Python: The Iterator Pattern

1. Iterator Pattern
Damian Gordon

2. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

3. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

4. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

5. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

6. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

7. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

8. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

9. Iterator Pattern
• The iterator pattern is a design pattern in which an iterator is
used to traverse a container and access the container's
elements.

10. Iterator Pattern
• In a programming language without patterns, an
iterator would have a next() method and a done()
method, and the iterator loops across all the containers
using these methods.

11. Iterator Pattern
• In a programming language without patterns, an
iterator would have a next() method and a done()
method, and the iterator loops across all the containers
using these methods.
WHILE NOT(iterator.done())
DO item = iterator.next()
# do more stuff
ENDWHILE;

12. Iterator Pattern
• The iterator pattern in Python has the following features:
Name Description
_ _next_ _() The next method returns the next
element from the container.
StopIteration() The StopIteration is an exception
that is raised when the last element is
reached.
_ _iter_ _() The iter method makes the object
iterable, and returns an iterator.

13. Iterator Pattern
• The iterator pattern has two parts:
–An iterable class (to create the iterator object)
–An iterator class (to traverse a container)

14. def __init__(self, VALUE):
self.VALUE = VALUE
# END Init
def __iter__(self):
return ITERATOR(self.VALUE)
# END Iter
class ITERABLE:
# END ITERABLE.

15. def __init__(self, VALUE):
self.VALUE = VALUE
self.index = 0
# END Init
def __iter__(self):
return self
# END Iter
def __next__(self):
if CONDITION:
VALUE = SELF.VALUE
self.index = self.index + 1
return VALUE
else:
raise StopIteration()
# ENDIF;
# END Next
class ITERATOR:
# END ITERATOR.

16. Iterator Pattern
• Let’s look at a simple example that counts numbers,
there’ll be three parts:
– The Iterable part
– The Iteration part
– The program execution part

17. Iterator Pattern
class MyCountIterable:
def __init__(self, Value):
self.Value = Value
# END Init
def __iter__(self):
return MyCountIteration(self.Value)
# END Iter
# END MyCountIterable.

18. Iterator Pattern
class MyCountIteration:
def __init__(self, Value):
self.Index = 0
self.Value = Value
# END Init
def __iter__(self):
# Iterators are iterables too.
return self
# END Iter
def __next__(self):
if self.Index < self.Value:
# THEN
Index = self.Index
self.Index += 1
return Index
else:
raise StopIteration()
# ENDIF;
# END Next
# END MyCountIteration.

19. Iterator Pattern
FirstCount = MyCountIterable(5)
list(FirstCount)
FirstCountIter = iter(FirstCount)
while True:
# DO
try:
print(next(FirstCountIter))
except StopIteration:
break
# ENDWHILE

20. Iterator Pattern
FirstCount = MyCountIterable(5)
list(FirstCount)
FirstCountIter = iter(FirstCount)
while True:
# DO
try:
print(next(FirstCountIter))
except StopIteration:
break
# ENDWHILE
0
1
2
3
4

21. Iterator Pattern
• Hang on…
• Couldn’t we just do:
• YES!
for Counter in range(0,5):
print(Counter)
# ENDFOR;

22. etc.