[Python-ideas] Inline assignments using "given" clauses

Tim Peters tim.peters at gmail.com
Fri May 11 12:45:13 EDT 2018

[Gustavo Carneiro]
>>> IMHO, all these toy examples don't translate well to the real world
>>> because they tend to use very short variable names while in real world [good
>>> written code] tends to select longer more descriptive variable names.

[Greg Ewing]
>> I don't believe that's always true. It depends on the context.
>> Sometimes, using long variable names can make code *harder*
>> to read.
>> I don't think there's anything unrealistic about this
>> example:
>>    if m given m = pattern.match(the_string):
>>       nugget = m.group(2)
>> Most people's short-term memory is good enough to remember
>> that "m" refers to the match object while they read the
>> next couple of lines. IMO, using a longer name would serve
>> no purpose and would just clutter things up.

[Nick Coghlan]
> I've been thinking about this problem, and I think for the If/elif/while
> cases it's actually possible to allow the "binding is the same as the
> condition" case to be simplified to:
>     if command =  pattern.match(the_string):
>         ...
>     elif command =  other_pattern.match(the_string):
>         ...
>     while data = read_data():

Unless there's some weird font problem on my machine, that looks like
a single "equals sign".  In which case we'd be reproducing C's
miserable confusion about whether:

    if (i = 1)

was a too-hastily-typed spelling of the intended:

    if (i == 1)

or whether they were thinking "equals" and typed "=" by mistake.

If so, that would get an instant -1 from any number of core devs, who
have vivid painful memories of being burned by that in C.  That's not
just speculation - it came up a number of times in the PEP 572

> Allowing this would be part of the definition of the if/elif/while statement
> headers, rather than a general purpose assignment expression.
> The restriction of the LHS to a simple name target would need to be in the
> AST generator rather than in the grammar, but it's hardly the only case
> where we do that kind of thing. Switching to the given expression form would
> then only be necessary in cases where the condition *wasn't* the same as the
> binding target.
> A similar enhancement could be made to conditional expressions (adjusting
> their grammar to permit "EXPR if NAME = EXPR else EXPR") and filter clauses
> in comprehensions (allowing "EXPR for TARGET in EXPR if NAME = EXPR"). In
> essence, "if", "elif", and "while" would all allow for an "implied given"
> clause in order to simplify the 90% case where the desired condition and the
> bound expression are the same.

Spell it ":=" (colon equals) instead, and a few core devs would stop
objecting ;-)

More information about the Python-ideas mailing list