Hi List comprehensions (and generator expressions) come in two 'flavours' at the moment: (1) [f(x) for x in L], which stands for map(f, L). Let's call this a 'map comprehension' (2) [f(x) for x in L if p(x)], which stands for map(f, filter(p, L)). Let's call this a 'map-filter comprehension'. Now if one wants to write simply filter(p, L) as a list comprehension, one has to write: (3) [x for x in L if p(x)]. This could be called a 'filter comprehension'. the 'x for x in L' is not very nice IMHO, but it is often handy to use such expressions over 'filter(...)', eg building the sublist of a given list consisting of all the items of a given type could be written as: filter(lambda x: isinstance(x, FilteringType), heterogeneous_list) or: [x for x in heterogenous_list if isinstance(x, FilteringType)] I still prefer the list comprehension over the lambda/filter combination, but neither feels very satisfying (to me :) (not that one cannot use partial in the filter version) Why not just drop the 'x for' at the start of a 'filter comprehension' (or generator expression)? Thus (3) could be written more simply as: (3') [x in L if p(x)] This is consistent with common mathematical notation: * { f(x) | x \in L } means the set of all f(x) for x in L * { f(x) | x \in L, p(x) } means the set of all f(x) for x in L satisfying predicate p. * { x \in L | p(x) } means the set of all x in L satisfying predicate p. -- Arnaud