Piping to head results in broken pipe in shell script called from python

I have a command I would to run to generate random string:

var=`< /dev/urandom tr -dc _A-Z-a-z-0-9 | head -c8`

When I run this command in interactive bash session I get absolutely no errors. But when I put this command into script and run it as a script I get Broken pipe error indicated by tr. I've read several related topics but still has no answer why script and interactive behavior is different and is there a way to control it with shell options or with something else?

Edit I:

In regards to comments given I found that indicating broken pipe errors can be controlled via:

 trap - SIGPIPE # to ignore errors

and

 trap "" SIGPIPE # to display errors

Edit II:

Well, I've provided incorrect information about reproduction conditions. Finally it seems that problem caused with the python wrapper that called the script with the os.system():

 python -c "import os; os.system('sh -c \"< /dev/urandom tr -dc _A-Z-a-z-0-9 | head -c8\"')"

Line given produces broken pipe errors independently of used OS.

Edit III:

This topic has been discussed here: https://mail.python.org/pipermail/python-dev/2005-September/056341.html

If one of the parent processes traps sigpipe, then the pipeline will inherit the ignore signal disposition, which will cause this problem you're experiencing.

This can be (safely) reproduced with:

( trap '' pipe; var=`< /dev/urandom tr -dc _A-Z-a-z-0-9 | head -c8 )

Normally, the head -c8 command will be done pretty soon at which point its stdin is closed. Since it's stdin is a pipe connected to the stdout of tr, it now no longer makes sense for tr to write to its stdout. Once it tries to, the system will kill it with SIGPIPE. Unless tr ignores this signal or has inherited the ignore (SIG_IGN) disposition for this signal from its parent. Then a write to tr's broken stdout will simply cause a regular error and set errno to EPIPE at which point tr will most likely stringify and outputs this error to its stderr and exit.