Introduction
This section looks at the underlying process of iteration.
This section looks at the underlying process of iteration.
Many different objects support iteration.
a = 'hello'
for c in a: ## Loop over characters in a
...
b = { 'name': 'Dave', 'password':'foo'}
for k in b: ## Loop over keys in dictionary
...
c = [1,2,3,4]
for i in c: ## Loop over items in a list/tuple
...
f = open('foo.txt')
for x in f: ## Loop over lines in a file
...
Consider the for
-statement.
for x in obj:
## statements
What happens under the hood?
_iter = obj.__iter__() ## Get iterator object
while True:
try:
x = _iter.__next__() ## Get next item
## statements ...
except StopIteration: ## No more items
break
All the objects that work with the for-loop
implement this low-level iteration protocol.
Example: Manual iteration over a list.
>>> x = [1,2,3]
>>> it = x.__iter__()
>>> it
<listiterator object at 0x590b0>
>>> it.__next__()
1
>>> it.__next__()
2
>>> it.__next__()
3
>>> it.__next__()
Traceback (most recent call last):
File "<stdin>", line 1, in ? StopIteration
>>>
Knowing about iteration is useful if you want to add it to your own objects. For example, making a custom container.
class Portfolio:
def __init__(self):
self.holdings = []
def __iter__(self):
return self.holdings.__iter__()
...
port = Portfolio()
for s in port:
...
Create the following list:
a = [1,9,4,25,16]
Manually iterate over this list. Call __iter__()
to get an iterator and call the __next__()
method to obtain successive elements.
>>> i = a.__iter__()
>>> i
<listiterator object at 0x64c10>
>>> i.__next__()
1
>>> i.__next__()
9
>>> i.__next__()
4
>>> i.__next__()
25
>>> i.__next__()
16
>>> i.__next__()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
>>>
The next()
built-in function is a shortcut for calling the __next__()
method of an iterator. Try using it on a file:
>>> f = open('portfolio.csv')
>>> f.__iter__() ## Note: This returns the file itself
<_io.TextIOWrapper name='portfolio.csv' mode='r' encoding='UTF-8'>
>>> next(f)
'name,shares,price\n'
>>> next(f)
'"AA",100,32.20\n'
>>> next(f)
'"IBM",50,91.10\n'
>>>
Keep calling next(f)
until you reach the end of the file. Watch what happens.
On occasion, you might want to make one of your own objects support iteration--especially if your object wraps around an existing list or other iterable. In a new file portfolio.py
, define the following class:
## portfolio.py
class Portfolio:
def __init__(self, holdings):
self._holdings = holdings
@property
def total_cost(self):
return sum([s.cost for s in self._holdings])
def tabulate_shares(self):
from collections import Counter
total_shares = Counter()
for s in self._holdings:
total_shares[s.name] += s.shares
return total_shares
This class is meant to be a layer around a list, but with some extra methods such as the total_cost
property. Modify the read_portfolio()
function in report.py
so that it creates a Portfolio
instance like this:
## report.py
...
import fileparse
from stock import Stock
from portfolio import Portfolio
def read_portfolio(filename):
'''
Read a stock portfolio file into a list of dictionaries with keys
name, shares, and price.
'''
with open(filename) as file:
portdicts = fileparse.parse_csv(file,
select=['name','shares','price'],
types=[str,int,float])
portfolio = [ Stock(d['name'], d['shares'], d['price']) for d in portdicts ]
return Portfolio(portfolio)
...
Try running the report.py
program. You will find that it fails spectacularly due to the fact that Portfolio
instances aren't iterable.
>>> import report
>>> report.portfolio_report('portfolio.csv', 'prices.csv')
... crashes ...
Fix this by modifying the Portfolio
class to support iteration:
class Portfolio:
def __init__(self, holdings):
self._holdings = holdings
def __iter__(self):
return self._holdings.__iter__()
@property
def total_cost(self):
return sum([s.shares*s.price for s in self._holdings])
def tabulate_shares(self):
from collections import Counter
total_shares = Counter()
for s in self._holdings:
total_shares[s.name] += s.shares
return total_shares
After you've made this change, your report.py
program should work again. While you're at it, fix up your pcost.py
program to use the new Portfolio
object. Like this:
## pcost.py
import report
def portfolio_cost(filename):
'''
Computes the total cost (shares*price) of a portfolio file
'''
portfolio = report.read_portfolio(filename)
return portfolio.total_cost
...
Test it to make sure it works:
>>> import pcost
>>> pcost.portfolio_cost('portfolio.csv')
44671.15
>>>
If making a container class, you often want to do more than just iteration. Modify the Portfolio
class so that it has some other special methods like this:
class Portfolio:
def __init__(self, holdings):
self._holdings = holdings
def __iter__(self):
return self._holdings.__iter__()
def __len__(self):
return len(self._holdings)
def __getitem__(self, index):
return self._holdings[index]
def __contains__(self, name):
return any([s.name == name for s in self._holdings])
@property
def total_cost(self):
return sum([s.shares*s.price for s in self._holdings])
def tabulate_shares(self):
from collections import Counter
total_shares = Counter()
for s in self._holdings:
total_shares[s.name] += s.shares
return total_shares
Now, try some experiments using this new class:
>>> import report
>>> portfolio = report.read_portfolio('portfolio.csv')
>>> len(portfolio)
7
>>> portfolio[0]
Stock('AA', 100, 32.2)
>>> portfolio[1]
Stock('IBM', 50, 91.1)
>>> portfolio[0:3]
[Stock('AA', 100, 32.2), Stock('IBM', 50, 91.1), Stock('CAT', 150, 83.44)]
>>> 'IBM' in portfolio
True
>>> 'AAPL' in portfolio
False
>>>
One important observation about this--generally code is considered "Pythonic" if it speaks the common vocabulary of how other parts of Python normally work. For container objects, supporting iteration, indexing, containment, and other kinds of operators is an important part of this.
Congratulations! You have completed the Iteration Protocol lab. You can practice more labs in LabEx to improve your skills.