Python subprocess stdout truncated by env variable $COLUMNS

I am calling a bash script in python (3.4.3) using subprocess:

import subprocess as sp
res = sp.check_output("myscript", shell=True)

and myscript contains a line:

ps -ef | egrep somecommand

It was not giving the same result as when myscript is directly called in a bash shell window. After much tinkering, I realized that when myscript is called in python, the stdout of "ps -ef" was truncated by the current $COLUMNS value of the shell window before being piped to "egrep". To me, this is crazy as simply by resizing the shell window, the command can give different results!

I managed to "solve" the problem by passing env argument to the subprocess call to specify a wide enough COLUMNS:

res = sp.check_output("myscript", shell=True, env={'COLUMNS':'100'})

However, this looks very dirty to me and I don't understand why the truncation only happens in python subprocess but not in a bash shell. Frankly I'm amazed that this behavior isn't documented in the official python doc unless it's in fact a bug -- I am using python 3.4.3. What is the proper way of avoiding this strange behavior?