[Python-ideas] "Assignment expression" with function call-alike syntax

[Terry Reedy]

On 5/22/2018 5:32 PM, Kirill Balunov wrote:
> Just one more variation on "assignment exression" syntax to make the 
> list more complete :) Sorry, if something similar has already been 
> suggested. The idea is to use function's call-like syntax in the from: 
> `this( name = expr )`.

Functions names should be verbs.  For this one, 'bind' or even better, 
'let' as in 'let(name=expr)'

  I'm not sure that such idea will find its
> supporters and whether it is possible to implement it in a general way, 
> but nevertheless. Below is a half- and slow-working prototype and some 
> silly examples just for the sake of feeling the idea:
> 
>     import sys
>     import ctypes
> 
>     def this(**assign):
>          assert len(assign) == 1
>          [(name, value)] = assign.items()
>          frame = sys._getframe(1)
>          frame.f_locals[name] = value
>          ctypes.pythonapi.PyFrame_LocalsToFast(ctypes.py_object(frame),
>                                                ctypes.c_int(0))
>          return value
> 
> Loop-and-a-half example with `dummy` function[1]:
> 
>     # in global scope everything works ok since locals is globals
>      >>> while len( this( val = dummy() ) ) >= 0:
>     ...     print(val)
>     [0, 1]
>     [0, 1, 2]
>     [0, 1, 2, 3]
>          ...
> 
> 
>     # needs to set the same local name to work inside function
>     def func():
>          val = ...
>     while len( this( val = dummy() ) ) >= 0:
>              print(val)
> 
> 
>      >>> # relies on the implicit underlying function's local `x`
>      >>> [[this(x = (x+10)), x//10, x*10] for x in [100, 200, 300]]
>     [[110, 11, 1100], [210, 21, 2100], [310, 31, 3100]]
> 
> 
> In this way it is somewhat possible to make an assignment in `while` and 
> `if`headers right now, which covers 90% cases, but it is damn 
> slow. Maybe be it is worth to make `this` magic call-alike object work 
> fast...on the other hand does anyone like `this`?
> 
> 
> With kind regards,
> -gdg
> 
> [1]:
> 
>     def dummy(ls = [0]):
>          ls.append(ls[-1]+1)
>          return ls
> 
> 
> 
> 
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-- 
Terry Jan Reedy