Description

domdfcoding

opened on Jun 1, 2023

Issue body actions

Bug report

Between Python 3.12 alpha 7 and beta 1 the behaviour of format specifiers containing a backslash (e.g. \n, \u2603) in f-strings changed.

Previously, the __format__ method would get e.g. the newline character, but it now receives a backslash and an n character (i.e. the string "\n")

The following script demonstrates this:

# issue.py

class F:
	def __format__(self, format_spec):
		print("spec:", repr(format_spec))
		return format_spec.join(["a", "b", "c"])

print("out: ", repr((f"{F():\n}")))
print("out: ", repr((f"{F():\u2603}")))

print("out: ", repr((format(F(), "\n"))))
print("out: ", repr((format(F(), "\u2603"))))

$ python3.11 issue.py
spec: '\n'
out:  'a\nb\nc'
spec: '☃'
out:  'a☃b☃c'
spec: '\n'
out:  'a\nb\nc'
spec: '☃'
out:  'a☃b☃c'

$ python3.12 issue.py
spec: '\\n'
out:  'a\\nb\\nc'
spec: '\\u2603'
out:  'a\\u2603b\\u2603c'
spec: '\n'
out:  'a\nb\nc'
spec: '☃'
out:  'a☃b☃c'

In both cases the format() function works correctly - I only encounter the issue with f-strings.

I suspect this is related to the PEP 701 changes to f-strings, but I couldn't find anything in the discussion of that PEP to suggest this was a deliberate change.

Your environment

• CPython versions tested on: Python 3.11.2, 3.12.0alpha7, 3.12.0beta1
• Operating system and architecture: Ubuntu 20.04 (AMD64)

Linked PRs

• gh-105194: Fix format specifier escaped characters in f-strings #105231
• [3.12] gh-105194: Fix format specifier escaped characters in f-strings (GH-105231) #105234

Activity

1. domdfcoding

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jun 1, 2023
2. AlexWaygood

   assigned

   pablogsal

   and

   lysnikolaou

   on Jun 1, 2023
3. pablogsal commented on Jun 1, 2023

   

   pablogsal

   on Jun 1, 2023

   Member

   More actions

   @isidentical @lysnikolaou can you take this one?
4. lysnikolaou commented on Jun 1, 2023

   

   lysnikolaou

   on Jun 1, 2023

   Member

   More actions

   Sure! Will probably have a look tomorrow morning.

   Reacted by Pablo Galindo Salgado
5. pablogsal

   added a commit that references this issue on Jun 2, 2023

   pythongh-105194: Fix format specifier escaped characters in f-strings

   a63b87a
6. pablogsal commented on Jun 2, 2023

   

   pablogsal

   on Jun 2, 2023

   Member

   More actions

   @lysnikolaou I gave this a go and I think I have it. Can you check #105231 ?
7. pablogsal

   added 3 commits that reference this issue on Jun 2, 2023

   fixup! pythongh-105194: Fix format specifier escaped characters in f-…

   9907c0f

   fixup! fixup! pythongh-105194: Fix format specifier escaped character…

   fd297bd

   gh-105194: Fix format specifier escaped characters in f-strings (#105231

   41de543
8. pablogsal

   closed this as completedon Jun 2, 2023
9. miss-islington

   added a commit that references this issue on Jun 2, 2023

   pythongh-105194: Fix format specifier escaped characters in f-strings (…

   e32ae2a
10. pablogsal

    added a commit that references this issue on Jun 2, 2023

    [3.12] gh-105194: Fix format specifier escaped characters in f-strings (

    103ae4e
11. domdfcoding commented on Jun 3, 2023

    

    domdfcoding

    on Jun 3, 2023

    ContributorAuthor

    More actions

    Thanks for fixing this.

Issue actions