Bash trap unset from function - linux

Question

Namely, http://fvue.nl/wiki/Bash:_Error_handling#Set_ERR_trap_to_exit
Why is it necessary to set -o errtrace to make trap set/unset from a function call work?
#!/usr/bin/env bash
function trapit {
echo 'trapped in a box'
}
function setTrap {
trap 'trapit' ERR
}
function unsetTrap {
trap - ERR
}
function foo_init {
fooOldErrtrace=$(set +o | grep errtrace)
set -o errtrace
trap 'echo trapped' ERR # Set ERR trap
}
function foo_deinit {
trap - ERR # Reset ERR trap
eval $fooOldErrtrace # Restore `errtrace' setting
unset fooOldErrtrace # Delete global variable
}
# foo_init
setTrap
echo 'set'
false
echo 'unset'
#foo_deinit
unsetTrap
false

Answer 1

According to man bash(5) functions not inherits ERR trap without the errtrace flag turned on. I dont know why ERR trap cant be inherited by default, but... it is so for now :)
You can test this behaviour with my sample code:
#!/usr/bin/env bash
trapit () {
echo 'some error trapped'
}
doerr1 () {
echo 'I am the first err generator and i return error status to the callee'
return 1
}
doerr2 () {
echo 'I am the second err generator and i produce an error inside my code'
fgrep a /etc/motttd
return 0
}
[[ $1 ]] && set -o errtrace
trap trapit ERR
doerr1
doerr2
echo 'We will produce an exception in the main program...'
cat /etc/ftab | fgrep a
echo 'OK, thats done, you see it :)'
If you pass any parameter to this script, errtrace flag will be turned on and you will see that exception was "catched" when doerr2 tried to do something awful.