Re: [Python-ideas] Map-then-filter in comprehensions

Some time ago I was trying to solve the same issue but using a new keyword "where", and I thought that new keyword is too much for just list comprehension filtering, so I've made it something like assignments in expresion, eg.

(x+y)**2 + (x-y)**2 where x=1, y=2

So for list comprehension I can write:

[stripped for line in lines if stripped where stripped=line.strip()]

or:

result = map(f, objs) where f=lambda x: x.return_something()

or:

it = iter(lines) while len(line) > 4 where line=next(it, '').strip(): print(line)

or:

lambda x, y: ( 0 if z == 0 else 1 if z > 0 else -1) where z = x + y

or even:

lambda something: d where (d, _)=something, d['a']=1

I even implemented it: https://github.com/thektulu/cpython/commit/9e669d63d292a639eb6ba2ecea3ed2c0c...

and it works nicely. I was thinking to reuse "with [expr] as [var]" but I also 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"...

The "where" keyword is from guards pattern in Haskell :)

2016-03-10 17:53 GMT+01:00 Pavol Lisy pavol.lisy@gmail.com:

2016-03-09 6:16 GMT+01:00, Sjoerd Job Postmus sjoerdjob@sjec.nl: [...]

...

Trying to word it in such a way:

"... considering each of the `for` or `if` clauses a block, nesting from left to right, and evaluating the expression to produce an element each time the innermost block is reached. The `with expr as target` should be considered equivalent to `target = expr`.

(And here we already see the downside of this idea). Normally a comprehension of the form

(expr1 for target1 in expr2 for target2 in expr3 if expr4)

boils down to

for target1 in expr2:
    for target2 in expr3:
        if expr4:
            yield expr1

The natural extension would be for

(expr1 for target1 in expr2 with expr3 as target2 if expr4)

to reduce as follows.

for target1 in expr2:
    with expr3 as target2:
        if expr4:
            yield expr1

Instead, it becomes

for target1 in expr2:
    target2 = expr3:
    if expr4:
        yield expr1

But of course we're not going to have context managers in comprehensions, are we? So this inconsistency is somewhat forgiveable.

If we want variable assignment we have already "for var in [expr]" syntax.

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