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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 open_url(url, headers=None):
 """ Opens a URL. If headers are passed as argument, no check is performed and the URL will be opened. @param url: the URL to open @type url: string @param headers: the headers to use @type headers: dictionary @return: a file-like object as returned by urllib2.urlopen. """

    request = urllib2.Request(url)
    if headers:
        for key, value in headers.items():
            request.add_header(key, value)
    return URL_OPENER.open(request)