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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 make_http_credentials(username=None, password=None):
 """Build auth part for api_url."""

    credentials = ''
    if username is None:
        return credentials
    if username is not None:
        if ':' in username:
            return credentials
        credentials += username
    if credentials and password is not None:
        credentials += ":%s" % password
    return "%s@" % credentials