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Comment by davepeck | Hacker News Reader

Yeah. `join` has forever been backwards to me in Python and I still sometimes get it wrong the first time out.

Comprehensions, though -- they are perfection. :-)

IMO comprehensions like `[x**2 for x in range(4)]` would be better written as `[for x in range(4): x**2]`.

That would make the mapping between a comprehension and the equivalent loop much clearer, especially once you use nested loops and/or conditionals.

For example, to flatten a list of lists `l = [[1, 2], [3], [4, 5, 6]]`:

    [item for sublist in l for item in sublist]

    [for sublist in l: for item in sublist: item]

That would make dict comprehensions confusing due to two occurrences of colons with different semantics.

Ah, yes - good point!

I think it's very deliberate that these sorts of expressions are inside-out from the corresponding statements. It mixes and matches modes of thinking in my mind - the same kind of mistake as command-query separation violations. When I read a list comprehension, I can read it left to right: "A list (`[`) of each `item` that I get, if I look `for` each `sublist` that is `in l`, then `for` each `item` which is `in` that `sublist`." And the entire description is - well - descriptive, not imperative. With the other order, I have to think in terms of explicitly following the steps, and then mentally translating the last `item` into an imperative "... and put it into the result list".

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