25 May 2014

Python and cmd.exe on Windows - a world of pain.

As I mentioned a few days ago on Hacker News in a Ruby thread, CPython support for Windows is, overall, extraordinarily good for a runtime with clear Unix roots. This said, occasionally you'll eventually hit a wall and find yourself cursing Guido & Bill under your breath. Yesterday was one of those times for me.

I'm currently working on a project using Python 3.3.5 on Windows 2008 r2, building a program that will talk to Weblogic 10.3 via the Jython-powered WLST interface (it actually does more than that: it leverages Jython to also instantiate several complex Java classes, launch VBS scripts and so on and so forth). In order to correctly set up WLST/Jython, I have to launch a batch file which in turns calls several other batch files in order to set up all sorts of environment variables. These are all pure-DOS batch files doing very little except creating or reading environment variables, but they're nested two or three levels deep from the entry-point batch.

For some reason, when I launched this batch with Popen(), variables were not set correctly. In fact, it looked like "call" statements in the batch were just silently ignored. I tried using shell=True, and it made no difference whatsoever. I put it down to some weird cmd.exe behaviour; tried to switch extension from .bat to .cmd and things started to move a bit more (so much for all those posts saying there is no difference between the two) but still some stuff wouldn't work, so I eventually settled for reimplementing the whole batch chain in Python (which is terrible and will likely bite me a year down the line as the version of Weblogic changes, but beggars can't be choosers).

The most frustrating thing, however, was that opening an interactive pipe to test and do exploratory programming was just too difficult. There are a lot of examples out there talking about .send(), .communicate() and stdin=subprocess.PIPE, but nobody seems to mention what I experienced: as soon as you call communicate() on a cmd.exe launched with Popen(), all pipes are closed and there is no obvious way to reopen them. I don't think this is due to cmd.exe, because the last output I got was always ">More ?"; I think this is just CPython being too eager to clean up.

Luckily, I found a solution in WinPexpect, a fork of Pexpect that actually deals with Windows weirdness. Processes launched with winpexpect.winspawn() actually keep their stdin pipes up long enough for me to figure out enough stuff to fully re-implement the batch chain.

The result is a Python script three times as long as the original batch and likely to break the first time Oracle changes a line here or there. It will do for now, but the experience left a sour taste in my mouth, so to speak; cmd.exe is a crappy shell, but it shouldn't be that hard to open a long-running prompt-like process piping stuff to it. If I'm missing an obviously-better solution, please let me know and I'll happily blog about it, because clearly Google and DuckDuckGo need to know about it.