[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Mateusz Bysiek]

Change by Mateusz Bysiek :


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nosy: +mbdevpl

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Barry A. Warsaw]

Change by Barry A. Warsaw :


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nosy: +barry

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Arcadiy Ivanov]

Arcadiy Ivanov  added the comment:

Sorry about that! I'll be sure to refresh next time before posting a reply.

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Eric V. Smith]

Eric V. Smith  added the comment:

Arcadiy: Somehow you're dropping Serhiy and me from the nosy list. I've 
re-added us.

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nosy: +eric.smith, serhiy.storchaka

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Arcadiy Ivanov]

Arcadiy Ivanov  added the comment:

> Although general-purpose mechanism that would allow pluggable constructs like 
> `sh`, `html`, `sql`

Strike that one, I didn't read into PEP-0501 deep enough before replying. 

Yes, `i"format"` is what I'm talking about.

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Arcadiy Ivanov]

Arcadiy Ivanov  added the comment:

@eric.smith

Thanks! I was looking for such a general-purpose proposal but could not find 
it. 

Although general-purpose mechanism that would allow pluggable constructs like 
`sh`, `html`, `sql` and the like is awesome and very desirable (especially sh 
in preference of Popen madness), string formatting/concatenation is IMO a 
fundamental concept (hence PEP-498 is in core with specialized opcodes instead 
of being deferred to be a general purpose pluggable PEP-501 implementation).

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Arcadiy Ivanov]

Arcadiy Ivanov  added the comment:

@serhiy.storchaka Of course a similar pattern can be implemented via a class 
(or even without one as I've shown below). 

But you can clearly notice that in your example:

1) There are tons of boilerplate (as in mine with lambdas).
2) It's going to be slower than a core implementation with specialized objects.
3) It can't be made to cooperate with a FORMAT_VALUE/BUILD_STRING - 
intermediate strings will still be produced.

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nosy:  -eric.smith, serhiy.storchaka

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Eric V. Smith]

Eric V. Smith  added the comment:

See also PEP 501.

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nosy: +eric.smith, serhiy.storchaka

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Arcadiy Ivanov]

Arcadiy Ivanov  added the comment:

As an example this is the current state of affairs:

>>> def x():
... foo = "foo"
... s1 = f"S1 value {foo}"
... s2 = f"S2 value {s1}"
... print(s2)
... 
>>> dis.dis(x)
  2   0 LOAD_CONST   1 ('foo')
  2 STORE_FAST   0 (foo)

  3   4 LOAD_CONST   2 ('S1 value ')
  6 LOAD_FAST0 (foo)
  8 FORMAT_VALUE 0
 10 BUILD_STRING 2
 12 STORE_FAST   1 (s1)

  4  14 LOAD_CONST   3 ('S2 value ')
 16 LOAD_FAST1 (s1)
 18 FORMAT_VALUE 0
 20 BUILD_STRING 2
 22 STORE_FAST   2 (s2)

  5  24 LOAD_GLOBAL  0 (print)
 26 LOAD_FAST2 (s2)
 28 CALL_FUNCTION1
 30 POP_TOP
 32 LOAD_CONST   0 (None)
 34 RETURN_VALUE

whereas an fl-string representation of:

>>> def x():
... foo = "foo"
... s1 = fl"S1 value {foo}"
... s2 = fl"S2 value {s1}"
... print(s2)
... 

would behave close to:

>>> def x():
... foo = "foo"
... s1 = lambda: f"S1 value {foo}"
... s2 = lambda: f"S2 value {s1()}"
... print(s2())
... 
>>> dis.dis(x)
  2   0 LOAD_CONST   1 ('foo')
  2 STORE_DEREF  0 (foo)

  3   4 LOAD_CLOSURE 0 (foo)
  6 BUILD_TUPLE  1
  8 LOAD_CONST   2 ( at 
0x7ff2ef1abd20, file "", line 3>)
 10 LOAD_CONST   3 ('x..')
 12 MAKE_FUNCTION8
 14 STORE_DEREF  1 (s1)

  4  16 LOAD_CLOSURE 1 (s1)
 18 BUILD_TUPLE  1
 20 LOAD_CONST   4 ( at 
0x7ff2ef1abae0, file "", line 4>)
 22 LOAD_CONST   3 ('x..')
 24 MAKE_FUNCTION8
 26 STORE_FAST   0 (s2)

  5  28 LOAD_GLOBAL  0 (print)
 30 LOAD_FAST0 (s2)
 32 CALL_FUNCTION0
 34 CALL_FUNCTION1
 36 POP_TOP
 38 LOAD_CONST   0 (None)
 40 RETURN_VALUE

Where LOAD_CONST, LOAD_CONST, MAKE_FUNCTION sequences are replaced with 
BUILD_LAZY_STRING (name's provisional) producing the same lambda-like formatter 
function that cooperates with FORMAT_VALUE and BUILD_STRING/BUILD_LAZY_STRING 
ops.

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components: +Interpreter Core -Library (Lib)
nosy:  -serhiy.storchaka

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

class FL:
def __init__(self, func):
self.func = func
def __str__(self):
return self.func()

extra = FL(lambda: f'{extra},waiters:{len(self._waiters)}')

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nosy: +serhiy.storchaka

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[issue32954] Lazy Literal String Interpolation (PEP-498-based fl-strings)

[Arcadiy Ivanov]

New submission from Arcadiy Ivanov :

I'd like to start a discussion on/gauge interest for introducing an enhancement 
to PEP-498 in a form of delayed/lazy/lambda f-string.

The proposed change is, conceptually, minor and would not differ from PEP-498 
syntactically at all except for string prefix.

E.g. 

 extra = f'{extra},waiters:{len(self._waiters)}'

becomes

 extra = fl'{extra},waiters:{len(self._waiters)}'

The proposal would produce a lambda-like object x('format'), where x.__str__() 
== f'format'.

This should come extremely useful in all cases where delayed or conditional 
string formatting and concatenation are desired, such as in cases of logging.

As an example, currently, the 

logger.debug("Format %s string", value) 

cannot be used with an f-string as follows 

logger.debug(f"Format {value} string") 

without an unconditional evaluation of all parameters due to current 
compilation prior to logging checking whether it's even enabled for debug:

>>> b = 1
>>> def a(x):
... return f"Foo {x} bar {b}"
... 
>>> dis.dis(a)
  2   0 LOAD_CONST   1 ('Foo ')
  2 LOAD_FAST0 (x)
  4 FORMAT_VALUE 0
  6 LOAD_CONST   2 (' bar ')
  8 LOAD_GLOBAL  0 (b)
 10 FORMAT_VALUE 0
 12 BUILD_STRING 4
 14 RETURN_VALUE

Additional great optimizations may be rendered by introducing an a fl-string is 
a case where

foo = "foo"
s1 = fl"S1 value ${foo}"
s2 = fl"S2 value of ${s1}"
print(s2)

may produce only one BUILD_STRING instruction, potentially dramatically 
increasing performance in heavy string-concat based applications. 

Even when a compiler will not be able to statically prove that a particular 
value is in fact an fl-string, an interpreter-level check or a JIT-based Python 
implementation may be able to optimize such concats ad-nauseam (with trap-based 
deopt), allowing to collapse an extremely long chains of formats into a single 
BUILD_STRING op without any intermediary string allocation/concatenation (very 
useful in cases such as web servers, templating engines etc).

I'll be happy to produce patches against 3.7/3.8 for this if a general concept 
is found useful/desirable by the community.

--
components: Library (Lib)
messages: 312909
nosy: arcivanov
priority: normal
severity: normal
status: open
title: Lazy Literal String Interpolation (PEP-498-based fl-strings)
type: enhancement
versions: Python 3.7, Python 3.8

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