With macOS Big Sur (11.y.z), the major version is 11 (instead of the earlier 10.15, 10.14, etc). Therefore, MACOSX_DEPLOYMENT_TARGET=11 has become a valid setting. It is accepted by system tools:

$ MACOSX_DEPLOYMENT_TARGET=11 clang a.c && echo $?
0

But it leads to failure in Python compilation in setup.py:

DYLD_FRAMEWORK_PATH=/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0 CC='clang' LDSHARED='clang -bundle -undefined dynamic_lookup -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk   ' OPT='-DNDEBUG -g -fwrapv -O3 -Wall'   _TCLTK_INCLUDES='' _TCLTK_LIBS=''       ./python.exe -E ./setup.py  build
running build
running build_ext
Traceback (most recent call last):
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/./setup.py", line 2614, in <module>
    main()
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/./setup.py", line 2584, in main
    setup(# PyPI Metadata (PEP 301)
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/Lib/distutils/core.py", line 148, in setup
    dist.run_commands()
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/Lib/distutils/dist.py", line 966, in run_commands
    self.run_command(cmd)
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/Lib/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/Lib/distutils/command/build.py", line 135, in run
    self.run_command(cmd_name)
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/Lib/distutils/cmd.py", line 313, in run_command
    self.distribution.run_command(command)
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/Lib/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/Lib/distutils/command/build_ext.py", line 340, in run
    self.build_extensions()
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/./setup.py", line 470, in build_extensions
    self.detect_modules()
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/./setup.py", line 1807, in detect_modules
    self.detect_readline_curses()
  File "/private/tmp/python@3.9-20201127-81873-q9b09s/Python-3.9.0/./setup.py", line 1038, in detect_readline_curses
    (tuple(int(n) for n in dep_target.split('.')[0:2])
AttributeError: 'int' object has no attribute 'split'
make: *** [sharedmods] Error 1

See the full report here: Homebrew/brew#9329

The problem is here: https://github.com/python/cpython/blob/master/setup.py#L1015
When MACOSX_DEPLOYMENT_TARGET=11, this call:
sysconfig.get_config_var('MACOSX_DEPLOYMENT_TARGET')
returns an int (with value 11) instead of a string (as is the case if 11.0, 10.15, etc).

And therefore the subsequent call to: dep_target.split('.')[0:2] fails, because split() cannot be called on an int.

Good catch.

New changeset 5291639 by FX Coudert in branch 'master':
bpo-42504: fix for MACOSX_DEPLOYMENT_TARGET=11 (GH-23556)
5291639

New changeset 09a698b by Miss Islington (bot) in branch '3.9':
bpo-42504: fix for MACOSX_DEPLOYMENT_TARGET=11 (GH-23556)
09a698b

FWIW I don't agree with this fix.

sys.get_config_var('MACOSX_DEPLOYMENT_TARGET') has always resulted in a string value. There's bound to be someone relying on this.

As I wrote in comment on the PR a nicer fix is to teach sysconfig (and distutils.sysconfig) to not convert the value of this key to an integer.

"It has always resulted in a string value": only MACOSX_DEPLOYMENT_TARGET always took the form of a non-integer. The code for sysconfig.get_config_var() has a pretty clear intent: it will try to cast its return value to an int.

I don't have a strong opinion whether this is fixed this way or another. I would think, however, that there are surely places in existing code that expect and rely on sysconfig.get_config_var() returning an int for integer-type values.

The value *for this key* is a dot separated sequence of integer labels in a string. The limit of that is a single integer label that could be converted to a Python int.

With the current patch get_config_var('MACOSX_DEPLOYMENT_TARGET') mostly returns a string, but sometimes an integer. And the last part is new behaviour, which means existing consumers of this won't be prepared to handle a non-string value (other than None).

I'll probably create an alternative PR this weekend.

sysconfig converts config values that look like integers to python ints because that's often useful, but in this case it isn't. This part of the code base is old and has not very well defined semantics.

I agree with Ronald that it would be safest if the sysconfigs (all of them) always returned deployment target as a string and I would be fine with a PR that did that. That doesn't mean that the already applied PR needs to be changed, though, right? Just extra insurance.

I'd prefer to (effectively) revert the PR and treat MACOSX_DEPLOYMENT_TARGET=11 the same as MACOSX_DEPLOYMENT_TARGET=11.0.

The same issue has crept up on the wheel and packaging repos (in the context of changing the platform tag in wheel names). As I wrote there: the ship has sailed w.r.t. those tags, there are already "macosx_11_0_univeral2" wheels on PyPI.

It seems the approach that "The code for sysconfig.get_config_var() has a pretty clear intent: it will try to cast its return value to an int." has been accepted as true even though other parts of the code assumed the returned value was a string, and the CPython code was updated appropriately. This is causing problems for downstream libraries like NumPy, see numpy/numpy#17906. If CPython does not wish to reconsider this decision, it should at least be documented clearly via a type hint and/or in the documentation for sysconfig.get_config_var https://docs.python.org/3/library/sysconfig.html#sysconfig.get_config_var

I believe this is causing issues when trying to install pyjq (doloopwhile/pyjq#54).

@ronaldoussoren are you still planning on creating a patch for this? Or should we plan on fixing it downstream?

I will create a PR later today, I've already committed the work in a branch of my clone of cpython.

see PR 24341.

New changeset 49926cf by Ronald Oussoren in branch 'master':
bpo-42504: Ensure that get_config_var('MACOSX_DEPLOYMENT_TARGET') is a string (GH-24341)
49926cf

New changeset d6675fe by Miss Islington (bot) in branch '3.9':
bpo-42504: Ensure that get_config_var('MACOSX_DEPLOYMENT_TARGET') is a string (GH-24341) (GH-24410)
d6675fe

New changeset b29d0a5 by Ned Deily in branch '3.8':
[3.8] bpo-41100: Support macOS 11 Big Sur and Apple Silicon Macs (bpo-25806)
b29d0a5