Why mininet python api works only once?

Question

I have the following script,

 mininet>py h2.sendCmd('python listen.py 10.0.0.2') & h3.sendCmd('python listen.py 10.0.0.3')

It works the first time, but I get a message in mininet shell as below,

 unsupported operand type(s) for &: 'NoneType' and 'NoneType'

And if I run the same script subsequently, it doesn't work anymore. Does anyone know why this behaviour occurs and how to get rid of it.

listen.py is a python script to listen on a socket and create a file to save the received data, it works perfectly when I run it on individual xTerm of hosts.

Answer 1

 mininet>py h2.sendCmd('python listen.py 10.0.0.2') & h3.sendCmd('python listen.py 10.0.0.3')

will cause the result (return values) of the two calls to be operated via bitwise and op

Try this

mininet> xterm h2 h3

And in the xterms, issue the commands