PythonUser$Dir must be set or Python will not start.

[gerph]

If PythonUser$Dir is not set, the python system will not start, and reports an error:

...
  File "$.python3.!Python3.python38.lib.riscospath", line 575, in canonicalise
    swi.swi('OS_FSControl','isbiii',37,path,buffer,0,0,256*4)
swi.error: Invalid empty filename

Invalid empty filename is error &CC, equivalent to Filing system 'Foo' must be given a filename.

From debugging the FS calls, the failing call is:

 8464af8: SWI     XOS_FSControl
          => r0  = &00000025         37  constant 37
             r1  = &003a4710    3819280  pointer to string path_name
                   "<PythonUser$Dir>"
             r2  = &003e5000    4083712  pointer to buffer
                   [&00000000, &00000000, &00000000, &00000000]
             r3  = &00000000          0  pointer to string var
             r4  = &00000000          0  pointer to string path
             r5  = &00000400       1024  size
          <= Error returned: (&000000cc) Invalid empty filename
             r5  = &00000400       1024  spare