tornado/template.py

#
# Copyright 2009 Facebook
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License 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.

"""A simple template system that compiles templates to Python code.

Basic usage looks like::

    t = template.Template("<html>{{ myvalue }}</html>")
    print(t.generate(myvalue="XXX"))

`Loader` is a class that loads templates from a root directory and caches
the compiled templates::

    loader = template.Loader("/home/btaylor")
    print(loader.load("test.html").generate(myvalue="XXX"))

We compile all templates to raw Python. Error-reporting is currently... uh,
interesting. Syntax for the templates::

    ### base.html
    <html>
      <head>
        <title>{% block title %}Default title{% end %}</title>
      </head>
      <body>
        <ul>
          {% for student in students %}
            {% block student %}
              <li>{{ escape(student.name) }}</li>
            {% end %}
          {% end %}
        </ul>
      </body>
    </html>

    ### bold.html
    {% extends "base.html" %}

    {% block title %}A bolder title{% end %}

    {% block student %}
      <li><span style="bold">{{ escape(student.name) }}</span></li>
    {% end %}

Unlike most other template systems, we do not put any restrictions on the
expressions you can include in your statements. ``if`` and ``for`` blocks get
translated exactly into Python, so you can do complex expressions like::

   {% for student in [p for p in people if p.student and p.age > 23] %}
     <li>{{ escape(student.name) }}</li>
   {% end %}

Translating directly to Python means you can apply functions to expressions
easily, like the ``escape()`` function in the examples above. You can pass
functions in to your template just like any other variable
(In a `.RequestHandler`, override `.RequestHandler.get_template_namespace`)::

   ### Python code
   def add(x, y):
      return x + y
   template.execute(add=add)

   ### The template
   {{ add(1, 2) }}

We provide the functions `escape() <.xhtml_escape>`, `.url_escape()`,
`.json_encode()`, and `.squeeze()` to all templates by default.

Typical applications do not create `Template` or `Loader` instances by
hand, but instead use the `~.RequestHandler.render` and
`~.RequestHandler.render_string` methods of
`tornado.web.RequestHandler`, which load templates automatically based
on the ``template_path`` `.Application` setting.

Variable names beginning with ``_tt_`` are reserved by the template
system and should not be used by application code.

Syntax Reference
----------------

Template expressions are surrounded by double curly braces: ``{{ ... }}``.
The contents may be any python expression, which will be escaped according
to the current autoescape setting and inserted into the output.  Other
template directives use ``{% %}``.

To comment out a section so that it is omitted from the output, surround it
with ``{# ... #}``.


To include a literal ``{{``, ``{%``, or ``{#`` in the output, escape them as
``{{!``, ``{%!``, and ``{#!``, respectively.


``{% apply *function* %}...{% end %}``
    Applies a function to the output of all template code between ``apply``
    and ``end``::

        {% apply linkify %}{{name}} said: {{message}}{% end %}

    Note that as an implementation detail apply blocks are implemented
    as nested functions and thus may interact strangely with variables
    set via ``{% set %}``, or the use of ``{% break %}`` or ``{% continue %}``
    within loops.

``{% autoescape *function* %}``
    Sets the autoescape mode for the current file.  This does not affect
    other files, even those referenced by ``{% include %}``.  Note that
    autoescaping can also be configured globally, at the `.Application`
    or `Loader`.::

        {% autoescape xhtml_escape %}
        {% autoescape None %}

``{% block *name* %}...{% end %}``
    Indicates a named, replaceable block for use with ``{% extends %}``.
    Blocks in the parent template will be replaced with the contents of
    the same-named block in a child template.::

        <!-- base.html -->
        <title>{% block title %}Default title{% end %}</title>

        <!-- mypage.html -->
        {% extends "base.html" %}
        {% block title %}My page title{% end %}

``{% comment ... %}``
    A comment which will be removed from the template output.  Note that
    there is no ``{% end %}`` tag; the comment goes from the word ``comment``
    to the closing ``%}`` tag.

``{% extends *filename* %}``
    Inherit from another template.  Templates that use ``extends`` should
    contain one or more ``block`` tags to replace content from the parent
    template.  Anything in the child template not contained in a ``block``
    tag will be ignored.  For an example, see the ``{% block %}`` tag.

``{% for *var* in *expr* %}...{% end %}``
    Same as the python ``for`` statement.  ``{% break %}`` and
    ``{% continue %}`` may be used inside the loop.

``{% from *x* import *y* %}``
    Same as the python ``import`` statement.

``{% if *condition* %}...{% elif *condition* %}...{% else %}...{% end %}``
    Conditional statement - outputs the first section whose condition is
    true.  (The ``elif`` and ``else`` sections are optional)

``{% import *module* %}``
    Same as the python ``import`` statement.

``{% include *filename* %}``
    Includes another template file.  The included file can see all the local
    variables as if it were copied directly to the point of the ``include``
    directive (the ``{% autoescape %}`` directive is an exception).
    Alternately, ``{% module Template(filename, **kwargs) %}`` may be used
    to include another template with an isolated namespace.

``{% module *expr* %}``
    Renders a `~tornado.web.UIModule`.  The output of the ``UIModule`` is
    not escaped::

        {% module Template("foo.html", arg=42) %}

    ``UIModules`` are a feature of the `tornado.web.RequestHandler`
    class (and specifically its ``render`` method) and will not work
    when the template system is used on its own in other contexts.

``{% raw *expr* %}``
    Outputs the result of the given expression without autoescaping.

``{% set *x* = *y* %}``
    Sets a local variable.

``{% try %}...{% except %}...{% else %}...{% finally %}...{% end %}``
    Same as the python ``try`` statement.

``{% while *condition* %}... {% end %}``
    Same as the python ``while`` statement.  ``{% break %}`` and
    ``{% continue %}`` may be used inside the loop.

``{% whitespace *mode* %}``
    Sets the whitespace mode for the remainder of the current file
    (or until the next ``{% whitespace %}`` directive). See
    `filter_whitespace` for available options. New in Tornado 4.3.
"""

import datetime
from io import StringIO
import linecache
import os.path
import posixpath
import re
import threading

from tornado import escape
from tornado.log import app_log
from tornado.util import ObjectDict, exec_in, unicode_type

from typing import Any, Union, Callable, List, Dict, Iterable, Optional, TextIO
import typing

if typing.TYPE_CHECKING:
    from typing import Tuple, ContextManager  # noqa: F401

_DEFAULT_AUTOESCAPE = "xhtml_escape"


class _UnsetMarker:
    pass


_UNSET = _UnsetMarker()


def filter_whitespace(mode: str, text: str) -> str:
    """Transform whitespace in ``text`` according to ``mode``.

    Available modes are:

    * ``all``: Return all whitespace unmodified.
    * ``single``: Collapse consecutive whitespace with a single whitespace
      character, preserving newlines.
    * ``oneline``: Collapse all runs of whitespace into a single space
      character, removing all newlines in the process.

    .. versionadded:: 4.3
    """
    if mode == "all":
        return text
    elif mode == "single":
        text = re.sub(r"([\t ]+)", " ", text)
        text = re.sub(r"(\s*\n\s*)", "\n", text)
        return text
    elif mode == "oneline":
        return re.sub(r"(\s+)", " ", text)
    else:
        raise Exception("invalid whitespace mode %s" % mode)


class Template(object):
    """A compiled template.

    We compile into Python from the given template_string. You can generate
    the template from variables with generate().
    """

    # note that the constructor's signature is not extracted with
    # autodoc because _UNSET looks like garbage.  When changing
    # this signature update website/sphinx/template.rst too.
    def __init__(
        self,
        template_string: Union[str, bytes],
        name: str = "<string>",
        loader: Optional["BaseLoader"] = None,
        compress_whitespace: Union[bool, _UnsetMarker] = _UNSET,
        autoescape: Optional[Union[str, _UnsetMarker]] = _UNSET,
        whitespace: Optional[str] = None,
    ) -> None:
        """Construct a Template.

        :arg str template_string: the contents of the template file.
        :arg str name: the filename from which the template was loaded
            (used for error message).
        :arg tornado.template.BaseLoader loader: the `~tornado.template.BaseLoader` responsible
            for this template, used to resolve ``{% include %}`` and ``{% extend %}`` directives.
        :arg bool compress_whitespace: Deprecated since Tornado 4.3.
            Equivalent to ``whitespace="single"`` if true and
            ``whitespace="all"`` if false.
        :arg str autoescape: The name of a function in the template
            namespace, or ``None`` to disable escaping by default.
        :arg str whitespace: A string specifying treatment of whitespace;
            see `filter_whitespace` for options.

        .. versionchanged:: 4.3
           Added ``whitespace`` parameter; deprecated ``compress_whitespace``.
        """
        self.name = escape.native_str(name)

        if compress_whitespace is not _UNSET:
            # Convert deprecated compress_whitespace (bool) to whitespace (str).
            if whitespace is not None:
                raise Exception("cannot set both whitespace and compress_whitespace")
            whitespace = "single" if compress_whitespace else "all"
        if whitespace is None:
            if loader and loader.whitespace:
                whitespace = loader.whitespace
            else:
                # Whitespace defaults by filename.
                if name.endswith(".html") or name.endswith(".js"):
                    whitespace = "single"
                else:
                    whitespace = "all"
        # Validate the whitespace setting.
        assert whitespace is not None
        filter_whitespace(whitespace, "")

        if not isinstance(autoescape, _UnsetMarker):
            self.autoescape = autoescape  # type: Optional[str]
        elif loader:
            self.autoescape = loader.autoescape
        else:
            self.autoescape = _DEFAULT_AUTOESCAPE

        self.namespace = loader.namespace if loader else {}
        reader = _TemplateReader(name, escape.native_str(template_string), whitespace)
        self.file = _File(self, _parse(reader, self))
        self.code = self._generate_python(loader)
        self.loader = loader
        try:
            # Under python2.5, the fake filename used here must match
            # the module name used in __name__ below.
            # The dont_inherit flag prevents template.py's future imports
            # from being applied to the generated code.
            self.compiled = compile(
                escape.to_unicode(self.code),
                "%s.generated.py" % self.name.replace(".", "_"),
                "exec",
                dont_inherit=True,
            )
        except Exception:
            formatted_code = _format_code(self.code).rstrip()
            app_log.error("%s code:\n%s", self.name, formatted_code)
            raise

    def generate(self, **kwargs: Any) -> bytes:
        """Generate this template with the given arguments."""
        namespace = {
            "escape": escape.xhtml_escape,
            "xhtml_escape": escape.xhtml_escape,
            "url_escape": escape.url_escape,
            "json_encode": escape.json_encode,
            "squeeze": escape.squeeze,
            "linkify": escape.linkify,
            "datetime": datetime,
            "_tt_utf8": escape.utf8,  # for internal use
            "_tt_string_types": (unicode_type, bytes),
            # __name__ and __loader__ allow the traceback mechanism to find
            # the generated source code.
            "__name__": self.name.replace(".", "_"),
            "__loader__": ObjectDict(get_source=lambda name: self.code),
        }
        namespace.update(self.namespace)
        namespace.update(kwargs)
        exec_in(self.compiled, namespace)
        execute = typing.cast(Callable[[], bytes], namespace["_tt_execute"])
        # Clear the traceback module's cache of source data now that
        # we've generated a new template (mainly for this module's
        # unittests, where different tests reuse the same name).
        linecache.clearcache()
        return execute()

    def _generate_python(self, loader: Optional["BaseLoader"]) -> str:
        buffer = StringIO()
        try:
            # named_blocks maps from names to _NamedBlock objects
            named_blocks = {}  # type: Dict[str, _NamedBlock]
            ancestors = self._get_ancestors(loader)
            ancestors.reverse()
            for ancestor in ancestors:
                ancestor.find_named_blocks(loader, named_blocks)
            writer = _CodeWriter(buffer, named_blocks, loader, ancestors[0].template)
            ancestors[0].generate(writer)
            return buffer.getvalue()
        finally:
            buffer.close()

    def _get_ancestors(self, loader: Optional["BaseLoader"]) -> List["_File"]:
        ancestors = [self.file]
        for chunk in self.file.body.chunks:
            if isinstance(chunk, _ExtendsBlock):
                if not loader:
                    raise ParseError(
                        "{% extends %} block found, but no " "template loader"
                    )
                template = loader.load(chunk.name, self.name)
                ancestors.extend(template._get_ancestors(loader))
        return ancestors


class BaseLoader(object):
    """Base class for template loaders.

    You must use a template loader to use template constructs like
    ``{% extends %}`` and ``{% include %}``. The loader caches all
    templates after they are loaded the first time.
    """

    def __init__(
        self,
        autoescape: str = _DEFAULT_AUTOESCAPE,
        namespace: Optional[Dict[str, Any]] = None,
        whitespace: Optional[str] = None,
    ) -> None:
        """Construct a template loader.

        :arg str autoescape: The name of a function in the template
            namespace, such as "xhtml_escape", or ``None`` to disable
            autoescaping by default.
        :arg dict namespace: A dictionary to be added to the default template
            namespace, or ``None``.
        :arg str whitespace: A string specifying default behavior for
            whitespace in templates; see `filter_whitespace` for options.
            Default is "single" for files ending in ".html" and ".js" and
            "all" for other files.

        .. versionchanged:: 4.3
           Added ``whitespace`` parameter.
        """
        self.autoescape = autoescape
        self.namespace = namespace or {}
        self.whitespace = whitespace
        self.templates = {}  # type: Dict[str, Template]
        # self.lock protects self.templates.  It's a reentrant lock
        # because templates may load other templates via `include` or
        # `extends`.  Note that thanks to the GIL this code would be safe
        # even without the lock, but could lead to wasted work as multiple
        # threads tried to compile the same template simultaneously.
        self.lock = threading.RLock()

    def reset(self) -> None:
        """Resets the cache of compiled templates."""
        with self.lock:
            self.templates = {}

    def resolve_path(self, name: str, parent_path: Optional[str] = None) -> str:
        """Converts a possibly-relative path to absolute (used internally)."""
        raise NotImplementedError()

    def load(self, name: str, parent_path: Optional[str] = None) -> Template:
        """Loads a template."""
        name = self.resolve_path(name, parent_path=parent_path)
        with self.lock:
            if name not in self.templates:
                self.templates[name] = self._create_template(name)
            return self.templates[name]

    def _create_template(self, name: str) -> Template:
        raise NotImplementedError()


class Loader(BaseLoader):
    """A template loader that loads from a single root directory."""

    def __init__(self, root_directory: str, **kwargs: Any) -> None:
        super().__init__(**kwargs)
        self.root = os.path.abspath(root_directory)

    def resolve_path(self, name: str, parent_path: Optional[str] = None) -> str:
        if (
            parent_path
            and not parent_path.startswith("<")
            and not parent_path.startswith("/")
            and not name.startswith("/")
        ):
            current_path = os.path.join(self.root, parent_path)
            file_dir = os.path.dirname(os.path.abspath(current_path))
            relative_path = os.path.abspath(os.path.join(file_dir, name))
            if relative_path.startswith(self.root):
                name = relative_path[len(self.root) + 1 :]
        return name

    def _create_template(self, name: str) -> Template:
        path = os.path.join(self.root, name)
        with open(path, "rb") as f:
            template = Template(f.read(), name=name, loader=self)
            return template


class DictLoader(BaseLoader):
    """A template loader that loads from a dictionary."""

    def __init__(self, dict: Dict[str, str], **kwargs: Any) -> None:
        super().__init__(**kwargs)
        self.dict = dict

    def resolve_path(self, name: str, parent_path: Optional[str] = None) -> str:
        if (
            parent_path
            and not parent_path.startswith("<")
            and not parent_path.startswith("/")
            and not name.startswith("/")
        ):
            file_dir = posixpath.dirname(parent_path)
            name = posixpath.normpath(posixpath.join(file_dir, name))
        return name

    def _create_template(self, name: str) -> Template:
        return Template(self.dict[name], name=name, loader=self)


class _Node(object):
    def each_child(self) -> Iterable["_Node"]:
        return ()

    def generate(self, writer: "_CodeWriter") -> None:
        raise NotImplementedError()

    def find_named_blocks(
        self, loader: Optional[BaseLoader], named_blocks: Dict[str, "_NamedBlock"]
    ) -> None:
        for child in self.each_child():
            child.find_named_blocks(loader, named_blocks)


class _File(_Node):
    def __init__(self, template: Template, body: "_ChunkList") -> None:
        self.template = template
        self.body = body
        self.line = 0

    def generate(self, writer: "_CodeWriter") -> None:
        writer.write_line("def _tt_execute():", self.line)
        with writer.indent():
            writer.write_line("_tt_buffer = []", self.line)
            writer.write_line("_tt_append = _tt_buffer.append", self.line)
            self.body.generate(writer)
            writer.write_line("return _tt_utf8('').join(_tt_buffer)", self.line)

    def each_child(self) -> Iterable["_Node"]:
        return (self.body,)


class _ChunkList(_Node):
    def __init__(self, chunks: List[_Node]) -> None:
        self.chunks = chunks

    def generate(self, writer: "_CodeWriter") -> None:
        for chunk in self.chunks:
            chunk.generate(writer)

    def each_child(self) -> Iterable["_Node"]:
        return self.chunks


class _NamedBlock(_Node):
    def __init__(self, name: str, body: _Node, template: Template, line: int) -> None:
        self.name = name
        self.body = body
        self.template = template
        self.line = line

    def each_child(self) -> Iterable["_Node"]:
        return (self.body,)

    def generate(self, writer: "_CodeWriter") -> None:
        block = writer.named_blocks[self.name]
        with writer.include(block.template, self.line):
            block.body.generate(writer)

    def find_named_blocks(
        self, loader: Optional[BaseLoader], named_blocks: Dict[str, "_NamedBlock"]
    ) -> None:
        named_blocks[self.name] = self
        _Node.find_named_blocks(self, loader, named_blocks)


class _ExtendsBlock(_Node):
    def __init__(self, name: str) -> None:
        self.name = name


class _IncludeBlock(_Node):
    def __init__(self, name: str, reader: "_TemplateReader", line: int) -> None:
        self.name = name
        self.template_name = reader.name
        self.line = line

    def find_named_blocks(
        self, loader: Optional[BaseLoader], named_blocks: Dict[str, _NamedBlock]
    ) -> None:
        assert loader is not None
        included = loader.load(self.name, self.template_name)
        included.file.find_named_blocks(loader, named_blocks)

    def generate(self, writer: "_CodeWriter") -> None:
        assert writer.loader is not None
        included = writer.loader.load(self.name, self.template_name)
        with writer.include(included, self.line):
            included.file.body.generate(writer)


class _ApplyBlock(_Node):
    def __init__(self, method: str, line: int, body: _Node) -> None:
        self.method = method
        self.line = line
        self.body = body

    def each_child(self) -> Iterable["_Node"]:
        return (self.body,)

    def generate(self, writer: "_CodeWriter") -> None:
        method_name = "_tt_apply%d" % writer.apply_counter
        writer.apply_counter += 1
        writer.write_line("def %s():" % method_name, self.line)
        with writer.indent():
            writer.write_line("_tt_buffer = []", self.line)
            writer.write_line("_tt_append = _tt_buffer.append", self.line)
            self.body.generate(writer)
            writer.write_line("return _tt_utf8('').join(_tt_buffer)", self.line)
        writer.write_line(
            "_tt_append(_tt_utf8(%s(%s())))" % (self.method, method_name), self.line
        )


class _ControlBlock(_Node):
    def __init__(self, statement: str, line: int, body: _Node) -> None:
        self.statement = statement
        self.line = line
        self.body = body

    def each_child(self) -> Iterable[_Node]:
        return (self.body,)

    def generate(self, writer: "_CodeWriter") -> None:
        writer.write_line("%s:" % self.statement, self.line)
        with writer.indent():
            self.body.generate(writer)
            # Just in case the body was empty
            writer.write_line("pass", self.line)


class _IntermediateControlBlock(_Node):
    def __init__(self, statement: str, line: int) -> None:
        self.statement = statement
        self.line = line

    def generate(self, writer: "_CodeWriter") -> None:
        # In case the previous block was empty
        writer.write_line("pass", self.line)
        writer.write_line("%s:" % self.statement, self.line, writer.indent_size() - 1)


class _Statement(_Node):
    def __init__(self, statement: str, line: int) -> None:
        self.statement = statement
        self.line = line

    def generate(self, writer: "_CodeWriter") -> None:
        writer.write_line(self.statement, self.line)


class _Expression(_Node):
    def __init__(self, expression: str, line: int, raw: bool = False) -> None:
        self.expression = expression
        self.line = line
        self.raw = raw

    def generate(self, writer: "_CodeWriter") -> None:
        writer.write_line("_tt_tmp = %s" % self.expression, self.line)
        writer.write_line(
            "if isinstance(_tt_tmp, _tt_string_types):" " _tt_tmp = _tt_utf8(_tt_tmp)",
            self.line,
        )
        writer.write_line("else: _tt_tmp = _tt_utf8(str(_tt_tmp))", self.line)
        if not self.raw and writer.current_template.autoescape is not None:
            # In python3 functions like xhtml_escape return unicode,
            # so we have to convert to utf8 again.
            writer.write_line(
                "_tt_tmp = _tt_utf8(%s(_tt_tmp))" % writer.current_template.autoescape,
                self.line,
            )
        writer.write_line("_tt_append(_tt_tmp)", self.line)


class _Module(_Expression):
    def __init__(self, expression: str, line: int) -> None:
        super().__init__("_tt_modules." + expression, line, raw=True)


class _Text(_Node):
    def __init__(self, value: str, line: int, whitespace: str) -> None:
        self.value = value
        self.line = line
        self.whitespace = whitespace

    def generate(self, writer: "_CodeWriter") -> None:
        value = self.value

        # Compress whitespace if requested, with a crude heuristic to avoid
        # altering preformatted whitespace.
        if "<pre>" not in value:
            value = filter_whitespace(self.whitespace, value)

        if value:
            writer.write_line("_tt_append(%r)" % escape.utf8(value), self.line)


class ParseError(Exception):
    """Raised for template syntax errors.

    ``ParseError`` instances have ``filename`` and ``lineno`` attributes
    indicating the position of the error.

    .. versionchanged:: 4.3
       Added ``filename`` and ``lineno`` attributes.
    """

    def __init__(
        self, message: str, filename: Optional[str] = None, lineno: int = 0
    ) -> None:
        self.message = message
        # The names "filename" and "lineno" are chosen for consistency
        # with python SyntaxError.
        self.filename = filename
        self.lineno = lineno

    def __str__(self) -> str:
        return "%s at %s:%d" % (self.message, self.filename, self.lineno)


class _CodeWriter(object):
    def __init__(
        self,
        file: TextIO,
        named_blocks: Dict[str, _NamedBlock],
        loader: Optional[BaseLoader],
        current_template: Template,
    ) -> None:
        self.file = file
        self.named_blocks = named_blocks
        self.loader = loader
        self.current_template = current_template
        self.apply_counter = 0
        self.include_stack = []  # type: List[Tuple[Template, int]]
        self._indent = 0

    def indent_size(self) -> int:
        return self._indent

    def indent(self) -> "ContextManager":
        class Indenter(object):
            def __enter__(_) -> "_CodeWriter":
                self._indent += 1
                return self

            def __exit__(_, *args: Any) -> None:
                assert self._indent > 0
                self._indent -= 1

        return Indenter()

    def include(self, template: Template, line: int) -> "ContextManager":
        self.include_stack.append((self.current_template, line))
        self.current_template = template

        class IncludeTemplate(object):
            def __enter__(_) -> "_CodeWriter":
                return self

            def __exit__(_, *args: Any) -> None:
                self.current_template = self.include_stack.pop()[0]

        return IncludeTemplate()

    def write_line(
        self, line: str, line_number: int, indent: Optional[int] = None
    ) -> None:
        if indent is None:
            indent = self._indent
        line_comment = "  # %s:%d" % (self.current_template.name, line_number)
        if self.include_stack:
            ancestors = [
                "%s:%d" % (tmpl.name, lineno) for (tmpl, lineno) in self.include_stack
            ]
            line_comment += " (via %s)" % ", ".join(reversed(ancestors))
        print("    " * indent + line + line_comment, file=self.file)


class _TemplateReader(object):
    def __init__(self, name: str, text: str, whitespace: str) -> None:
        self.name = name
        self.text = text
        self.whitespace = whitespace
        self.line = 1
        self.pos = 0

    def find(self, needle: str, start: int = 0, end: Optional[int] = None) -> int:
        assert start >= 0, start
        pos = self.pos
        start += pos
        if end is None:
            index = self.text.find(needle, start)
        else:
            end += pos
            assert end >= start
            index = self.text.find(needle, start, end)
        if index != -1:
            index -= pos
        return index

    def consume(self, count: Optional[int] = None) -> str:
        if count is None:
            count = len(self.text) - self.pos
        newpos = self.pos + count
        self.line += self.text.count("\n", self.pos, newpos)
        s = self.text[self.pos : newpos]
        self.pos = newpos
        return s

    def remaining(self) -> int:
        return len(self.text) - self.pos

    def __len__(self) -> int:
        return self.remaining()

    def __getitem__(self, key: Union[int, slice]) -> str:
        if isinstance(key, slice):
            size = len(self)
            start, stop, step = key.indices(size)
            if start is None:
                start = self.pos
            else:
                start += self.pos
            if stop is not None:
                stop += self.pos
            return self.text[slice(start, stop, step)]
        elif key < 0:
            return self.text[key]
        else:
            return self.text[self.pos + key]

    def __str__(self) -> str:
        return self.text[self.pos :]

    def raise_parse_error(self, msg: str) -> None:
        raise ParseError(msg, self.name, self.line)


def _format_code(code: str) -> str:
    lines = code.splitlines()
    format = "%%%dd  %%s\n" % len(repr(len(lines) + 1))
    return "".join([format % (i + 1, line) for (i, line) in enumerate(lines)])


def _parse(
    reader: _TemplateReader,
    template: Template,
    in_block: Optional[str] = None,
    in_loop: Optional[str] = None,
) -> _ChunkList:
    body = _ChunkList([])
    while True:
        # Find next template directive
        curly = 0
        while True:
            curly = reader.find("{", curly)
            if curly == -1 or curly + 1 == reader.remaining():
                # EOF
                if in_block:
                    reader.raise_parse_error(
                        "Missing {%% end %%} block for %s" % in_block
                    )
                body.chunks.append(
                    _Text(reader.consume(), reader.line, reader.whitespace)
                )
                return body
            # If the first curly brace is not the start of a special token,
            # start searching from the character after it
            if reader[curly + 1] not in ("{", "%", "#"):
                curly += 1
                continue
            # When there are more than 2 curlies in a row, use the
            # innermost ones.  This is useful when generating languages
            # like latex where curlies are also meaningful
            if (
                curly + 2 < reader.remaining()
                and reader[curly + 1] == "{"
                and reader[curly + 2] == "{"
            ):
                curly += 1
                continue
            break

        # Append any text before the special token
        if curly > 0:
            cons = reader.consume(curly)
            body.chunks.append(_Text(cons, reader.line, reader.whitespace))

        start_brace = reader.consume(2)
        line = reader.line

        # Template directives may be escaped as "{{!" or "{%!".
        # In this case output the braces and consume the "!".
        # This is especially useful in conjunction with jquery templates,
        # which also use double braces.
        if reader.remaining() and reader[0] == "!":
            reader.consume(1)
            body.chunks.append(_Text(start_brace, line, reader.whitespace))
            continue

        # Comment
        if start_brace == "{#":
            end = reader.find("#}")
            if end == -1:
                reader.raise_parse_error("Missing end comment #}")
            contents = reader.consume(end).strip()
            reader.consume(2)
            continue

        # Expression
        if start_brace == "{{":
            end = reader.find("}}")
            if end == -1:
                reader.raise_parse_error("Missing end expression }}")
            contents = reader.consume(end).strip()
            reader.consume(2)
            if not contents:
                reader.raise_parse_error("Empty expression")
            body.chunks.append(_Expression(contents, line))
            continue

        # Block
        assert start_brace == "{%", start_brace
        end = reader.find("%}")
        if end == -1:
            reader.raise_parse_error("Missing end block %}")
        contents = reader.consume(end).strip()
        reader.consume(2)
        if not contents:
            reader.raise_parse_error("Empty block tag ({% %})")

        operator, space, suffix = contents.partition(" ")
        suffix = suffix.strip()

        # Intermediate ("else", "elif", etc) blocks
        intermediate_blocks = {
            "else": set(["if", "for", "while", "try"]),
            "elif": set(["if"]),
            "except": set(["try"]),
            "finally": set(["try"]),
        }
        allowed_parents = intermediate_blocks.get(operator)
        if allowed_parents is not None:
            if not in_block:
                reader.raise_parse_error(
                    "%s outside %s block" % (operator, allowed_parents)
                )
            if in_block not in allowed_parents:
                reader.raise_parse_error(
                    "%s block cannot be attached to %s block" % (operator, in_block)
                )
            body.chunks.append(_IntermediateControlBlock(contents, line))
            continue

        # End tag
        elif operator == "end":
            if not in_block:
                reader.raise_parse_error("Extra {% end %} block")
            return body

        elif operator in (
            "extends",
            "include",
            "set",
            "import",
            "from",
            "comment",
            "autoescape",
            "whitespace",
            "raw",
            "module",
        ):
            if operator == "comment":
                continue
            if operator == "extends":
                suffix = suffix.strip('"').strip("'")
                if not suffix:
                    reader.raise_parse_error("extends missing file path")
                block = _ExtendsBlock(suffix)  # type: _Node
            elif operator in ("import", "from"):
                if not suffix:
                    reader.raise_parse_error("import missing statement")
                block = _Statement(contents, line)
            elif operator == "include":
                suffix = suffix.strip('"').strip("'")
                if not suffix:
                    reader.raise_parse_error("include missing file path")
                block = _IncludeBlock(suffix, reader, line)
            elif operator == "set":
                if not suffix:
                    reader.raise_parse_error("set missing statement")
                block = _Statement(suffix, line)
            elif operator == "autoescape":
                fn = suffix.strip()  # type: Optional[str]
                if fn == "None":
                    fn = None
                template.autoescape = fn
                continue
            elif operator == "whitespace":
                mode = suffix.strip()
                # Validate the selected mode
                filter_whitespace(mode, "")
                reader.whitespace = mode
                continue
            elif operator == "raw":
                block = _Expression(suffix, line, raw=True)
            elif operator == "module":
                block = _Module(suffix, line)
            body.chunks.append(block)
            continue

        elif operator in ("apply", "block", "try", "if", "for", "while"):
            # parse inner body recursively
            if operator in ("for", "while"):
                block_body = _parse(reader, template, operator, operator)
            elif operator == "apply":
                # apply creates a nested function so syntactically it's not
                # in the loop.
                block_body = _parse(reader, template, operator, None)
            else:
                block_body = _parse(reader, template, operator, in_loop)

            if operator == "apply":
                if not suffix:
                    reader.raise_parse_error("apply missing method name")
                block = _ApplyBlock(suffix, line, block_body)
            elif operator == "block":
                if not suffix:
                    reader.raise_parse_error("block missing name")
                block = _NamedBlock(suffix, block_body, template, line)
            else:
                block = _ControlBlock(contents, line, block_body)
            body.chunks.append(block)
            continue

        elif operator in ("break", "continue"):
            if not in_loop:
                reader.raise_parse_error(
                    "%s outside %s block" % (operator, set(["for", "while"]))
                )
            body.chunks.append(_Statement(contents, line))
            continue

        else:
            reader.raise_parse_error("unknown operator: %r" % operator)