[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20

[Łukasz Langa]

> On 25 Apr, 2018, at 5:20 PM, Chris Angelico <rosuav at gmail.com> wrote:
> 
> On Thu, Apr 26, 2018 at 10:11 AM, Yury Selivanov
> <yselivanov.ml at gmail.com> wrote:
>> Just yesterday this snippet was used on python-dev to show how great the
>> new syntax is:
>> 
>>          my_func(arg, buffer=(buf := [None]*get_size()), size=len(buf))
>> 
>> To my eye this is an anti-pattern.  One line of code was saved, but the
>> other line becomes less readable.  The fact that 'buf' can be used after
>> that line means that it will be harder for a reader to trace the origin of
>> the variable, as a top-level "buf = " statement would be more visible.
> 
> Making 'buf' more visible is ONLY a virtue if it's going to be used
> elsewhere. Otherwise, the name 'buf' is an implementation detail of
> the fact that this function wants both a buffer and a size.

You're claiming that `:=` is nicer in this situation because it's less
prominent than regular assignment and thus doesn't suggest that the name
stays visible later.

But as others said, `:=` *does* make the name visible later until the
enclosing scope ends.  In fact, a large part of its appeal is that you
can use the result later (as in the `re.search()` example).  Will it be
visible enough to the reaser in those cases then?

There seems to be a conflict there.

The question of assignment visibility also makes me think about
unintentional name shadowing::

    buf = some_value

    ...  # 20 lines

    my_func(arg, buffer=(buf := [None]*get_size()), size=len(buf))

    ...  # 20 lines

    buf  # <-- What value does this have?


Even if we're not using the call pattern, there can be plenty of logic
tests which aren't very obvious::

    buf = some_value

    ...  # 20 lines

    if node.parent is not None and (buf := node.parent.buffer):
        ... # 10 lines

    ...  # 20 lines

    buf  # <-- What value does this have?


This is even more interesting because now `buf` isn't rebound
*always*.

So if I'm confused about an unexpected change in value of `buf`, I'll
skim the code, fail to find the assignment, and then grep for `buf =`
and also fail to find the assignment.  Yes, I could have searched for
just `buf` instead but that will give me too many false positives,
especially if I'm using a primitive text editor search or don't know
about \b in regular expressions.

Debugging this can be confusing.  I know it can since a similar
annoyance can be observed with the magic pseudo-scope of `except`::

    err = some_value
    try:
        ...
    except Error as err:
        ...

    err  # <-- now sometimes it's not defined


Just like Barry, I debugged a few cases of this in the past and within
larger functions this can be hard to find.

-- Ł

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