Kristján Valur Jónsson wrote:

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I've never understood the need to have a distinction betwen statements and expressions, not when expressions can have side effects.

Alex Martelli responded:

If you're interested in understanding it better, research Query-Command Separation (QCS), e.g. starting at http://en.wikipedia.org/wiki/Command-query_separation

Either you missed Kristján's point, or your answer was so subtle that I missed yours. QCS makes it easy to determine which pieces of code (queries) are free of side-effects. I see value in that for both debugging and optimization. What I don't see is how that relates to expressions vs statements **when expressions can have side effects.** (Actually, in Python, I would say that statements are far *more* likely to be free of side-effects, as they are often there for flow control.) -jJ