.find_all() returns list of all found elements, so:
input_tag = soup.find_all(attrs={"name" : "stainfo"})
input_tag is a list (probably containing only one element). Depending on what you want exactly you either should do:
output = input_tag[0]['value']
or use .find() method which returns only one (first) found element:
input_tag = soup.find(attrs={"name": "stainfo"})
output = input_tag['value']