Program chaining on Windows

[Barry]

> On 23 Aug 2020, at 20:58, Rob Cliffe via Python-list <python-list at python.org> wrote:
> 
> On WIndows 10, running Python programs in a DOS box, I would like one Python program to chain to another.  I.e. the first program to be replaced by the second (*not* waiting for the second to finish, as with e.g. os.system).  This doesn't seem a lot to ask, but so far I have been unable to so this.  I am using Python 3.8.3.  Some attempts so far (may be nonsensical):
> 
> ATTEMPT #1
> ----------------
> # File X1.py
> import os
> print("This is X1")
> os.execl('C:\\Python38\\python.exe', 'X2.py')

I think you need to have this (untested):

os.execl('C:\\Python38\\python.exe', 'C:\\Python38\\python.exe', 'X2.py')

You must put the path as the first arg, it is not done for you.

You version ran python without an arg so it thinks you want a interactive session.

Barry


> 
> # File X2.py
> print("This is X2")
> 
> When I type "X1.py", it prints "This is X1", then starts the Python interactive interpreter.  Furthermore:
>     TLDR: Weird behaviour
>     Long version:  If I attempt to exit the interpreter with ctl-Z, this apparently succeeds (displaying the Windows command prompt), but in reality it is still in the interpreter, e.g. if I type "dir" it responds "<built-in function dir>" followed by the ">>>" interpreter prompt.  And this cycle (ctl-Z etc.) can be repeated ad nauseam.  If instead I try to exit from the interpreter with "exit()", the cursor moves to the next line and the interpreter waits for more input (but without displaying the prompt). If I try "exit()" again, the whole DOS box disappears.
> 
> ATTEMPT #2
> -----------------
> # File X1.py
> import os
> print("This is X1")
> os.execl("C:\\Python38\\python.exe X2.py", '')

Arg 1 is a path to a program, not a command line.
So”python.exe” or maybe “X2.py”.


> 
> This raises ValueError: execv() arg 2 first element cannot be empty
> 
> ATTEMPT #3
> ----------------
> 
> import os, sys
> print("This is X1")
> os.execl("%s X2.py" % sys.executable, "X2.py")
> 
> This raises FileNotFoundError: [Errno 2] No such file or directory
> 
> ATTEMPT #4
> ----------------
> # File X1.py
> import os, sys
> print("This is X1")
> os.execv(sys.executable, ['X2.py'])
> 
> This behaves the same as, or similarly to, Attempt #1.
> 
> ATTEMPT #5
> ----------------
> # File X1.py
> import os
> print("This is X1")
> os.popen('python X2.py')
> 
> # File X2.py as previously
> 
>     TLDR: Really weird behaviour!
>     Long version: If I type "X1.py", it displays "This is X1" followed by the DOS prompt.  If I type in a DOS command, it is executed, and the DOS prompt displayed.  However, if I type in another DOS command, nothing happens except that the cursor moves to the next line and waits for input (no prompt).  If I type in a further DOS command, it is executed.  If I type still another DOS command, I see
> 
> Exception ignored in: <io.TextIOWrapper name='<stdout>' mode='w' encoding='cp1252'>
> OSError: [Errno 22] Invalid argument
> 
> and the cursor moves to the next line (no prompt).  If I type in one more DOS command, it is executed, and we appear to be back to normal DOS operation.
> 
> ATTEMPT #6
> -----------------
> # File X1.py
> import subprocess, sys
> print("This is X1")
> subprocess.Popen('X2.py', executable=sys.executable)
> 
> This behaves the same as, or similarly to, Attempt #1.
> 
> ATTEMPT #7
> -----------------
> # File X1.py
> import subprocess, sys
> print("This is X1")
> subprocess.Popen('-c X2.py', executable=sys.executable)    # added -c
> 
> # File X2.py
> print("This is X2")
> 
> Some progress (maybe).  This prints "This is X1", then the DOS prompt followed by "This is X2", then the cursor moves to the next line and sits waiting for input (no prompt).  If I then type something in, this is interpreted as a DOS command, and finally the DOS prompt is displayed.  To find out more about what is happening:
> 
> ATTEMPT #8
> ----------------
> # File X1.py as above
> 
> # File X2.py
> print("This is X2")
> input("Press Enter to continue X2")
> input("Press Enter to quit X2")
> 
> If I type "X1.py", it displays:
> 
> This is X1
> C:\Python38>This is X2
> Press Enter to continue X2
> 
> Now:
>     TLDR: More weird behaviour, as if Windows and X2.py were taking turns to collect lines from the console.
>     Long version: Now if I type something in and press Enter, it is interpreted as a *DOS command".  Then the DOS prompt is displayed. Now if I (type something and) hit Enter, I see
> 
> Press Enter to quit X2
> 
> Now if I type something and hit Enter, it is interpreted as a DOS command, and the DOS prompt is displayed again.  Now if I type in a DOS command and press Enter, it is ignored but the cursor moves to the next line and waits for input (no prompt).  Now if I type another DOS command, it is executed.  Finally we appear to be done (the DOS prompt is displayed and we are back to normal DOS operation).
> 
> 
> Am I missing something?  Is there a way in Windows for one Python program to "chain" to another (or indeed to any executable) without waiting for the latter to finish?
> Thanks in advance
> Rob Cliffe
> 
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