Why not allow empty code blocks?

[Steven D'Aprano]

On Sun, 24 Jul 2016 12:06 am, BartC wrote:

> pass can only do so much. If doesn't help here:
> 
>   for x in sequence:
>       print("Something")
>   print("Something else")
> 
> Was the second print meant to be indented as well or not?

True. But once you start wondering about code the programmer *hasn't*
written, you could drive yourself crazy:

    ...but what if the second print is supposed to follow 
    an "if" clause, which has been left out...?

*Most* for-loops have non-empty blocks. Only a tiny minority have empty
blocks for any appreciable period of time. (This is not 1970, and we don't
write timing loops like "for x in range(1000): pass".)

*Most* code is correctly indented. So the interpreter should assume that
indentation *is correct*, unless there is a reason to doubt that. What sort
of reasons could there be?

- if the indentation increases when it shouldn't:

    print("Hello")
        print("World!")  # unexpected indent

- if it dedents to a value that doesn't align with a previous block:

    for x in sequence:
        print(x)
      print("dedented")  # half-way between current and previous block

- if it doesn't indent when you expect it to:

    for x in sequence:
    print(x)


Yes, that last one *could* be an empty block. But empty blocks are rare. Its
more likely to be an error. If it's not an error, than the programmer can
just insert a "pass" statement to satisfy the interpreter.


> Perhaps rather than 'pass', the language ought to have provided an
> optional 'end' keyword to mark the end of a block. Then there would be
> less speculation about what was meant:

It can't be optional. If it were optional, we'd be no better off:

    for x in sequence:
    print(x)

The "end" is missing, but is it missing from the end of the empty block, or
the end of the non-empty block?

    for x in sequence:
    end
    print(x)

    for x in sequence:
    print(x)
    end


In any case, using "end" instead of "pass" is a poor tradeoff. Instead of
needing to use "pass" (say) one time in a thousand when it is needed, you
would need to use "end" 999 times in a thousand when it *isn't* needed.


>   for x in sequence:
>       print("Something")
>   end
>   print("Something else")
> 
> (And no speculation at all if 'end' was mandatory. Python already
> provides 'else' (and 'except'?) which can do a similar job in some
> circumstances.)





-- 
Steven
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.