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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def oauth_flow(s, oauth_url, username=None, password=None, sandbox=False):
 """s should be a requests session"""

    r = s.get(oauth_url)
    if r.status_code >= 300:
        raise RuntimeError(r.text)

    params = urlparse.parse_qs(urlparse.urlparse(r.url).query)

    data = {"un":username,
            "width":2560,
            "height":1440,
            "hasRememberUn":True,
            "startURL":params['startURL'],
            "loginURL":"",
            "loginType":6,
            "useSecure":True,
            "local":"",
            "lt":"OAUTH",
            "qs":"r=https%3A%2F%2Flocalhost%3A8443%2Fsalesforce%2F21",
            "locale":"",
            "oauth_token":"",
            "oauth_callback":"",
            "login":"",
            "serverid":"",
            "display":"popup",
            "username":username,
            "pw":password,
            "Login":""}

    base = "https://login.salesforce.com" if not sandbox else "https://test.salesforce.com"
    r2 = s.post(base, data)
    m = re.search("window.location.href\s*='(.[^']+)'", r2.text)
    assert m is not None, "Couldn't find location.href expression in page %s (Username or password is wrong)" % r2.url

    u3 = "https://" +  urlparse.urlparse(r2.url).hostname + m.group(1)
    r3 = s.get(u3)

    m = re.search("window.location.href\s*='(.[^']+)'", r3.text)

    assert m is not None, "Couldn't find location.href expression in page %s:\n%s" % (r3.url, r3.text)

    return m.group(1)