Generator expression vs list comprehension — what is the difference?

Generator expression and list comprehension are similar in syntax but differ in behavior and memory consumption.

• List comprehension creates and immediately returns a complete list in memory. This is convenient when you need to quickly obtain all elements, but can lead to high memory usage with large data volumes.

• Generator expression returns an iterator that generates elements one by one during iteration. This saves memory because elements are not stored all at once but are computed as needed.

Example:

# List comprehension
squares_list = [x*x for x in range(10)]  # immediately creates a list of 10 elements

# Generator expression
gen = (x*x for x in range(10))  # creates a generator, elements are computed during iteration
for val in gen:
    print(val)

In summary, if memory efficiency is important and data is processed step-by-step, it is better to use generator expressions. If a full list is needed immediately — use list comprehension.