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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 comprehension [x*2 for x in range(256)] 
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So what's the difference between Generator Expressions and List Comprehensions? The generator yields one item at a time and generates item only when in demand. Whereas, in a list comprehension, Python reserves memory for the whole list. Thus we can say that the generator expressions are memory efficient than the lists.
The main advantage of generator over a list is that it take much less memory. We can check how much memory is taken by both types using sys. getsizeof() method.
List comprehensions are eager but generators are lazy. In list comprehensions all objects are created right away, it takes longer to create and return the list. In generator expressions, object creation is delayed until request by next() . Upon next() generator object is created and returned immediately.
Conclusions. List comprehensions are often not only more readable but also faster than using “for loops.” They can simplify your code, but if you put too much logic inside, they will instead become harder to read and understand.
John's answer is good (that list comprehensions are better when you want to iterate over something multiple times). However, it's also worth noting that you should use a list if you want to use any of the list methods. For example, the following code won't work:
def gen(): return (something for something in get_some_stuff()) print gen()[:2] # generators don't support indexing or slicing print [5,6] + gen() # generators can't be added to lists Basically, use a generator expression if all you're doing is iterating once. If you want to store and use the generated results, then you're probably better off with a list comprehension.
Since performance is the most common reason to choose one over the other, my advice is to not worry about it and just pick one; if you find that your program is running too slowly, then and only then should you go back and worry about tuning your code.
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