Commits on May 16, 2020

1. py: Implement partial PEP-498 (f-string) support …

   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.

   

   klardotsh committed May 16, 2020
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