def __init__(self, request, response, urls, code=None):

        if isinstance(urls, string_types):
            urls = [urls]

        abs_urls = []
        for url in urls:
            # Note that urljoin will "do the right thing" whether url is:
            #  1. a complete URL with host (e.g. "http://www.example.com/test")
            #  2. a URL relative to root (e.g. "/dummy")
            #  3. a URL relative to the current path
            # Note that any query string in request is discarded.
            url = request.uri.relative(url).unicode()
            abs_urls.append(url)
        self.urls = urls = abs_urls

        # RFC 2616 indicates a 301 response code fits our goal; however,
        # browser support for 301 is quite messy. Do 302/303 instead. See
        # http://ppewww.ph.gla.ac.uk/~flavell/www/post-redirect.html
        if code is None:
            if request.protocol >= (1, 1):
                code = 303
            else:
                code = 302
        else:
            if code < 300 or code > 399:
                raise ValueError("status code must be between 300 and 399.")

        super(redirect, self).__init__(request, response, code)

        if code in (300, 301, 302, 303, 307):
            response.headers["Content-Type"] = "text/html"
            # "The ... URI SHOULD be given by the Location field
            # in the response."
            response.headers["Location"] = urls[0]

            # "Unless the request method was HEAD, the entity of the response
            # SHOULD contain a short hypertext note with a hyperlink to the
            # new URI(s)."
            msg = {300: "This resource can be found at <a href='%s'>%s</a>.",
                   301: ("This resource has permanently moved to "
                         "<a href='%s'>%s</a>."),
                   302: ("This resource resides temporarily at "
                         "<a href='%s'>%s</a>."),
                   303: ("This resource can be found at "
                         "<a href='%s'>%s</a>."),
                   307: ("This resource has moved temporarily to "
                         "<a href='%s'>%s</a>."),
                   }[code]
            response.body = "<br />\n".join([msg % (u, u) for u in urls])
            # Previous code may have set C-L, so we have to reset it
            # (allow finalize to set it).
            response.headers.pop("Content-Length", None)
        elif code == 304:
            # Not Modified.
            # "The response MUST include the following header fields:
            # Date, unless its omission is required by section 14.18.1"
            # The "Date" header should have been set in Response.__init__

            # "...the response SHOULD NOT include other entity-headers."
            for key in ("Allow", "Content-Encoding", "Content-Language",
                        "Content-Length", "Content-Location", "Content-MD5",
                        "Content-Range", "Content-Type", "Expires",
                        "Last-Modified"):
                if key in response.headers:
                    del response.headers[key]

            # "The 304 response MUST NOT contain a message-body."
            response.body = None
            # Previous code may have set C-L, so we have to reset it.
            response.headers.pop("Content-Length", None)
        elif code == 305:
            # Use Proxy.
            # urls[0] should be the URI of the proxy.
            response.headers["Location"] = urls[0]
            response.body = None
            # Previous code may have set C-L, so we have to reset it.
            response.headers.pop("Content-Length", None)
        else:
            raise ValueError("The %s status code is unknown." % code)
The constructor creates a new instance and modifies the
dataset/ETHPy150Open circuits/circuits/circuits/web/errors.py/redirect.__init__