This argument is forwarded on to `Popen.__init__`, like most of the other arguments. It can be `None`, just like the `executable` parameter for all the other `subprocess` functions:

```pycon
>>> import subprocess
>>> subprocess.check_call(["python", "-c", "1/1"], executable=None)
0
>>> subprocess.check_call(["python", "-c", "1/0"], executable=None)
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ZeroDivisionError: division by zero
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\AppData\Local\Programs\Python\Python310\lib\subprocess.py", line 369, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['python', '-c', '1/0']' returned non-zero exit status 1.
```