Class Template

Lib/string/__init__.py:64–190

A string class for supporting $-substitutions.

class Template:
    """A string class for supporting $-substitutions."""

    delimiter = '$'
    # r'[a-z]' matches to non-ASCII letters when used with IGNORECASE, but
    # without the ASCII flag.  We can't add re.ASCII to flags because of
    # backward compatibility.  So we use the ?a local flag and [a-z] pattern.
    # See https://bugs.python.org/issue31672
    idpattern = r'(?a:[_a-z][_a-z0-9]*)'
    braceidpattern = None
    flags = None  # default: re.IGNORECASE

    pattern = _TemplatePattern  # use a descriptor to compile the pattern

    def __init_subclass__(cls):
        super().__init_subclass__()
        cls._compile_pattern()

    @classmethod
    def _compile_pattern(cls):
        import re  # deferred import, for performance

        pattern = cls.__dict__.get('pattern', _TemplatePattern)
        if pattern is _TemplatePattern:
            delim = re.escape(cls.delimiter)
            id = cls.idpattern
            bid = cls.braceidpattern or cls.idpattern
            pattern = fr"""
            {delim}(?:
              (?P<escaped>{delim})  |   # Escape sequence of two delimiters
              (?P<named>{id})       |   # delimiter and a Python identifier
              {{(?P<braced>{bid})}} |   # delimiter and a braced identifier
              (?P<invalid>)             # Other ill-formed delimiter exprs
            )
            """
        if cls.flags is None:
            cls.flags = re.IGNORECASE
        pat = cls.pattern = re.compile(pattern, cls.flags | re.VERBOSE)
        return pat

    def __init__(self, template):
        self.template = template

    # Search for $$, $identifier, ${identifier}, and any bare $'s

    def _invalid(self, mo):
        i = mo.start('invalid')
        lines = self.template[:i].splitlines(keepends=True)
        if not lines:
            colno = 1
            lineno = 1
        else:
            colno = i - len(''.join(lines[:-1]))
            lineno = len(lines)
        raise ValueError('Invalid placeholder in string: line %d, col %d' %
                         (lineno, colno))

    def substitute(self, mapping=_sentinel_dict, /, **kws):