# documentação Django 2.0.8

# Código fonte para django.urls.base

    from threading import local
    from urllib.parse import urlsplit, urlunsplit

    from django.utils.encoding import iri_to_uri
    from django.utils.functional import lazy
    from django.utils.translation import override

    from .exceptions import NoReverseMatch, Resolver404
    from .resolvers import get_ns_resolver, get_resolver
    from .utils import get_callable

    # SCRIPT_NAME prefixes for each thread are stored here. If there's no entry for
    # the current thread (which is the only one we ever access), it is assumed to
    # be empty.
    _prefixes = local()

    # Overridden URLconfs for each thread are stored here.
    _urlconfs = local()

    def resolve(path, urlconf=None):
        if urlconf is None:
            urlconf = get_urlconf()
        return get_resolver(urlconf).resolve(path)

    def reverse(viewname, urlconf=None, args=None, kwargs=None, current_app=None):
        if urlconf is None:
            urlconf = get_urlconf()
        resolver = get_resolver(urlconf)
        args = args or []
        kwargs = kwargs or {}

        prefix = get_script_prefix()

        if not isinstance(viewname, str):
            view = viewname
        else:
            parts = viewname.split(':')
            parts.reverse()
            view = parts[0]
            path = parts[1:]

            if current_app:
                current_path = current_app.split(':')
                current_path.reverse()
            else:
                current_path = None

            resolved_path = []
            ns_pattern = ''
            ns_converters = {}
            while path:
                ns = path.pop()
                current_ns = current_path.pop() if current_path else None
                # Lookup the name to see if it could be an app identifier.
                try:
                    app_list = resolver.app_dict[ns]
                    # Yes! Path part matches an app in the current Resolver.
                    if current_ns and current_ns in app_list:
                        # If we are reversing for a particular app, use that
                        # namespace.
                        ns = current_ns
                    elif ns not in app_list:
                        # The name isn't shared by one of the instances (i.e.,
                        # the default) so pick the first instance as the default.
                        ns = app_list[0]
                except KeyError:
                    pass

                if ns != current_ns:
                    current_path = None

                try:
                    extra, resolver = resolver.namespace_dict[ns]
                    resolved_path.append(ns)
                    ns_pattern = ns_pattern + extra
                    ns_converters.update(resolver.pattern.converters)
                except KeyError as key:
                    if resolved_path:
                        raise NoReverseMatch(
                            "%s is not a registered namespace inside '%s'" %
                            (key, ':'.join(resolved_path))
                        )
                    else:
                        raise NoReverseMatch("%s is not a registered namespace" % key)
            if ns_pattern:
                resolver = get_ns_resolver(ns_pattern, resolver, tuple(ns_converters.items()))

        return iri_to_uri(resolver._reverse_with_prefix(view, prefix, *args, **kwargs))

    reverse_lazy = lazy(reverse, str)

    def clear_url_caches():
        get_callable.cache_clear()
        get_resolver.cache_clear()
        get_ns_resolver.cache_clear()

    def set_script_prefix(prefix):
        """
        Set the script prefix for the current thread.
        """
        if not prefix.endswith('/'):
            prefix += '/'
        _prefixes.value = prefix

    def get_script_prefix():
        """
        Return the currently active script prefix. Useful for client code that
        wishes to construct their own URLs manually (although accessing the request
        instance is normally going to be a lot cleaner).
        """
        return getattr(_prefixes, "value", '/')

    def clear_script_prefix():
        """
        Unset the script prefix for the current thread.
        """
        try:
            del _prefixes.value
        except AttributeError:
            pass

    def set_urlconf(urlconf_name):
        """
        Set the URLconf for the current thread (overriding the default one in
        settings). If urlconf_name is None, revert back to the default.
        """
        if urlconf_name:
            _urlconfs.value = urlconf_name
        else:
            if hasattr(_urlconfs, "value"):
                del _urlconfs.value

    def get_urlconf(default=None):
        """
        Return the root URLconf to use for the current thread if it has been
        changed from the default one.
        """
        return getattr(_urlconfs, "value", default)

    def is_valid_path(path, urlconf=None):
        """
        Return True if the given path resolves against the default URL resolver,
        False otherwise. This is a convenience method to make working with "is
        this a match?" cases easier, avoiding try...except blocks.
        """
        try:
            resolve(path, urlconf)
            return True
        except Resolver404:
            return False

    def translate_url(url, lang_code):
        """
        Given a URL (absolute or relative), try to get its translated version in
        the `lang_code` language (either by i18n_patterns or by translated regex).
        Return the original URL if no translated version is found.
        """
        parsed = urlsplit(url)
        try:
            match = resolve(parsed.path)
        except Resolver404:
            pass
        else:
            to_be_reversed = "%s:%s" % (match.namespace, match.url_name) if match.namespace else match.url_name
            with override(lang_code):
                try:
                    url = reverse(to_be_reversed, args=match.args, kwargs=match.kwargs)
                except NoReverseMatch:
                    pass
                else:
                    url = urlunsplit((parsed.scheme, parsed.netloc, url, parsed.query, parsed.fragment))
        return url