In the function main I have

```
def main():
    #...
    def button_callback(button,a, byMouse,label):
            #...
            date = date - oneday
            while(date.isoweekday()>5): date = date -oneday
            #...
    oneday = datetime.timedelta(1)
    date = datetime.date.today()

```

python complains: local variable 'date' referenced before assignment, which is expected. In other parts of main() I pay attention not to assign but to modify, thus I have for example 

```
def main():
    # other part of main()
    def clear_callback( button,byMouse,aaa):
        a_cats.clear()

    a_cats = set(["GT","GR"])

```

which python is happy about (it woudn't be if I set e.g. a\_cats = a\_cats.clear() ).

Is there a way to modify a datetime object without explicitly using "=", so that I can avoid using global variables ?