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 email@example.com 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:
- assign to a variable then test and use
- 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 Pythonfirstname.lastname@example.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/