[Python] Using fork & exec in Python

I wrote a little test program to play around with fork() and exec() in Python, and I got some puzzling results. The program just forks a new process to run whatever is given as the command-line parameters in argv. Here is my code:

import sys
import os

print("Start: " + str(os.getpid()))
sys.stdout.flush()

if len(sys.argv) > 1:
pid = os.fork()
if pid == 0:
os.execvp(sys.argv[1], sys.argv[1:])
elif pid > 0:
print("Fork: " + str(pid))
sys.stdout.flush()
x = os.wait()
if x[0] != pid: # NEEDED
os.waitpid(pid,0) # NEEDED
print("End: " + str(x[0]))
else:
print("Fork failed", file=sys.stderr)
sys.exit(1)
else:
print("End")

sys.stdout.flush()


I run this with the following command:

python test.py python test.py ps


In other words, fork a process which runs another copy of the test program which forks yet another process which runs ps. The output I get looks like this:

Start: 999773
Fork: 999777
Start: 999777
Fork: 999781
PID TTY TIME CMD
999773 ? 00:00:00 python3
999777 ? 00:00:00 python3
999781 ? 00:00:00 ps
End: 999781
End: 999774


So, process 999773 creates process 999777 which creates process 999781, which runs ps and shows processes 999773, 999777 amd 999781. Process 999781 terminates, so the parent process (999777) prints a message saying that 999781 terminated and then exits. Process 999773 then prints a message saying that process 999774 terminated. Where did 999774 come from?

Then if I remove the two lines with the comment NEEDED, so that it waits for any process to terminate before terminating itself, I get the following output:

Start: 1000822
Fork: 1000830
End: 1000824
Python path configuration:
PYTHONHOME = (not set)
PYTHONPATH = (not set)
program name = '/usr/bin/python3'
isolated = 0
environment = 1
user site = 1
import site = 1
sys._base_executable = '/usr/bin/python3'
sys.base_prefix = '/usr'
sys.base_exec_prefix = '/usr'
sys.executable = '/usr/bin/python3'
sys.prefix = '/usr'
sys.exec_prefix = '/usr'
sys.path = [
'/usr/lib/python38.zip',
'/usr/lib/python3.8',
'/usr/lib/python3.8/lib-dynload',
]
Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
Python runtime state: core initialized
ModuleNotFoundError: No module named 'encodings'

Current thread 0x00007f27b84f7740 (most recent call first):
<no Python frame>


Can anyone explain what on earth is going on here?

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