The getenvironment() function in "Modules/_winapi.c" needs to sort variables in the environment block and remove duplicates case insensitively¹.

The sort order used to matter with SetEnvironmentVairableW(). As soon as it reached a name in the environment block that compared greater than the target name, it would insert a new variable. Nowadays, SetEnvironmentVairableW() searches the entire environment block before inserting a new value. Regardless, at the very least, getenvironment() is not well-behaved and is not setting the environment in the documented sort order that users, and possibly other programs, expect.

Case-insensitive sorting in Windows uses upper case. The variable names in the mapping can be added to a list and sorted with a key function that's based on LCMapStringEx(), with the flag LCMAP_UPPERCASE. Loop over the sorted list to create the environment block. Remove duplicates by skipping a name that compares equal to the previously stored name according to CompareStringOrdinal().

I have a question, how to determine which name should be stored if they are duplicated with case insensitive?

which name should be stored if they are duplicated with case insensitive?

Ideally os.environ would preserve the original case of the process environment, and os.environ.copy() would return a copy that's also case insensitive. That would prevent most problems with duplicates keys. See msg387676 in bpo-28824, and msg414319 in bpo-15373.

In msg390038 I suggested keeping the first key that's encountered. However, dicts preserve insertion order nowadays, so one could assume that the last one is the one that the caller wants to keep.

This issue can be reproduced on msys2 (also cygwin, I think) easily:

msys2 shell $ ABC=1 abc=2 python
Python 3.11.0b1 (main, May  7 2022, 22:58:47) [MSC v.1931 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> print(os.environ["ABC"], os.environ["abc"])
2 2
>>> import subprocess
>>> subprocess.run(["bash", "-c", "echo $ABC $abc"])
1 2
CompletedProcess(args=['bash', '-c', 'echo $ABC $abc'], returncode=0)
>>> 
>>> os.environ["abc"] = "3"
>>> print(os.environ["ABC"], os.environ["abc"])
3 3
>>> subprocess.run(["bash", "-c", "echo $ABC $abc"])
3 2
CompletedProcess(args=['bash', '-c', 'echo $ABC $abc'], returncode=0)

msys2 shell $ ABC=1 abc=2 cmd
cmd > echo %ABC% %abc%
1 1
cmd > bash -c "echo $ABC $abc"
1 2
cmd > set abc=3
cmd > echo %ABC% %abc%
3 3
cmd > bash -c "echo $ABC $abc"
3 2

msys2 shell $ ABC=1 abc=2 powershell
PS > echo $Env:ABC $Env:abc
1
1
PS > bash -c "echo $ABC $abc"
1 2
PS > $Env:abc = 3
PS > echo $Env:ABC $Env:abc
3
3
PS > bash -c 'echo $ABC $abc'
3 2

Python behaves differently from both cmd.exe and powershell.

Case-sensitive environment variables are in nt.environ, but they don't affect the real environment:

msys2 shell $ ABC=1 abc=2 python
Python 3.11.0b1 (main, May  7 2022, 22:58:47) [MSC v.1931 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import nt, os
>>> print(nt.environ["ABC"], nt.environ["abc"])
1 2
>>> print(os.environ["ABC"], os.environ["abc"])
2 2

This is because environment vairiables are not sorted correctly in os._convertenviron:

https://github.com/python/cpython/blob/v3.11.0b1/Lib/os.py#L750

Related to GH-91018, GH-73010.

This PR is opened for a while, could you please help me review it by any chance? @eryksun @zooba