In some cases, adding a condition to a comprehension is somewhat like defining another function and then using filter:

Conditional Comprehensions

T = "cavernous"
print [c for c in T if c > 'k']
def bigrk(letter):
  return letter>'k'
print filter(bigrk,T)
print [c+c for c in T if c > 'k']

Notice how the filter function makes a string rather than a list (something that a comprehension alone cannot do). In the last line, the comprehension expression does two things: it filters (larger than 'k' letters only are selected) and doubles (c+c). Because the last line does two things, Python's filter function won't do the job.