On 30/05/2018 17:05, Peter O'Connor wrote:
On Thu, May 24, 2018 at 2:49 PM, Steven D'Aprano <email@example.com mailto:firstname.lastname@example.org> 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".
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] ) or a_gen = (g( a:=f(a, x) ) for a in  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]