[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part)
steve at pearwood.info
Sun Jun 24 10:52:04 EDT 2018
On Sun, Jun 24, 2018 at 05:24:12PM +0300, Ivan Pozdeev via Python-Dev wrote:
> An expression is intuitively thought to be self-contained i.e. without
> side effects.
> if I write `a=b+1`, I'm not expecting it to do anything except assigning
a = d.pop(1)
a = d.setdefault(key, 0)
chars_written = file.write(text)
> Expressions with side effects has long since proven to be problematic
> because of the implicit (thus hard to see and track) links they create
> (and because the result depends on the order of evaluation).
If you're going to take a hard-core functional approach to side-effects,
I think you are using the wrong language. Nearly everything in Python
*could* have side-effects (even if usually it won't).
Even your own example of "b+1" (depending on what b.__add__ does).
> Moreover, Python's other design elements have been consistently
> discouraging expressions with side effects, too (e.g. mutator methods
> intentionally return None instead of the new value, making them useless
> in expressions),
I don't think that's the reason why mutator methods return None. They
return None rather than self to avoid confusion over whether they return
a copy or not.
> so the proposition is in direct conflict with the
> language's design.
Python is full of operations with side-effects.
Besides, they're not quite useless:
(alist.append() or alist)
is functionally equivalent to alist.append returning self. Just a bit
Methods (and functions) all return a value, even if that value is None,
so they can be used in expressions. If Guido wanted Pascal style
procedures, which cannot be used in expressions, we would have them by
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