Bash coproc with python

Question

I am playing with the coproc of Bash and I am failing to understand something.

I started with the following example:

Example#1

$ coproc MY_BASH { bash; }
[1] 95244
$ echo 'ls -l; echo EOD' >&"${MY_BASH[1]}"
$ is_done=false; while [[ "$is_done" != "true" ]]; do
>   read var <&"${MY_BASH[0]}"
>   if [[ $var == "EOD" ]]; then
>      is_done="true"
>   else
>      echo $var
>   fi
> done
total 0
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file10.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file1.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file2.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file3.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file4.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file5.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file6.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file7.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file8.txt
-rw-rw-r-- 1 username username 0 Nov 11 13:00 file9.txt
$ 

Here we can see that the current Bash shell is able to create a coprocess and interact with it.

Example 2

In this case, I switch from a bash coprocess to a python coprocess:

$ coproc MY_BASH { python; }
[1] 95244
$ echo 'print("hello"); print("EOD");' >&"${MY_BASH[1]}"
$ is_done=false; while [[ "$is_done" != "true" ]]; do
>   read var <&"${MY_BASH[0]}"
>   if [[ $var == "EOD" ]]; then
>      is_done="true"
>   else
>      echo $var
>   fi
> done

In this scenario the program hangs and gets blocked.

I have the impression that I am forgetting to send something in the input.

Any help to better understand what is going on will be appreciated.

Answer 1

There are a couple of problems here…

First, you need to use the -i option of Python so that it enters interactive mode and reads commands from stdin for execution. So:

coproc MY_BASH { python -i ; }

Next, Python does not delimit commands with semicolons, so you need to use newlines to separate your commands:

echo $'print("Hello, World!")\nprint("EOD")' >&"${MY_BASH[1]}"