changeWords(“I can't eat") -> “I can not eat"
changeWords(“I don't like swimming.”) -> “I do not like swimming.”
changeWords(“I shouldn't do that.”) -> “I should not do that.”

def stringChange(a):
a = ""
for line in stringChange("a"):
    line = text.replace("a","can't","can not")
    if not line: break
    return line

>>> def changeWords(s):
        for old, new in (
                ("can't", "can not"),
                ("shouldn't", "should not"),
                ("don't", "do not"),
            ):
            s = s.replace(old, new)
        return s

>>> changeWords("I don't know how to do this")
'I do not know how to do this'