On 11 March 2016 at 12:58, Franklin? Lee leewangzhong+python@gmail.com wrote:
On Thu, Mar 10, 2016 at 5:20 PM, Michał Żukowski thektulu.pp@gmail.com wrote:
2016-03-10 17:53 GMT+01:00 Pavol Lisy pavol.lisy@gmail.com:
We could discuss if "with expr as var" syntax is more beautiful. (or if it not against There should be one-- and preferably only one --obvious way to do it.)
But why omit context manager semantics in "with expr as var" assignment syntax?
I personally don't like idea that semantics could be context sensitive in this way. (and we could already do pretty complex things in comprehension)
I was thinking to reuse "with [expr] as [var]" but Ialso don't like idea of context sensitive semantics, and I even thought that maybe someone, someday would want to write "content = fp.read() with open('foo.txt') as fp"...
This is my understanding of `with Expr() as Name: Block()`, in pseudocode (ignoring exception handling).
_context = Expr() Name = _context.__enter__(...) exec(Block, {"Name": Name}) _context.__exit__(...)
with doesn't create a new scope - it's more akin to a try/finally statement with a particular form (PEP 343 defines the full expansion, and that expansion is still broadly accurate, although some the specific details are different these days)
The only compound statements in Python that create new scopes are "def" and "class".
It's also worth reading PEP 343 for Guido's explanation for why the context management assignment syntax isn't "with VAR = EXPR": it's because EXPR *isn't* the thing being assigned, but rather a stepping stone towards obtaining that thing. This characteristic is true of all of the places where "as" is currently used instead of "=":
with CM_EXPR as VAR: # VAR = CM_EXPR.__enter__(), not CM_EXPR
except EXC_TYPE_EXPR as VAR: # VAR = the caught exception, not the exception type constraint # There's an implied "del VAR" at the end of the suite, but still not a full implicit scope
import MODULE_REFERENCE as VAR from MODULE_REFERENCE import NAME as VAR # Here, the LHS isn't even a normal expression - it's a module name, or a module name plus an attribute name
Cheers, Nick.