How to prevent accidentally calling pip when I am not in a virtualenv?

I wrote the following script called pipand added it to my ~/bin (which is before pip in my $PATH):

# This script makes sure I don't accidentally install pip without virtualenv
# This script requires $PIP to be set to the absolute path of pip to execute pip
# if $PIP is not set, it will write a message
if [ -z "$PIP" ]; then
   echo "you are not in a virtual env"
   echo "use virtual env or"
   # propose the second item in $PATH
   echo "  export PIP="`type -ap pip|sed -n 2p`
   echo "to cleanup use"
   echo "  unset PIP"
else
    # execute pip
    exec $PIP "$@"
fi

Is there a better way?

python virtualenv pip
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