[Python-Dev] PEP 572: Assignment Expressions (original) (raw)

Anthony Flury anthony.flury at btinternet.com
Fri Apr 20 21:38:04 EDT 2018

I am entirely new to this list, but if I can I would like share my comments :

I do think this proposal := has merit in my opinion; it does make some code more readable.
I think readability is only improved if :
- target is restricted to a simple name - I don't see a benefit in more complex targets
- chaining is not allowed - I think the construct :
            while (line := input.read_row()) is not None:                     process_line(line)

        Is readable, but :

            while (current_line := line := input.read_row()) is not None:                     line = process_line(line)

      is not obvious - and certainly isn't any more obvious than :

            while (line := input.read_row()) is not None:                 current_line = line                     line = process_line(line)
The current expectations of how comprehensions work should also be honored; I don't claim to have fully followed all of the discussions around this, but it seems to me that comprehensions work in a particular way because of a concerted effect (especially in Python
1. to make them that way. They are self contained and don't leak values in their containing scope. Similarly I think that setting variables within a comprehension is just for the benefit of readable code within the comprehension - i.e. :
  
  stuff = [[y, x/y] for x in range(5) for y in [f(x)]]

can become :

stuff = [[y := f(x), x/y] for x in range(5)]

So - overall from me a conditional +1 - conditions as above; if they are not possible then -1 from me.