream is None:
            if context.environment.is_async:
                raise RuntimeError(
                    "Async mode requires a body stream "
                    "to be passed to a template module.  Use "
                    "the async methods of the API you are "
                    "using."
                )
            body_stream = list(template.root_render_func(context))
        self._body_stream = body_stream
        self.__dict__.update(context.get_exported())
        self.__name__ = template.name

    def __html__(self):
        return Markup(concat(self._body_stream))

    def __str__(self):
        return concat(self._body_stream)

    def __repr__(self):
        if self.__name__ is None:
            name = "memory:%x" % id(self)
        else:
            name = repr(self.__name__)
        return "<%s %s>" % (self.__class__.__name__, name)


class TemplateExpression(object):
    """The :meth:`jinja2.Environment.compile_expression` method returns an
    instance of this object.  It encapsulates the expression-like access
    to the template with an expression it wraps.
    """

    def __init__(self, template, undefined_to_none):
        self._template = template
        self._undefined_to_none = undefined_to_none

    def __call__(self, *args, **kwargs):
        context = self._template.new_context(dict(*args, **kwargs))
        consume(self._template.root_render_func(context))
        rv = context.vars["result"]
        if self._undefined_to_none and isinstance(rv, Undefined):
            rv = None
        return rv


@implements_iterator
class TemplateStream(object):
    """A template stream works pretty much like an ordinary python generator
    but it can buffer multiple items to reduce the number of total iterations.
    Per default the output is unbuffered which means that for every unbuffered
    instruction in the template one unicode string is yielded.

    If buffering is enabled with a buffer size of 5, five items are combined
    into a new unicode string.  This is mainly useful if you are streaming
    big templates to a client via WSGI which flushes after each iteration.
    """

    def __init__(self, gen):
        self._gen = gen
        self.disable_buffering()

    def dump(self, fp, encoding=None, errors="strict"):
        """Dump the complete stream into a file or file-like object.
        Per default unicode strings are written, if you want to encode
        before writing specify an `encoding`.

        Example usage::

            Template('Hello {{ name }}!').stream(name='foo').dump('hello.html')
        """
        close = False
        if isinstance(fp, string_types):
            if encoding is None:
                encoding = "utf-8"
            fp = open(fp, "wb")
            close = True
        try:
            if encoding is not None:
                iterable = (x.encode(encoding, errors) for x in self)
            else:
                iterable = self
            if hasattr(fp, "writelines"):
                fp.writelines(iterable)
            else:
                for item in iterable:
                    fp.write(item)
        finally:
            if close:
                fp.close()

    def disable_buffering(self):
        """Disable the output buffering."""
        self._next = partial(next, self._gen)
        self.buffered = False

    def _buffered_generator(self, size):
        buf = []
        c_size = 0
        push = buf.append

        while 1:
            try:
                while c_size < size:
                    c = next(self._gen)
                    push(c)
                    if c:
                        c_size += 1
            except StopIteration:
                if not c_size:
                    return
            yield concat(buf)
            del buf[:]
            c_size = 0

    def enable_buffering(self, size=5):
        """Enable buffering.  Buffer `size` items before yielding them."""
        if size <= 1:
            raise ValueError("buffer size too small")

        self.buffered = True
        self._next = partial(next, self._buffered_generator(size))

    def __iter__(self):
        return self

    def __next__(self):
        return self._next()


# hook in default template class.  if anyone reads this comment: ignore that
# it's possible to use custom templates ;-)
Environment.template_class = Template