[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Documentation task added as issue #25179.

Thanks to Martin for the great code reviews.

--
resolution:  -> fixed
status: open -> closed

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[issue24965] Implement PEP 498: Literal String Formatting

[Roundup Robot]

Roundup Robot added the comment:

New changeset a10d37f04569 by Eric V. Smith in branch 'default':
Issue #24965: Implement PEP 498 "Literal String Interpolation". Documentation 
is still needed, I'll open an issue for that.
https://hg.python.org/cpython/rev/a10d37f04569

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

I changed the generated code to call:
format(x [, spec])

instead of:
x.__format__(spec)

The reason is that the correct way to call __format__ is actually:
type(x).__format__(x, spec)

That is, the __format__ lookup is done on the type, not the instance. From the 
earlier example, the disassembled code is now:

>>> dis.dis("f'a={a}'")
  1   0 LOAD_CONST   0 ('')
  3 LOAD_ATTR0 (join)
  6 LOAD_CONST   1 ('a=')
  9 LOAD_GLOBAL  1 (format)
 12 LOAD_NAME2 (a)
 15 CALL_FUNCTION1 (1 positional, 0 keyword pair)
 18 BUILD_LIST   2
 21 CALL_FUNCTION1 (1 positional, 0 keyword pair)
 24 RETURN_VALUE

The simplest way to make the lookup correctly is just to call format() itself, 
which does the right thing.

I still have a concept of adding opcodes to handle FormattedValue and JoinedStr 
nodes, but that's an optimization for later, if ever.

--
Added file: http://bugs.python.org/file40515/pep-498-10.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Cleaned up the error handling in fstring_expression_compile so it's easier to 
verify and more robust in the face of future changes.

Added a test for an un-doubled '}', which is an error in a top-level literal 
(and ends a nested expression). Modified existing tests to match.

--
Added file: http://bugs.python.org/file40502/pep-498-9.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Hopefully the last version.

--
Added file: http://bugs.python.org/file40495/pep-498-8.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

> Martin Panter added the comment:
> 
> Another strange error message (though maybe the new test changes you 
> mentioned caught this):
> 
 f'{3:{10}'  # Actually missing a closing bracket '}'
>   File "", line 1
> SyntaxError: f-string: unexpected '}'

Yes, I found that one, too. Sorry to waste your time on this, but I literally 
just finished the test changes 15 minutes ago.

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[issue24965] Implement PEP 498: Literal String Formatting

[Martin Panter]

Martin Panter added the comment:

Another strange error message (though maybe the new test changes you mentioned 
caught this):

>>> f'{3:{10}'  # Actually missing a closing bracket '}'
  File "", line 1
SyntaxError: f-string: unexpected '}'

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[issue24965] Implement PEP 498: Literal String Formatting

[Martin Panter]

Martin Panter added the comment:

I left a few more comments on Reitveld. Checking the error messages does make 
me feel a lot more comfortable though.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

I rewrote the format_spec parser to recursively call the f-string parser, so 
any oddness in what's allowed in a format_spec is gone.

It took way longer than I thought, but the code is better for it.

--
Added file: http://bugs.python.org/file40480/pep-498-6.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Simplified error handling, fixed 2 memory leaks.

All tests now pass with no leaks.

This should be the final version.

--
Added file: http://bugs.python.org/file40484/pep-498-7.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Yury Selivanov]

Changes by Yury Selivanov :


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[issue24965] Implement PEP 498: Literal String Formatting

[Martin Panter]

Martin Panter added the comment:

Regarding the null terminator, I was mainly smoke testing your code. :) Maybe 
it would be too hard to support properly. Although I could imagine someone 
doing things like this:

>>> d = {b"key\x00": "value"}
>>> f"key={d[b'key\x00']}"  # Oops, escape code at wrong level
  File "", line 1
(d[b'key
   ^
SyntaxError: EOL while scanning string literal
>>> rf"key={d[b'key\x00']}"  # Corrected
'key=value'

I also finished quickly reading over the C code, with a couple more review 
comments. But I am not familiar with the files involved to thoroughly review.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

I'll probably ensure that all of the parsing errors contain "format string" or 
"f-string" or similar. That way the regex check is easier, and the user can 
search for it more easily.

It remains to be seen how these are referenced in the documentation. "f-string" 
seems much easier to say and search for, but seems too slangy for the docs. But 
"format string" seems ambiguous and hard to search for. I guess time will tell.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

After discussing it with Guido, I've removed the ability to combine 'f' with 
'u'.

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[issue24965] Implement PEP 498: Literal String Formatting

[Jelle Zijlstra]

Jelle Zijlstra added the comment:

Thanks! Here are a few more cases I came across with the existing 
implementation:

>>> f"{'a\\'b'}"
  File "", line 1
SyntaxError: missing '}' in format string expression

I believe this is valid and should produce "a'b".

>>> f"{x!s!s}"
  File "", line 1
SyntaxError: single '}' encountered in format string

Could use a better error message.

>>> x = 3
>>> f"{x!s{y}}"
'3y}'

Not sure how this happened.

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[issue24965] Implement PEP 498: Literal String Formatting

[Jelle Zijlstra]

Jelle Zijlstra added the comment:

Is this behavior intentional?

>>> str = len
>>> x = 'foo'
>>> f'{x!s}'
'3'
>>> '{!s}'.format(x)
'foo'

Or similarly:

>>> import builtins
>>> del builtins.repr
>>> f'{x!r}'
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'repr' is not defined

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[issue24965] Implement PEP 498: Literal String Formatting

[Jelle Zijlstra]

Jelle Zijlstra added the comment:

I've started working on implementing this feature in Cython and I'd like to 
confirm a few edge cases:

- f'{ {1: 2\N{RIGHT CURLY BRACKET}[1]}' == '2' (string escape rules work even 
within the expressions)
- f'{ '''foo''' }' is a syntax error
- f'{ """foo 'bar'""" }' is a syntax error

--
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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

This patch fixes triple-quoted strings, plus a few bugs. I'm going to commit it 
tomorrow, barring any unforeseen issues.

--
Added file: http://bugs.python.org/file40447/pep-498-5.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Yes, Jelle, you are correct in all 3 cases. Remember that the steps are to 
extract the string from the source code, decode backslash escapes, and only 
then treat it as an f-string.

For the first case, without the 'f' prefix:
'{ {1: 2\N{RIGHT CURLY BRACKET}[1]}' == '{ {1: 2}[1]}'

Then, applying the 'f':
f'{ {1: 2}[1]}' == '2'.

For the last 2, since they're syntax errors without the 'f', they're also 
syntax errors with the 'f'.

I'll have a new version, with tests for all of these cases, posted in the next 
few hours. You can leverage the tests.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Agreed on checking the error messages better. Especially since even the 
simplest of errors (like leaving out a quote) results in a syntax error in 
parsing the string, not parsing inside the f-string.

I'll look at it eventually.

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[issue24965] Implement PEP 498: Literal String Formatting

[Martin Panter]

Martin Panter added the comment:

Regarding wrong error messages, I’ve learnt the hard way that it is often best 
to use assertRaisesRegex() instead of assertRaises(), to ensure that the actual 
exception you have in mind is being triggered, rather than a typo or something. 
Though that might upset your assertSyntaxErrors() helper.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

This one has been fixed:
>>> f"{'a\\'b'}"
"a'b"

This one was a bug that I previously fixed, that Martin pointed out:
>>> f"{x!s!s}"
  File "", line 1
SyntaxError: invalid character following conversion character

And this is the same bug:
>>> f"{x!s{y}}"
  File "", line 1
SyntaxError: invalid character following conversion character

I'm wrapping up my new code plus tests. I'll post it Real Soon Now.

Thanks for your help.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Both of those are known (to me!) byproducts of the current implementation. If 
my crazy idea of adding opcodes to speed up f-strings flies, then this issue 
will go away. I consider this a corner case that doesn't need to be addressed 
before committing this code. I wouldn't emulate it one way or the other just 
yet.

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[issue24965] Implement PEP 498: Literal String Formatting

[Martin Panter]

Martin Panter added the comment:

I’m actually trying out your patch now. A couple strange errors and 
observations:

>>> f"{'{'}"  # Why is this allowed in an outer format expression--
'{'
>>> f"{3:{'{'}>10}"  # --but not inside a format specifier?
SyntaxError: nesting of '{' in format specifier is not allowed
>>> opening = "{"; f"{3:{opening}>10}"  # Workaround
'{3'
>>> f"{3:{'}'}<10}"  # Error message is very strange!
SyntaxError: missing '}' in format string expression
>>> f"{\x00}"  # It seems this is treated as a null terminator
  File "", line 1
(
^
SyntaxError: unexpected EOF while parsing
>>> f"{'s'!\x00:.<10}"  # Default conversion is the null character?
's.'

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Thanks again, Martin. I've found 4 bugs so far, based on your suggested tests. 
The ones I haven't fixed are: 'fur' strings don't work (something in the 
lexer), and triple quoted strings don't work correctly. I'm working on both of 
those, and should have an updated patch in the next day or so.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

It turns out 'fur' strings aren't a thing, because 'ur' strings aren't.

>From tokenizer.c:
/* ur"" and ru"" are not supported */

And the PEP:
https://www.python.org/dev/peps/pep-0414/#exclusion-of-raw-unicode-literals

I'll add a test to make sure this fails.

So I just need to work on the triple-quoted string problem.

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[issue24965] Implement PEP 498: Literal String Formatting

[Martin Panter]

Martin Panter added the comment:

Another version of that AST that is better for my digestion:

f'a={a}'

Module(body=[Expr(
value=JoinedStr(values=[
Str(s='a='),
FormattedValue(
value=Name(id='a', ctx=Load()),
conversion=0,
format_spec=None,
),
]),
)])

I have been reading over the test cases, and left a bunch of suggestions for 
more edge cases etc. Some of them might reflect that I haven’t completely 
learnt how the inner Python expression syntax, outer string escaping syntax, 
{{curly bracket}} escaping, automatic concatenation, etc, are all meant to fit 
together.

--
nosy: +martin.panter

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Thanks, Martin. I've posted my replies. I'll add some more tests, and work on 
the triple quoted string bug.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

I discussed it with Guido and added 'F' to the PEP.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

This version does dynamic allocation for the expression list, and fixes some 
memory leaks and early decrefs.

I think it's complete, but I'll take some more passes through it checking for 
leaks.

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Changes by Eric V. Smith :


Added file: http://bugs.python.org/file40430/pep-498-4.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

The good news is that the performance is pretty good, and finally I have a case 
where I can beat %-formatting:

$ ./python.bat -mtimeit -s 'a=2' "'%s' % a"
100 loops, best of 3: 0.883 usec per loop

$ ./python.bat -mtimeit -s 'a=2' '"{}".format(a)'
100 loops, best of 3: 1.16 usec per loop

$ ./python.bat -mtimeit -s 'a=2' 'f"{a}"'
100 loops, best of 3: 0.792 usec per loop

This example is mildly contrived, and the performance of f-strings is slightly 
worse than %-formatting once the f-strings contains both expressions and 
literals.

I could speed it up significantly (I think) by adding opcodes for 2 things: 
calling __format__ and joining the strings together. Calling __format__ in an 
opcode could be a win because I could optimize for known types (str, int, 
float). Having a join opcode would be a win because I could use 
_PyUnicodeWriter instead of ''.join.

I'm inclined to check this code in as-is, then optimize it later, if we think 
it's needed and if I get motivated.

For reference, here's the ast and opcodes for f'a={a}':

>>> ast.dump(ast.parse("f'a={a}'"))
"Module(body=[Expr(value=JoinedStr(values=[Str(s='a='), 
FormattedValue(value=Name(id='a', ctx=Load()), conversion=0, 
format_spec=None)]))])"

>>> dis.dis("f'a={a}'")
  1   0 LOAD_CONST   0 ('')
  3 LOAD_ATTR0 (join)
  6 LOAD_CONST   1 ('a=')
  9 LOAD_NAME1 (a)
 12 LOAD_ATTR2 (__format__)
 15 LOAD_CONST   0 ('')
 18 CALL_FUNCTION1 (1 positional, 0 keyword pair)
 21 BUILD_LIST   2
 24 CALL_FUNCTION1 (1 positional, 0 keyword pair)
 27 RETURN_VALUE

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Changes by Eric V. Smith :


Removed file: http://bugs.python.org/file40302/pep-498.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Changes by Eric V. Smith :


Removed file: http://bugs.python.org/file40316/pep-498-1.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Changes by Eric V. Smith :


Removed file: http://bugs.python.org/file40317/pep-498-2.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

This implements the accepted PEP 498. The only other real change I plan on 
making is to do dynamic memory allocation when building the expressions that 
make up a JoinedStr AST node. The code has all of the places to do that already 
laid out, it's just a matter of hooking it up.

There's one nit where I accept 'f' and 'F', but the PEP just says 'f'. I'm not 
sure if we should accept the upper case version. I'd think not, but all of the 
other ones (b, r, and u) do.

I need to do one more scan for memory leaks. I've rearranged some code since 
the last time I checked for leaks, and that's always a recipe for some sneaking 
in.

And I need to write some more tests, mostly for syntax errors, but also for a 
few edge conditions.

Comments welcome.

--
Added file: http://bugs.python.org/file40421/pep-498-3.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

On Sep 09, 2015, at 11:57 PM, Eric V. Smith wrote:

>There's one nit where I accept 'f' and 'F', but the PEP just says 'f'. I'm
>not sure if we should accept the upper case version. I'd think not, but all
>of the other ones (b, r, and u) do.

I think it should be consistent with the other prefixes.  Shouldn't be a big
deal to amend the PEP to describe this.

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[issue24965] Implement PEP 498: Literal String Formatting

[Roundup Robot]

Roundup Robot added the comment:

New changeset a0194ec4195c by Eric V. Smith in branch 'default':
Removed Implementation Limitations section. While the version of the code on 
http://bugs.python.org/issue24965 has the 255 expression limitation, I'm going 
to remove this limit. The i18n section was purely speculative. We can worry 
about it if/when we add i18n and i-strings.
https://hg.python.org/peps/rev/a0194ec4195c

--
nosy: +python-dev

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Fixed validate_exprs bug.

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Added file: http://bugs.python.org/file40316/pep-498-1.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Yury Selivanov]

Changes by Yury Selivanov :


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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

Make sure f-strings are identified as literals in error messages.

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Added file: http://bugs.python.org/file40317/pep-498-2.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

New submission from Eric V. Smith:

See PEP 498.

 f'New for Python {sys.version.split()[0]}'
'New for Python 3.6.0a0'

--
assignee: eric.smith
components: Interpreter Core
files: pep-498.diff
keywords: patch
messages: 249362
nosy: eric.smith
priority: normal
severity: normal
status: open
title: Implement PEP 498: Literal String Formatting
type: enhancement
versions: Python 3.6
Added file: http://bugs.python.org/file40302/pep-498.diff

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[issue24965] Implement PEP 498: Literal String Formatting

[Barry A. Warsaw]

Changes by Barry A. Warsaw ba...@python.org:


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[issue24965] Implement PEP 498: Literal String Formatting

[Eric V. Smith]

Eric V. Smith added the comment:

One thing I've done in this implementation is to build up a string to pass to 
str.format(), instead of using the original string. This new string uses 
positional parameters instead of named parameters.

I had originally proposed to add a string.interpolate() to do the heavy lifting 
here, which would have meant I could use the original string (as seen in the 
source code), and not build up a new string and pass it to str.format(). I 
still might do that, but for now, the approach using str.format() is good 
enough.

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