I'm not sure that it is expected since Popen.send_signal does contain the following check:

```
def send_signal(self, sig):
    """Send a signal to the process."""
    # Skip signalling a process that we know has already died.
    if self.returncode is None:
        os.kill(self.pid, sig)
```

Additionally, the following program does not raise a ProcessLookupError despite the program already having exited:

```
import subprocess
import time

proc = subprocess.Popen(["sh", "-c", "exit 0"])

time.sleep(5)

proc.terminate()
```