[spamizbad]

for-ins leak iteration variables into the function scope, which can introduce subtle bugs if you're not careful.

set/dict/generator comprehensions do not.

Example:

   for foo in bar:
     baz(foo)
   
   print foo  # Will actually print something

Whereas

   whatever = {baz(foo) for foo in bar}
   print foo # NameError: name 'foo' is not defined

does not put foo into the outer scope.

list comprehensions in python 2 (but not 3) will leak variables however.