[Python-Dev] Parsing f-strings from PEP 498 -- Literal String Interpolation (original) (raw)

Eric V. Smith eric at trueblade.com
Fri Nov 4 03:56:35 EDT 2016

On 11/3/2016 3:06 PM, Fabio Zadrozny wrote:

Hi Python-Dev,

I'm trying to get my head around on what's accepted in f-strings -- https://www.python.org/dev/peps/pep-0498/ seems very light on the details on what it does accept as an expression and how things should actually be parsed (and the current implementation still doesn't seem to be in a state for a final release, so, I thought asking on python-dev would be a reasonable option).

In what way do you think the implementation isn't ready for a final release?

I was thinking there'd be some grammar for it (something as https://docs.python.org/3.6/reference/grammar.html), but all I could find related to this is a quote saying that f-strings should be something as:

f ' { <optional !s, !r, or !a> } So, given that, is it safe to assume that would be equal to the "test" node from the official grammar?

No. There are really three phases here:

  1. The f-string is tokenized as a regular STRING token, like all other strings (f-, b-, u-, r-, etc).
  2. The parser sees that it's an f-string, and breaks it into expression and text parts.
  3. For each expression found, the expression is compiled with PyParser_ASTFromString(..., Py_eval_input, ...).

Step 2 is the part that limits what types of expressions are allowed. While scanning for the end of an expression, it stops at the first '!', ':', or '}' that isn't inside of a string and isn't nested inside of parens, braces, and brackets.

The nesting-tracking is why these work:

f'{(lambda x:3)}' '<function at 0x000000000296E560>' f'{(lambda x:3)!s:.20}' '<function a'

But this doesn't:

f'{lambda x:3}' File "", line 1 (lambda x) ^ SyntaxError: unexpected EOF while parsing

Also, backslashes are not allowed anywhere inside of the expression. This was a late change right before beta 1 (I think), and differs from the PEP and docs. I have an open item to fix them.

I initially thought it would obviously be, but the PEP says that using a lamda inside the expression would conflict because of the colon (which wouldn't happen if a proper grammar was actually used for this parsing as there'd be no conflict as the lamda would properly consume the colon), so, I guess some pre-parser steps takes place to separate the expression to then be parsed, so, I'm interested on knowing how exactly that should work when the implementation is finished -- lots of plus points if there's actually a grammar to back it up :)

I've considered using the grammar and tokenizer to implement f-string parsing, but I doubt it will ever happen. It's a lot of work, and everything that produced or consumed tokens would have to be aware of it. As it stands, if you don't need to look inside of f-strings, you can just treat them as regular STRING tokens.

I hope that helps.

Eric.

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