As explained on the BeautifulSoup documentation
You may use this :
soup = BeautifulSoup(html)
results = soup.findAll("td", {"valign" : "top"})
EDIT :
To return tags that have only the valign=”top” attribute, you can check for the length of the tag attrs
property :
from BeautifulSoup import BeautifulSoup
html="<td valign="top">.....</td>\
<td width="580" valign="top">.......</td>\
<td>.....</td>"
soup = BeautifulSoup(html)
results = soup.findAll("td", {"valign" : "top"})
for result in results :
if len(result.attrs) == 1 :
print result
That returns :
<td valign="top">.....</td>