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