Re: [Tutor] Criticism / Suggestions

[Kent Johnson]

Bill Kranec wrote:
Hi Kent,
- The above code depends on roundlist which is not even defined 
anywhere in the module (just shown in the comment). This suggests 
again that the init code from the comment should be part of 
Tournament, and roundlist should be an attribute of Tournament. For a 
class to depend on some external variable is bad design and will break 
if the client code is in a different module from the class code 
(roundlist will be in a different namespace than Tournament).

I have always been uneasy about this, but I wanted to be able to define 
multiple tournament objects off of the same roundlist, to avoid 
generating the list every time a new object is created.  I think what I 
really want to do is have a separate Round class, from which Tournament 
inherits the list of rounds.  I have started to implement something like 
this in my most recent version, but haven't finished it yet.  ( I need 
to understand class inheritance better. )
Composition is a better solution than inheritance. You can have a separate Round class and pass an 
instance of Round to the constructor for Tournament. Tournament can save the instance and refer to 
it as needed.


- Tournament.next() doesn't return a value, it prints it directly. It 
should return self.key or str( self.key )

Does the next() call in an iterator object need to return a value, and 
if so, why? ( My reasoning was that next() only needs to increment the 
iterator. )
To be an iterator it needs to return a value. This is the value that will be assigned to the loop 
variable.

For example, this class looks like an iterator but it doesn't return any value 
from next():
class not_an_iterator:
  def __init__(self):
    self.i = 0
  def __iter__(self):
    return self
  def next(self):
    if self.i < 5:
      print 'In next - i =', self.i
      self.i += 1
    else:
      raise StopIteration
If I use the class in a loop, the loop variable doesn't receive any value:
for i in not_an_iterator():
  print 'In loop - i =', i
prints:
In next - i = 0
In loop - i = None
In next - i = 1
In loop - i = None
In next - i = 2
In loop - i = None
In next - i = 3
In loop - i = None
In next - i = 4
In loop - i = None
For the loop variable to get the values from next(), I have to return it:
class is_an_iterator:
  def __init__(self):
    self.i = 0
  def __iter__(self):
    return self
  def next(self):
    if self.i < 5:
      print 'In next - i =', self.i
      old_i = self.i
      self.i += 1
      return old_i
    else:
      raise StopIteration
for i in is_an_iterator():
  print 'In loop - i =', i
prints:
In next - i = 0
In loop - i = 0
In next - i = 1
In loop - i = 1
In next - i = 2
In loop - i = 2
In next - i = 3
In loop - i = 3
In next - i = 4
In loop - i = 4
Incidentally, generator functions make it much easier to do this because they maintain state between 
calls automatically. I could write is_an_iterator as a generator like this:

def is_a_generator():
  i = 0
  while i < 5:
    yield i
    i += 1
for i in is_a_generator():
  print 'In loop - i =', i
Kent
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