https://github.com/django/django/blob/2.1.5/django/urls/base.py#L27-L90

base.py

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))