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Filename : lazy_regex.py
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# Copyright (C) 2006, 2008-2011, 2017 Canonical Ltd
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

"""Lazily compiled regex objects.

This module defines a class which creates proxy objects for regex
compilation.  This allows overriding re.compile() to return lazily compiled
objects.

We do this rather than just providing a new interface so that it will also
be used by existing Python modules that create regexs.
"""

import re

from . import errors


class InvalidPattern(errors.BzrError):
    _fmt = "Invalid pattern(s) found. %(msg)s"

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


class LazyRegex:
    """A proxy around a real regex, which won't be compiled until accessed."""

    # These are the parameters on a real _sre.SRE_Pattern object, which we
    # will map to local members so that we don't have the proxy overhead.
    _regex_attributes_to_copy = [
        "__copy__",
        "__deepcopy__",
        "findall",
        "finditer",
        "match",
        "scanner",
        "search",
        "split",
        "sub",
        "subn",
    ]

    # We use slots to keep the overhead low. But we need a slot entry for
    # all of the attributes we will copy
    __slots__ = [
        "_real_regex",
        "_regex_args",
        "_regex_kwargs",
    ] + _regex_attributes_to_copy

    def __init__(self, args, kwargs):
        """Create a new proxy object, passing in the args to pass to re.compile.

        :param args: The `*args` to pass to re.compile
        :param kwargs: The `**kwargs` to pass to re.compile
        """
        self._real_regex = None
        self._regex_args = args
        self._regex_kwargs = kwargs

    def _compile_and_collapse(self):
        """Actually compile the requested regex."""
        self._real_regex = self._real_re_compile(
            *self._regex_args, **self._regex_kwargs
        )
        for attr in self._regex_attributes_to_copy:
            setattr(self, attr, getattr(self._real_regex, attr))

    def _real_re_compile(self, *args, **kwargs):
        """Thunk over to the original re.compile."""
        try:
            return re.compile(*args, **kwargs)
        except re.error as e:
            # raise InvalidPattern instead of re.error as this gives a
            # cleaner message to the user.
            raise InvalidPattern('"' + args[0] + '" ' + str(e))

    def __getstate__(self):
        """Return the state to use when pickling."""
        return {
            "args": self._regex_args,
            "kwargs": self._regex_kwargs,
        }

    def __setstate__(self, dict):
        """Restore from a pickled state."""
        self._real_regex = None
        self._regex_args = dict["args"]
        self._regex_kwargs = dict["kwargs"]

    def __getattr__(self, attr):
        """Return a member from the proxied regex object.

        If the regex hasn't been compiled yet, compile it
        """
        if self._real_regex is None:
            self._compile_and_collapse()
        # Once we have compiled, the only time we should come here
        # is actually if the attribute is missing.
        return getattr(self._real_regex, attr)


def lazy_compile(*args, **kwargs):
    """Create a proxy object which will compile the regex on demand.

    :return: a LazyRegex proxy object.
    """
    return LazyRegex(args, kwargs)