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# Licensed under the Apache License: http://www.apache.org/licenses/LICENSE-2.0
# For details: https://github.com/nedbat/coveragepy/blob/master/NOTICE.txt
"""A simple Python template renderer, for a nano-subset of Django syntax.
For a detailed discussion of this code, see this chapter from 500 Lines:
http://aosabook.org/en/500L/a-template-engine.html
"""
# Coincidentally named the same as http://code.activestate.com/recipes/496702/
from __future__ import annotations
import re
from typing import (
Any, Callable, Dict, List, NoReturn, Optional, Set, Union, cast,
)
class TempliteSyntaxError(ValueError):
"""Raised when a template has a syntax error."""
pass
class TempliteValueError(ValueError):
"""Raised when an expression won't evaluate in a template."""
pass
class CodeBuilder:
"""Build source code conveniently."""
def __init__(self, indent: int = 0) -> None:
self.code: List[Union[str, CodeBuilder]] = []
self.indent_level = indent
def __str__(self) -> str:
return "".join(str(c) for c in self.code)
def add_line(self, line: str) -> None:
"""Add a line of source to the code.
Indentation and newline will be added for you, don't provide them.
"""
self.code.extend([" " * self.indent_level, line, "\n"])
def add_section(self) -> CodeBuilder:
"""Add a section, a sub-CodeBuilder."""
section = CodeBuilder(self.indent_level)
self.code.append(section)
return section
INDENT_STEP = 4 # PEP8 says so!
def indent(self) -> None:
"""Increase the current indent for following lines."""
self.indent_level += self.INDENT_STEP
def dedent(self) -> None:
"""Decrease the current indent for following lines."""
self.indent_level -= self.INDENT_STEP
def get_globals(self) -> Dict[str, Any]:
"""Execute the code, and return a dict of globals it defines."""
# A check that the caller really finished all the blocks they started.
assert self.indent_level == 0
# Get the Python source as a single string.
python_source = str(self)
# Execute the source, defining globals, and return them.
global_namespace: Dict[str, Any] = {}
exec(python_source, global_namespace)
return global_namespace
class Templite:
"""A simple template renderer, for a nano-subset of Django syntax.
Supported constructs are extended variable access::
{{var.modifier.modifier|filter|filter}}
loops::
{% for var in list %}...{% endfor %}
and ifs::
{% if var %}...{% endif %}
Comments are within curly-hash markers::
{# This will be ignored #}
Lines between `{% joined %}` and `{% endjoined %}` will have lines stripped
and joined. Be careful, this could join words together!
Any of these constructs can have a hyphen at the end (`-}}`, `-%}`, `-#}`),
which will collapse the white space following the tag.
Construct a Templite with the template text, then use `render` against a
dictionary context to create a finished string::
templite = Templite('''
<h1>Hello {{name|upper}}!</h1>
{% for topic in topics %}
<p>You are interested in {{topic}}.</p>
{% endif %}
''',
{"upper": str.upper},
)
text = templite.render({
"name": "Ned",
"topics": ["Python", "Geometry", "Juggling"],
})
"""
def __init__(self, text: str, *contexts: Dict[str, Any]) -> None:
"""Construct a Templite with the given `text`.
`contexts` are dictionaries of values to use for future renderings.
These are good for filters and global values.
"""
self.context = {}
for context in contexts:
self.context.update(context)
self.all_vars: Set[str] = set()
self.loop_vars: Set[str] = set()
# We construct a function in source form, then compile it and hold onto
# it, and execute it to render the template.
code = CodeBuilder()
code.add_line("def render_function(context, do_dots):")
code.indent()
vars_code = code.add_section()
code.add_line("result = []")
code.add_line("append_result = result.append")
code.add_line("extend_result = result.extend")
code.add_line("to_str = str")
buffered: List[str] = []
def flush_output() -> None:
"""Force `buffered` to the code builder."""
if len(buffered) == 1:
code.add_line("append_result(%s)" % buffered[0])
elif len(buffered) > 1:
code.add_line("extend_result([%s])" % ", ".join(buffered))
del buffered[:]
ops_stack = []
# Split the text to form a list of tokens.
tokens = re.split(r"(?s)({{.*?}}|{%.*?%}|{#.*?#})", text)
squash = in_joined = False
for token in tokens:
if token.startswith("{"):
start, end = 2, -2
squash = (token[-3] == "-")
if squash:
end = -3
if token.startswith("{#"):
# Comment: ignore it and move on.
continue
elif token.startswith("{{"):
# An expression to evaluate.
expr = self._expr_code(token[start:end].strip())
buffered.append("to_str(%s)" % expr)
else:
# token.startswith("{%")
# Action tag: split into words and parse further.
flush_output()
words = token[start:end].strip().split()
if words[0] == "if":
# An if statement: evaluate the expression to determine if.
if len(words) != 2:
self._syntax_error("Don't understand if", token)
ops_stack.append("if")
code.add_line("if %s:" % self._expr_code(words[1]))
code.indent()
elif words[0] == "for":
# A loop: iterate over expression result.
if len(words) != 4 or words[2] != "in":
self._syntax_error("Don't understand for", token)
ops_stack.append("for")
self._variable(words[1], self.loop_vars)
code.add_line(
"for c_{} in {}:".format(
words[1],
self._expr_code(words[3])
)
)
code.indent()
elif words[0] == "joined":
ops_stack.append("joined")
in_joined = True
elif words[0].startswith("end"):
# Endsomething. Pop the ops stack.
if len(words) != 1:
self._syntax_error("Don't understand end", token)
end_what = words[0][3:]
if not ops_stack:
self._syntax_error("Too many ends", token)
start_what = ops_stack.pop()
if start_what != end_what:
self._syntax_error("Mismatched end tag", end_what)
if end_what == "joined":
in_joined = False
else:
code.dedent()
else:
self._syntax_error("Don't understand tag", words[0])
else:
# Literal content. If it isn't empty, output it.
if in_joined:
token = re.sub(r"\s*\n\s*", "", token.strip())
elif squash:
token = token.lstrip()
if token:
buffered.append(repr(token))
if ops_stack:
self._syntax_error("Unmatched action tag", ops_stack[-1])
flush_output()
for var_name in self.all_vars - self.loop_vars:
vars_code.add_line(f"c_{var_name} = context[{var_name!r}]")
code.add_line("return ''.join(result)")
code.dedent()
self._render_function = cast(
Callable[
[Dict[str, Any], Callable[..., Any]],
str
],
code.get_globals()["render_function"],
)
def _expr_code(self, expr: str) -> str:
"""Generate a Python expression for `expr`."""
if "|" in expr:
pipes = expr.split("|")
code = self._expr_code(pipes[0])
for func in pipes[1:]:
self._variable(func, self.all_vars)
code = f"c_{func}({code})"
elif "." in expr:
dots = expr.split(".")
code = self._expr_code(dots[0])
args = ", ".join(repr(d) for d in dots[1:])
code = f"do_dots({code}, {args})"
else:
self._variable(expr, self.all_vars)
code = "c_%s" % expr
return code
def _syntax_error(self, msg: str, thing: Any) -> NoReturn:
"""Raise a syntax error using `msg`, and showing `thing`."""
raise TempliteSyntaxError(f"{msg}: {thing!r}")
def _variable(self, name: str, vars_set: Set[str]) -> None:
"""Track that `name` is used as a variable.
Adds the name to `vars_set`, a set of variable names.
Raises an syntax error if `name` is not a valid name.
"""
if not re.match(r"[_a-zA-Z][_a-zA-Z0-9]*$", name):
self._syntax_error("Not a valid name", name)
vars_set.add(name)
def render(self, context: Optional[Dict[str, Any]] = None) -> str:
"""Render this template by applying it to `context`.
`context` is a dictionary of values to use in this rendering.
"""
# Make the complete context we'll use.
render_context = dict(self.context)
if context:
render_context.update(context)
return self._render_function(render_context, self._do_dots)
def _do_dots(self, value: Any, *dots: str) -> Any:
"""Evaluate dotted expressions at run-time."""
for dot in dots:
try:
value = getattr(value, dot)
except AttributeError:
try:
value = value[dot]
except (TypeError, KeyError) as exc:
raise TempliteValueError(
f"Couldn't evaluate {value!r}.{dot}"
) from exc
if callable(value):
value = value()
return value
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