如何从Python中的一组regexp匹配中提取第一个非null匹配？

Question

Suppose I have a regular expression (a)|(b)|(c)|(d) . If I apply it to text 'foobar' I get a match object

>>> compiled = re.compile('(a)|(b)|(c)|(d)')
>>> compiled.search('foobar').groups()
(None, 'b', None, None)

How do I extract the 'b' from here? Or in general, how do I extract the first match from an unknown number of groups (can happen when the regexp was built dynamically)?

Answer 1

>>> g = (None, 'b', None, None)
>>> next(x for x in g if x is not None)
'b'

>>> g = (None, None, None)
>>> next((x for x in g if x is not None), "default")  # try this with filter :)
'default'

>>> g = (None, None, None)  # so you know what happens, and what you could catch
>>> next(x for x in g if x is not None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

Answer 2

reduce(lambda x, y : (x, y)[x is None], groups, None)

Answer 3

filter(lambda x : x is not None, groups)[0]

Answer 4

>>> g = (None,'b',None,None)
>>> filter(None,g)
('b',)
>>> h = (None,None,None)
>>> filter(None,h)
()