I was looking into format issues and came across msg99839 . The example causes segfault in master, 3.7 and 3.6 branches. This used to pass in 3.7 and 3.6. I searched for open issues and cannot come across an issue for this. I guess this is caused due to issue33954 which adds an assert as I can see from the segfault. Compiling in release mode works fine but debug build fails. Are asserts removed in release builds?

$ python3.7
Python 3.7.1rc2 (v3.7.1rc2:6c06ef7dc3, Oct 13 2018, 05:10:29)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> format(float(123), "00")
'123.0'

Master

master $ ./python.exe
Python 3.8.0a0 (heads/35559:c1b4b0f616, Dec 22 2018, 15:00:08)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> format(float(123), "00")
Assertion failed: (0 <= min_width), function _PyUnicode_InsertThousandsGrouping, file Objects/unicodeobject.c, line 9394.

Python 3.6

cpython git:(5241ecff16) ./python.exe
Python 3.6.8rc1+ (remotes/upstream/3.6:5241ecff16, Dec 22 2018, 15:05:57)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> format(float(123), "00")
Assertion failed: (0 <= min_width), function _PyUnicode_InsertThousandsGrouping, file Objects/unicodeobject.c, line 9486.
[1]    33859 abort      ./python.exe

Python 3.7

cpython git:(c046d6b618) ./python.exe
Python 3.7.2rc1+ (remotes/upstream/3.7:c046d6b618, Dec 22 2018, 15:07:24)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> format(float(123), "00")
Assertion failed: (0 <= min_width), function _PyUnicode_InsertThousandsGrouping, file Objects/unicodeobject.c, line 9369.
[1]    42710 abort      ./python.exe


Latest master, 3.6 and 3.7 branch has this bug in debug mode with this being last Python 3.6 bug fix release. Commenting out the assert line gives me the correct result but I have limited knowledge of the C code and I guess release builds remove asserts where it cannot be reproduced? I am tagging issue33954 devs who might have a better understanding of this and there might be limited bandwidth for someone to look into this along with other cases since it's holiday season.

# Release mode works fine

./python.exe -c 'print(format(float(123), "00"))'
123.0