How to debug python CLI that takes stdin?

Question

I'm trying to debug a Python CLI I wrote that can take its arguments from stdin. A simple test case would have the output of

echo "test" | python mytool.py 

be equivalent to the output of

python mytool.py test 

I'd like to debug some issues with this tool, so I tried to run this:

echo "test" | pdb mytool.py 

But I get this output, then pdb exits:

> /path/to/mytool.py(5)<module>() -> ''' (Pdb) *** NameError: name 'test' is not defined (Pdb) 

The same thing occurs when I add -m python to the shebang, and if I run pdb.set_trace() inside the script.

What's going on here?

Answer 1

Another option is to create you own Pdb object, and set there the stdin and stdout. My proof of concept involves 2 terminals, but for sure some work can be merged some kind of very unsecure network server.

1. Create two fifos:

   mkfifo fifo_stdin mkfifo fifo_stdout 

2. In one terminal, open stdout on background, and write to stdin:

   cat fifo_stdout & cat > fifo_stdin 

3. In your python code/console create the pdb object, and use it:

   import pdb mypdb=pdb.Pdb(stdin=open('fifo_stdin','r'), stdout=open('fifo_stdout','w')) ... mypdb.set_trace() ... 

4. Profit!

You should be able to use pdb on the first console.

The only drawback is having to use your custom pdb, but some monkey patching at init (PYTHONSTARTUP or similar) can help:

import pdb mypdb=pdb.Pdb(stdin=open('fifo_stdin','r'), stdout=open('fifo_stdout','w')) pdb.set_trace=mypdb.set_trace