[Tutor] default parameters in classes

[Terry Carroll]

On Mon, 12 Mar 2007, J Greg wrote:

> Perhaps I don't understand. In the example below, the
> first instance correctly creates a fresh dictionary.
> But the second instance seems to inherit the now dirty
> dictionary from the first instance. 
> 
> What am I missing?

This is a very common Python "gotcha," passing mutable objects as default 
arguments to a function.  If you do a google search on "python gotchas" it 
will show up in pretty much every list of gotchas.

The idiom to get around this is to instead use a default value of None, 
and then, if you actually see None in the function, set it equal to the 
real default argument you want, i.e.:

class newobj:
    def __init__(self, labl, d = None ):   # changed line
        if d is None:                      # added line
            d = {}                         # added line
        print labl, d, '\n'
        # print the dictionary as passed, or freshly created
        if d.has_key('obj1'):
            d['obj2'] = 'very dirty'
        d['obj1'] = 'dirty'

Now your code:

obj1 = newobj('obj1:')      # ok
obj2 = newobj('obj2:')      # gets the dirty dictionary from obj1

dd = dict(obj3=True)
obj3 = newobj('obj3:', dd)  # ok
obj4 = newobj('obj4:')      # get a dirty dictionary from obj2

Gets:

obj1: {}

obj2: {}

obj3: {'obj3': True}

obj4: {}


Is that more what you were expecting?