On Thu, May 24, 2018 at 2:49 PM, Steven D'Aprano
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()