Implement partial PEP-498 (f-string) support #2690
Merged
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This implements (most of) the PEP-498 spec for f-strings, with two
exceptions:
- raw f-strings (`fr` or `rf` prefixes) raise `NotImplementedError`
- one special corner case does not function as specified in the PEP
(more on that in a moment)
This is implemented in the core as a syntax translation, brute-forcing
all f-strings to run through `String.format`. For example, the statement
`x='world'; print(f'hello {x}')` gets translated *at a syntax level*
(injected into the lexer) to `x='world'; print('hello {}'.format(x))`.
While this may lead to weird column results in tracebacks, it seemed
like the fastest, most efficient, and *likely* most RAM-friendly option,
despite being implemented under the hood with a completely separate
`vstr_t`.
Since [string concatenation of adjacent literals is implemented in the
lexer](micropython@534b7c3),
two side effects emerge:
- All strings with at least one f-string portion are concatenated into a
single literal which *must* be run through `String.format()` wholesale,
and:
- Concatenation of a raw string with interpolation characters with an
f-string will cause `IndexError`/`KeyError`, which is both different
from CPython *and* different from the corner case mentioned in the PEP
(which gave an example of the following:)
```python
x = 10
y = 'hi'
assert ('a' 'b' f'{x}' '{c}' f'str<{y:^4}>' 'd' 'e') == 'ab10{c}str< hi >de'
```
The above-linked commit detailed a pretty solid case for leaving string
concatenation in the lexer rather than putting it in the parser, and
undoing that decision would likely be disproportionately costly on
resources for the sake of a probably-low-impact corner case. An
alternative to become complaint with this corner case of the PEP would
be to revert to string concatenation in the parser *only when an
f-string is part of concatenation*, though I've done no investigation on
the difficulty or costs of doing this.
A decent set of tests is included. I've manually tested this on the
`unix` port on Linux and on a Feather M4 Express (`atmel-samd`) and
things seem sane.
This turns the "edge case" into a parse-time error.
This should reclaim *most* code space added to handle f-strings. However, there may be some small code growth as parse_string_literal takes a new parameter (which will always be 0, so hopefully the optimizer eliminates it)
f13e19f to
21bee01
Compare
* string_pep498_fstring.py: Not compatible with python3.5 * cmd_parsetree.py: enumerated constants changed
21bee01 to
ef195d6
Compare
tannewt
approved these changes
Mar 10, 2020
There was a problem hiding this comment.
Thanks for dusting this off! Thanks to @klardotsh for the original PR. I know everyone will be excited for the f-string future! @brettcannon asked me about this at PyCon last year.
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
I've taken a shot at updating #2054: