On 7 Sep 2017, at 13:11, Paul Moore p.f.moore@gmail.com wrote:

On 7 September 2017 at 11:43, Denis Krienbühl denis@href.ch wrote:

...

What I would love to see is the following syntax instead, which to me is much cleaner:

if computation() as result: do_something_with_result(result)

Hi - thanks for your suggestion! This has actually come up quite a lot in the past. Here's a couple of links to threads you might want to read (it's not surprising if you missed these, it's not that easy to come up with a good search term for this topic).

https://mail.python.org/pipermail/python-ideas/2012-January/013461.html https://mail.python.org/pipermail/python-ideas/2009-March/003423.html (This thread includes a note by Guido that he intentionally left out this functionality)

In summary, it's a reasonably commonly suggested idea, but there's not enough benefit to warrant adding it to the language.

Paul

I see, thank you for digging those threads up. I’ll read them to learn a thing or two :)

On Thu, Sep 07, 2017 at 04:36:40PM +0200, Jason H wrote:

I also often wonder why we are left doing an assignment and test. You have two options:

1. assign to a variable then test and use
2. repeat the function call

Personally, I don't see what's wrong with the "assign then test" idiom.

x = something() if x: do_stuff()

I would offer that 'with' [sh|c]ould be used: with test() as x: handle_truthy(x) else: handle_falsey() # do we provide x here too? Because None vs False?

This would cause confusing errors and mysterious behaviour, depending on whether the test() object was a context manager or not. Which should take priority? If you see:

with spam() as x: do_stuff

is that a context manager with block (like "with open(...) as f") or your boolean if test in disguise?

Having "with" sometimes be a disguised "if" and sometimes a regular "with" will make it really, really hard to reason about code.

sorry, it’s a bit more difficult. this works: https://gist.github.com/flying-sheep/86dfcc1bdd71a33fa3483b83e254084c

Philipp A. flying-sheep@web.de schrieb am Do., 7. Sep. 2017 um 21:18 Uhr:

Sadly it’s hard to create a context manager that skips its body like this:

with unpack(computation()) as result: do_something_with_result(result)

You can do it with some hackery like described here: https://stackoverflow.com/a/12594789/247482

class unpack: def __init__(self, pred): self.pred = pred

def __enter__(self):
    if self.pred:
        return self.pred
    # else skip the with block’s body
    sys.settrace(lambda *args, **kw: None)
    frame = inspect.currentframe(1)
    frame.f_trace = self.trace

def trace(self, frame, event, arg):
    raise

def __exit__(self, type, value, traceback):
    return True  # suppress the exception

Steven D'Aprano steve@pearwood.info schrieb am Do., 7. Sep. 2017 um 18:26 Uhr:

...

On Thu, Sep 07, 2017 at 04:36:40PM +0200, Jason H wrote:

...

I also often wonder why we are left doing an assignment and test. You

have two options:

...

1. assign to a variable then test and use
2. repeat the function call

Personally, I don't see what's wrong with the "assign then test" idiom.

x = something() if x: do_stuff()

...

I would offer that 'with' [sh|c]ould be used: with test() as x: handle_truthy(x) else: handle_falsey() # do we provide x here too? Because None vs False?

This would cause confusing errors and mysterious behaviour, depending on whether the test() object was a context manager or not. Which should take priority? If you see:

with spam() as x: do_stuff

is that a context manager with block (like "with open(...) as f") or your boolean if test in disguise?

Having "with" sometimes be a disguised "if" and sometimes a regular "with" will make it really, really hard to reason about code.

-- Steve _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/