<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_dois(query_str, count=100):
 """Search ScienceDirect through the API for articles. See http://api.elsevier.com/content/search/fields/scidir for constructing a query string to pass here. Example: 'abstract(BRAF) AND all("colorectal cancer")' """

    url = '%s/%s' % (elsevier_search_url, query_str)
    params = {'query': query_str,
              'count': count,
              'httpAccept': 'application/xml',
              'sort': '-coverdate',
              'field': 'doi'}
    res = requests.get(url, params)
    if not res.status_code == 200:
        return None
    tree = ET.XML(res.content, parser=UTB())
    doi_tags = tree.findall('atom:entry/prism:doi', elsevier_ns)
    dois = [dt.text for dt in doi_tags]
    return dois