[Python-Dev] Changes to PEP 498 (f-strings) (original) (raw)

Eric V. Smith eric at trueblade.com
Tue Aug 30 13:55:07 EDT 2016

After a long discussion on python-ideas (starting at https://mail.python.org/pipermail/python-ideas/2016-August/041727.html) I'm proposing the following change to PEP 498: backslashes inside brackets will be disallowed. The point of this is to disallow convoluted code like:

d = {'a': 4} f'{d['a']}' '4'

In addition, I'll disallow escapes to be used for brackets, as in:

f'\x7bd["a"]}' '4'

(where chr(0x7b) == "{").

Because we're so close to 3.6 beta 1, my plan is to:

  1. Modify the PEP to reflect these restrictions.
  2. Modify the code to prevent any backslashes inside f-strings.

This is a more restrictive change than the PEP will describe, but it's much easier to implement. After beta 1, and hopefully before beta 2, I will implement the restrictions as I've outlined above (and as they will be documented in the PEP). The net effects are:

a. Some code that works in the alphas won't work in beta 1. I'll document this. b. All code that's valid in beta 1 will work in beta 2, and some f-strings that are syntax errors in beta 1 will work in beta 2.

I've discussed this issue with Ned and Guido, who are okay with these changes.

The python-ideas thread I referenced above has some discussion about further changes to f-strings. Those proposals are outside the scope of 3.6, but the changes I'm putting forth here will allow for those additional changes, should we decide to make them. That's a discussion for 3.7, however.

I'm sending this email out just to notify people of this upcoming change. I hope this won't generate much discussion. If you feel the need to discuss this issue further, please use the python-ideas thread (where some people are already ignoring it!).

Eric.

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