Difference between "findAll" and "find_all" in BeautifulSoup
In BeautifulSoup version 4, the methods are exactly the same; the mixed-case versions (findAll, findAllNext, nextSibling, etc.) have all been renamed to conform to the Python style guide, but the old names are still available to make porting easier. See Method Names for a full list.
In new code, you should use the lowercase versions, so find_all, etc.
In your example however, you are using BeautifulSoup version 3 (discontinued since March 2012, don't use it if you can help it), where only findAll() is available. Unknown attribute names (such as .find_all, which only is available in BeautifulSoup 4) are treated as if you are searching for a tag by that name. There is no <find_all> tag in your document, so None is returned for that.
from the source code of BeautifulSoup:
http://bazaar.launchpad.net/~leonardr/beautifulsoup/bs4/view/head:/bs4/element.py#L1260
def find_all(self, name=None, attrs={}, recursive=True, text=None, limit=None, **kwargs):# ...# ...findAll = find_all # BS3findChildren = find_all # BS2