[Edu-sig] teaching Python

[John Posner]

Bill Punch<punch at cse.msu.edu>  said:

<snip>

> Default mutables are very confusing indeed. The recommended procedure is
> to assign the default to be None, then check for that value in the
> function and do the assignment there, such as:
>
> def myFun(myList=None):
>      if myList == None:
>          myList = []
>       ...
>    

+1. This issue comes up on python-list about once a month.

> So is the example of replicating a mutable as Mark showed.  To round out
> the top three, which are the ones that get even my best students, look
> at the below:
>
> myVar = 27
>
> def myFun ():
>      print myVar
>      myVar += 1
>      return myVar
>
> print myFun
>
> What happens when you run this?
>    

Oops, you meant *print myFun()* there. On the bright side, an error 
occurs, warning you of your errant ways:

   UnboundLocalError: local variable 'myVar' referenced before assignment


Perhaps more gotcha-esque is this similar situation, in which no error 
occurs:

#----------------------------------
class MyClass:
     var = 101    # class attribute

     def try_to_modify_class_attribute(self):
         self.var += 13

     def show(self):
         print self.var

obj1 = MyClass()
obj2 = MyClass()

obj1.try_to_modify_class_attribute()
obj1.show()  # output: 114
obj2.show()  # output: 101
#----------------------------------

-John