[docs] [issue37816] f-string documentation not fully accurate

Glenn Linderman report at bugs.python.org
Sat Aug 10 21:06:41 EDT 2019

New submission from Glenn Linderman <v+python at g.nevcal.com>:

I noticed the following description for f-strings:

> Escape sequences are decoded like in ordinary string literals (except when a literal is also marked as a raw string). After decoding, the grammar for the contents of the string is:

followed by lots of stuff, followed by

> Backslashes are not allowed in format expressions and will raise an error:
> f"newline: {ord('\n')}"  # raises SyntaxError

If f-strings are processed AS DESCRIBED, the \n would never seen by the format expression... but in fact, it does produce an error.

PEP 498 has a more accurate description, that the {} parsing actually happens before the escape processing. The string or raw-string escape processing is done only to the literal string pieces.

So the string content is first parsed for format expressions enclosed in {}, on the way, converting {{ and }} to { and } in the literal parts of the string, and then the literal parts are treated exactly like independent string literals (with regular or raw-string escaping performed), to be concatenated with the evaluation of the format expressions, and subsequent literal parts.

The evaluation of the format expressions presently precludes the use of # and \ for comments and escapes, but otherwise tries to sort of by the same as a python expression possibly followed by a format mini-language part.  I am not the expert here, just the messenger.

assignee: docs at python
components: Documentation
messages: 349370
nosy: docs at python, v+python
priority: normal
severity: normal
status: open
title: f-string documentation not fully accurate
versions: Python 3.6, Python 3.7, Python 3.8, Python 3.9

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