On 26/03/2016 14:30, Chris Angelico wrote:
On Sun, Mar 27, 2016 at 1:09 AM, BartC <b...@freeuk.com> wrote:
I'm surprised that both C and Python allow statements that apparently do
nothing. In both, an example is:
x
on a line by itself. This expression is evaluated, but then any result
discarded. If there was a genuine use for this (for example, reporting any
error with the evaluation), then it would be simple enough to require a
keyword in front.
Tell me, which of these is a statement that "does nothing"?
foo
foo.bar
foo["bar"]
foo.__call__
foo()
int(foo)
All of them are expressions to be evaluated and the result discarded.
I'm sure you'll recognize "foo()" as useful code, but to the
interpreter, they're all the same.
Out of that lot, only foo() is something that is commonly written both
as an expression and standalone statement.
And any one of them could raise an
exception rather than emit a value; for instance, consider these code
blocks:
# Personally, I prefer doing it the other way, but
# if you have a big Py2 codebase, this will help
# port it to Py3.
try: raw_input
except NameError: raw_input = input
try: int(sys.argv[1])
except IndexError:
print("No argument given")
except ValueError:
print("Not an integer")
In each case, the "dummy evaluation" of an expression is used as a way
of asking "Will this throw?". That's why this has to be squarely in
the hands of linters, not the main interpreter; there's nothing that
can't in some way be useful.
But notice that to render such an evaluation 'useful', you've put 'try:'
in front, which is also a cue to the reader.
Now you can't just do that anyway (try expects an except block to
follow). But my suggestion was to have required a keyword in front of
such expressions. Then no linter is needed. And stops some
head-scratching from people who have to read the code.
--
Bartc
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