Generator Expressions vs. List Comprehension

Question

When should you use generator expressions and when should you use list comprehensions in Python?

# Generator expression
(x*2 for x in range(256))

# List com         


        

Answer 1

When creating a generator from a mutable object (like a list) be aware that the generator will get evaluated on the state of the list at time of using the generator, not at time of the creation of the generator:

>>> mylist = ["a", "b", "c"]
>>> gen = (elem + "1" for elem in mylist)
>>> mylist.clear()
>>> for x in gen: print (x)
# nothing

If there is any chance of your list getting modified (or a mutable object inside that list) but you need the state at creation of the generator you need to use a list comprehension instead.