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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:
# statementsWhat 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
breakAll 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('Data/portfolio.csv')
>>> f.__iter__() # Note: This returns the file itself
<_io.TextIOWrapper name='Data/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_sharesThis 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('Data/portfolio.csv', 'Data/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_sharesAfter 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('Data/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_sharesNow, try some experiments using this new class:
>>> import report
>>> portfolio = report.read_portfolio('Data/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.
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