SyntaxWarnings in `test_fstring`

[AlexWaygood]

Bug report

Running python -m test test_fstring on main results in several SyntaxWarnings being emitted.

If I just run python -m test test_fstring locally, I get this:

C:\Users\alexw\coding\cpython>python -m test test_fstring
Running Release|x64 interpreter...
0:00:00 Run tests sequentially
0:00:00 [1/1] test_fstring
C:\Users\alexw\coding\cpython\Lib\test\test_fstring.py:775: SyntaxWarning: invalid escape sequence '\_'
  exec('f"{F():¯\_(ツ)_/¯}"', {'F': CustomFormat})

== Tests result: SUCCESS ==

1 test OK.

Total duration: 14.5 sec
Tests result: SUCCESS

If I run python -We -m test test_fstring -vv, I get a different selection of SyntaxWarnings:

======================================================================
ERROR: test_fstring_backslash_before_double_bracket (test.test_fstring.TestCase.test_fstring_backslash_before_double_bracket) (case="f'\\{{\\}}'", expected_result='\\{\\}')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_fstring.py", line 1041, in test_fstring_backslash_before_double_bracket
    result = eval(case)
             ^^^^^^^^^^
  File "<string>", line 1
    f'\{{\}}'
            ^
SyntaxError: invalid escape sequence '\{'

======================================================================
ERROR: test_fstring_backslash_before_double_bracket (test.test_fstring.TestCase.test_fstring_backslash_before_double_bracket) (case="f'\\{{'", expected_result='\\{')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_fstring.py", line 1041, in test_fstring_backslash_before_double_bracket
    result = eval(case)
             ^^^^^^^^^^
  File "<string>", line 1
    f'\{{'
         ^
SyntaxError: invalid escape sequence '\{'

======================================================================
ERROR: test_fstring_backslash_before_double_bracket (test.test_fstring.TestCase.test_fstring_backslash_before_double_bracket) (case="f'\\{{{1+1}'", expected_result='\\{2')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_fstring.py", line 1041, in test_fstring_backslash_before_double_bracket
    result = eval(case)
             ^^^^^^^^^^
  File "<string>", line 1
    f'\{{{1+1}'
              ^
SyntaxError: invalid escape sequence '\{'

======================================================================
ERROR: test_fstring_backslash_before_double_bracket (test.test_fstring.TestCase.test_fstring_backslash_before_double_bracket) (case="f'\\}}{1+1}'", expected_result='\\}2')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_fstring.py", line 1041, in test_fstring_backslash_before_double_bracket
    result = eval(case)
             ^^^^^^^^^^
  File "<string>", line 1
    f'\}}{1+1}'
              ^
SyntaxError: invalid escape sequence '\}'

======================================================================
ERROR: test_fstring_backslash_before_double_bracket (test.test_fstring.TestCase.test_fstring_backslash_before_double_bracket) (case="f'{1+1}\\}}'", expected_result='2\\}')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_fstring.py", line 1041, in test_fstring_backslash_before_double_bracket
    result = eval(case)
             ^^^^^^^^^^
  File "<string>", line 1
    f'{1+1}\}}'
              ^
SyntaxError: invalid escape sequence '\}'

----------------------------------------------------------------------
Ran 80 tests in 14.399s

FAILED (errors=5)
test test_fstring failed
test_fstring failed (5 errors)

== Tests result: FAILURE ==

1 test failed:
    test_fstring

Total duration: 14.6 sec
Tests result: FAILURE

Your environment

Python 3.13.0a0 (heads/main:ab3823a97b, Jun 19 2023, 14:46:42) [MSC v.1932 64 bit (AMD64)] on win32

Linked PRs

• gh-105915: Add 'r' prefix to not emit SyntaxWarning in test_fstring #105940
• [3.12] gh-105915: Add 'r' prefix to not emit SyntaxWarning in test_fstring (GH-105940) #105942
• gh-105915: Fix SyntaxWarning becoming a SyntaxError with -We in test_fstring #105943
• [3.12] gh-105915: Fix SyntaxWarning becoming a SyntaxError with -We in test_fstring (GH-105943) #105945

[sunmy2019]

The first should be

-            exec('f"{F():¯\_(ツ)_/¯}"', {'F': CustomFormat})
+            exec(r'f"{F():¯\_(ツ)_/¯}"', {'F': CustomFormat})

I am wondering why the second test is reporting errors.

[sobolevn]

I cannot reproduce this locally on macOS (using 155577d), output for me:

» ./python.exe -m test test_fstring   
0:00:00 load avg: 1.06 Run tests sequentially
0:00:00 load avg: 1.06 [1/1] test_fstring

== Tests result: SUCCESS ==

1 test OK.

Total duration: 1.6 sec
Tests result: SUCCESS

[AlexWaygood]

I cannot reproduce this locally on macOS (using 155577d), output for me:

» ./python.exe -m test test_fstring   
0:00:00 load avg: 1.06 Run tests sequentially
0:00:00 load avg: 1.06 [1/1] test_fstring

== Tests result: SUCCESS ==

1 test OK.

Total duration: 1.6 sec
Tests result: SUCCESS

I'm certain this reproduced consistently for me when I filed the report, but I can also no longer reproduce the first issue. I have no idea why this is.

I can still reproduce the test failures when running the test with -We, however.

[lysnikolaou]

Related: #105938. Having a look.

[lysnikolaou]

Fixed in #105938 and #105940.