def weight_for_p():
    global weight_p
    isnt_num=True  #loop once
    while isnt_num:    #loop if weight_p isn't a number
        print "How much does your pork weigh(kg)?"
        try:
            isnt_num=False #don't loop again
            weight_p = float(raw_input(">"))
        except ValueError: #if there is a value error, do this
            isnt_num=True       #loop again
            print "you suck at inputting numbers" #print that you suck at inputing numbersA few questions:
What do you need this for? I'm not going to assume anything, but I hope you aren't just copy and pasting this into an assignment.
Why do all of your questions involve meat in some way? Am I the only one here who knows python?

if you already have one thread dedicated to you asking for python help (here), please don't open a duplicate. that being said, i'm glad people were able to help you out =)