What is the difference between a list comprehension and a generator expression?

A list comprehension builds the whole list in memory immediately; a generator expression returns a lazy iterator that produces items one at a time as you consume them. Generators use constant memory and can start yielding before the source is exhausted, but you can only iterate them once and you cannot index them or take their length.

Square brackets give me a list that is fully materialized before the next line runs, while parentheses give me a generator that computes each item on demand. For a few hundred items the difference is noise, so I use a list because it is easier to debug and I can look at it twice. The moment the source is a file, a database cursor, or an API paginator, I switch to a generator. On a reconciliation job I worked on, reading a large export into a list pushed the container past its memory limit and it kept getting killed, and changing one pair of brackets to parentheses dropped peak memory to a few megabytes because we only ever held one row. The catch is that a generator is exhausted after one pass. I have watched someone log the count of a generator and then loop over it, and the loop body never executed. If I need the data more than once, I materialize it deliberately and accept the memory.