[Python-ideas] Descouraging the implicit string concatenation

[Terry Reedy]

On 3/14/2018 8:18 AM, Facundo Batista wrote:
> Hello!
> 
> What would you think about formally descouraging the following idiom?
> 
>      long_string = (
>          "some part of the string "
>          "with more words, actually is the same "
>          "string that the compiler puts together")

-1

> We should write the following, instead:
> 
>      long_string = (
>          "some part of the string " +
>          "with more words, actually is the same " +
>          "string that the compiler puts together")
> 

> Note that there's no penalty in adding the '+' between the strings,
> those are resolved at compilation time.

Not always, as Eryk Sun discussed.

When f-strings are thrown into the mix, as when creating multi-line 
messages with interpolated values, not even true in 3.7.  Compare the 
following:

 >>> def f():
	x = input('x:')
	print(f'a{x}bc' 'def')

 >>> dis.dis(f)
   2           0 LOAD_GLOBAL              0 (input)
               2 LOAD_CONST               1 ('x:')
               4 CALL_FUNCTION            1
               6 STORE_FAST               0 (x)

   3           8 LOAD_GLOBAL              1 (print)
              10 LOAD_CONST               2 ('a')
              12 LOAD_FAST                0 (x)
              14 FORMAT_VALUE             0
              16 LOAD_CONST               3 ('bcdef')
              18 BUILD_STRING             3
              20 CALL_FUNCTION            1
              22 POP_TOP
              24 LOAD_CONST               0 (None)
              26 RETURN_VALUE

 >>> def f():
	x = input('x:')
	print(f'a{x}bc' + 'def')

 >>> dis.dis(f)
   2           0 LOAD_GLOBAL              0 (input)
               2 LOAD_CONST               1 ('x:')
               4 CALL_FUNCTION            1
               6 STORE_FAST               0 (x)

   3           8 LOAD_GLOBAL              1 (print)
              10 LOAD_CONST               2 ('a')
              12 LOAD_FAST                0 (x)
              14 FORMAT_VALUE             0
              16 LOAD_CONST               3 ('bc')
              18 BUILD_STRING             3
              20 LOAD_CONST               4 ('def')
              22 BINARY_ADD
              24 CALL_FUNCTION            1
              26 POP_TOP
              28 LOAD_CONST               0 (None)
              30 RETURN_VALUE

The '+' prevents compile time concatenation of 'bc' and 'def' and adds a 
load and binary add.

-- 
Terry Jan Reedy