On 30/05/2018 17:05, Peter O'Connor wrote:

On Thu, May 24, 2018 at 2:49 PM, Steven D'Aprano <steve@pearwood.info> wrote:
On Thu, May 24, 2018 at 02:06:03PM +0200, Peter O'Connor wrote:
> We could use given for both the in-loop variable update and the variable
> initialization:
>    smooth_signal =  [average given average=(1-decay)*average + decay*x for
> x in signal] given average=0.

So in your example, the OUTER "given" creates a local variable in the
current scope, average=0, but the INNER "given" inside the comprehension
exists inside a separate, sub-local comprehension scope, where you will
get an UnboundLocalError when it tries to evaluate (1-decay)*average the
first time.

You're right, having re-thought it, it seems that the correct way to write it would be to define both of them in the scope of the comprehension:

    smooth_signal =  [average given average=(1-decay)*average + decay*x for x in signal given average=0.] 

This makes sense and follows a simple rule: "B given A" just causes A to be executed before B - that holds true whether B is a variable or a loop declaration like "for x in x_gen".

So 

    a_gen = (g(a) given a=f(a, x) for x in x_gen given a=0)

would be a compact form of:

    def a_gen_func(x_gen):
        a=0
        for x in x_gen:
            a = f(a, x)
            yield g(a)
    
    a_gen = a_gen_func()
[There is a typo here - a_gen_func is defined to take 1 argument but is called with none.]

After - *I think* - understanding this, I would try to make the one-line clearer by parenthesizing it thus (whether or not the grammar required it):

    a_gen = ( ((g(a) given a=f(a, x)) for x in x_gen) given a=0)

Even then, it would make my head spin if I came across it.  I hope no-one would write code like that.

I'm not keen on given, but I must admit that ISTM that this example shows something that can only be done with given: putting some initialisation, viz. "a=0", into a generator expression.  With :=, it would need a trick:
    a_gen = (g( a:=f(a, x) ) for x in [x_gen, a:=0][0] )
or
    a_gen = (g( a:=f(a, x) ) for a in [0] for x in x_gen] )

Of course, in the case of a list comprehension (as opposed to a genexp), the initialisation could be done separately:
    a = 0
    a_list = [g( a:=f(a, x) ) for x in x_gen]

Rob Cliffe