import re
def get_attr(str, attr):
    m = re.search(attr + r'=(\w+)', str)
    return None if not m else m.group(1)

str = 'type=weaksubj len=1 word1=wrestle pos1=verb stemmed1=y priorpolarity=negative'

print get_attr(str, 'word1')  # wrestle
print get_attr(str, 'type')   # weaksubj
print get_attr(str, 'foo')    # None

wrestle
weaksubj
None