Why not allow empty code blocks?

[Steven D'Aprano]

On Sun, 24 Jul 2016 08:35 pm, BartC wrote:

> I didn't want to get into this subject again, but Python's indentation
> scheme is fragile.

*shrug*

Okay, it's fragile. In 20 (give or take a couple) years of programming in
Python, do you know how many times this fragility has been an *actual*
problem? Why don't you take a guess?


> Given an otherwise correctly typed program that compiles with no errors,
> then it is very easy (if Backspace or Delete is inadvertently pressed
> for example), for an indent to disappear without your noticing, 

Not really. It depends on the editor, but typically you need to have your
text insertion point somewhere in the indent.

And when you do accidentally press Delete, what happens? The high-level
structure of the code changes. Typically things will no longer align:

def f():
    for x in seq:
        do_this()
    do_that()
        do_more()

which is a SyntaxError. It requires quite the coincidence before you can
accidentally delete an indent and have the code still run. Far more likely
is that accidentally pressing delete will break the code in a way that
can't be detected until runtime:

def f():
    for x in seq:
        do_this()
        d_that()
        do_more()

Conclusion: if you're the sort of person who habitually presses the Delete
or Backspace key without paying attention, you're going to have a bad time.

(Do other professions make arguments like this? Do carpenters, say, argue
against nail guns because "well if you accidentally hold a loaded nail gun
to your head, then press the trigger, bad things will happen"? Or is it
just programmers who make a common practice of arguing that the programming
language should protect the user from sheer carelessness?)


> but a 
> program still compiles. And still runs without execution errors. But
> might now be subtly wrong.

That's certainly *theoretically* possible, but its really not likely. The
only way it could happen is:

- you have a block with at least two lines;
- your insertion point is in the indent of the last line of the block;
- but not the actual text part;
- your editor is configured to dedent on Delete/Backspace, not just 
  delete a single space; or you're using tabs to indent;
- and the block isn't followed by another block (e.g. if...else...)



> Deleting any other character than a leading space or tab I think is more
> likely to result in an error that would be noticed, generate a compile
> error, or execute error ('variable not initialised'), or that goes wrong
> more obviously.

pi = 314159  # oops accidentally hit delete


[...]
> But my suggestion is simply that you can write:
> 
>       for x in sequence:
>       end
> 
> instead of:
> 
>       for x in sequence:
>           pass
> 
> The first now allows statements to be added or removed from the body of
> the loop without needing to change the 'end'; it wouldn't look out of
> place as a trailing 'pass' would.

You know, I suspect that you've probably spent more time talking about the
effort required to delete and insert "pass" than the *actual* effort spent
by a hundred programmers deleting and inserting "pass" in their code.


> But thinking about it some more, it wouldn't work. All the blocks that
> don't now use 'end' would look odd. I think it would either have to be
> all or nothing. I guess nothing.


 

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