http://docs.sqlalchemy.org/en/latest/dialects/mssql.html#dialect-mssql-pyodbc-connect
You could use the example towards the end of the documentation's section like this:
import urllib
from sqlalchemy import create_engine
connection_string = '127.0.0.1;Database=MyDatabase;MultiSubnetFailover=True'
engine_string = 'mssql+pyodbc:///?odbc_connect={}'.format(urllib.quote_plus(connection_string))
engine = create_engine(engine_string)
Update from comments
For newer versions of Microsoft ODBC Driver for SQL Server, you may need to use MultiSubnetFailover=Yes
instead of True