python-mastery/Exercises/ex2_3.md

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Exercise 2.3

Objectives:

• Iterate like a pro

Files Modified: None.

Iteration is an essential Python skill. In this exercise, we look at a number of common iteration idioms.

Start the exercise by grabbing some rows of data from a CSV file.

>>> import csv
>>> f = open('Data/portfolio.csv')
>>> f_csv = csv.reader(f)
>>> headers = next(f_csv)
>>> headers
['name', 'shares', 'price']
>>> rows = list(f_csv)
>>> from pprint import pprint
>>> pprint(rows)
[['AA', '100', '32.20'],
 ['IBM', '50', '91.10'],
 ['CAT', '150', '83.44'],
 ['MSFT', '200', '51.23'],
 ['GE', '95', '40.37'],
 ['MSFT', '50', '65.10'],
 ['IBM', '100', '70.44']]
>>>

(a) Basic Iteration and Unpacking

The for statement iterates over any sequence of data. For example:

>>> for row in rows:
        print(row)

['AA', '100', '32.20']
['IBM', '50', '91.10']
['CAT', '150', '83.44']
['MSFT', '200', '51.23']
['GE', '95', '40.37']
['MSFT', '50', '65.10']
['IBM', '100', '70.44']
>>>

Unpack the values into separate variables if you need to:

>>> for name, shares, price in rows:
        print(name, shares, price)

AA 100 32.20
IBM 50 91.10
CAT 150 83.44
MSFT 200 51.23
GE 95 40.37
MSFT 50 65.10
IBM 100 70.44
>>>

It’s somewhat common to use _ or __ as a throw-away variable if you don’t care about one or more of the values. For example:

>>> for name, _, price in rows:
        print(name, price)

AA 32.20
IBM 91.10
CAT 83.44
MSFT 51.23
GE 40.37
MSFT 65.10
IBM 70.44
>>>

If you don’t know how many values are being unpacked, you can use * as a wildcard. Try this experiment in grouping the data by name:

>>> from collections import defaultdict
>>> byname = defaultdict(list)
>>> for name, *data in rows:
        byname[name].append(data)

>>> byname['IBM']
[['50', '91.10'], ['100', '70.44']]
>>> byname['CAT']
[['150', '83.44']]
>>> for shares, price in byname['IBM']:
        print(shares, price)

50 91.10
100 70.44
>>>

(b) Counting with enumerate()

enumerate() is a useful function if you ever need to keep a counter or index while iterating. For example, suppose you wanted an extra row number:

>>> for rowno, row in enumerate(rows):
        print(rowno, row)

0 ['AA', '100', '32.20']
1 ['IBM', '50', '91.10']
2 ['CAT', '150', '83.44']
3 ['MSFT', '200', '51.23']
4 ['GE', '95', '40.37']
5 ['MSFT', '50', '65.10']
6 ['IBM', '100', '70.44']
>>>

You can combine this with unpacking if you’re careful about how you structure it:

>>> for rowno, (name, shares, price) in enumerate(rows):
        print(rowno, name, shares, price)

0 AA 100 32.20
1 IBM 50 91.10
2 CAT 150 83.44
3 MSFT 200 51.23
4 GE 95 40.37
5 MSFT 50 65.10
6 IBM 100 70.44
>>> 

(c) Using the zip() function

The zip() function is most commonly used to pair data. For example, recall that you created a headers variable:

>>> headers
['name', 'shares', 'price']
>>>

This might be useful to combine with the other row data:

>>> row = rows[0]
>>> row
['AA', '100', '32.20']
>>> for col, val in zip(headers, row):
        print(col, val)

name AA
shares 100
price 32.20
>>>

Or maybe you can use it to make a dictionary:

>>> dict(zip(headers, row))
{'name': 'AA', 'shares': '100', 'price': '32.20'}
>>>

Or maybe a sequence of dictionaries:

>>> for row in rows:
        record = dict(zip(headers, row))
        print(record)

{'name': 'AA', 'shares': '100', 'price': '32.20'}
{'name': 'IBM', 'shares': '50', 'price': '91.10'}
{'name': 'CAT', 'shares': '150', 'price': '83.44'}
{'name': 'MSFT', 'shares': '200', 'price': '51.23'}
{'name': 'GE', 'shares': '95', 'price': '40.37'}
{'name': 'MSFT', 'shares': '50', 'price': '65.10'}
{'name': 'IBM', 'shares': '100', 'price': '70.44'}
>>>

(d) Generator Expressions

A generator expression is almost exactly the same as a list comprehension except that it does not create a list. Instead, it creates an object that produces the results incrementally–typically for consumption by iteration. Try a simple example:

>>> nums = [1,2,3,4,5]
>>> squares = (x*x for x in nums)
>>> squares
<generator object <genexpr> at 0x37caa8>
>>> for n in squares:
        print(n)

1
4
9
16
25
>>>

You will notice that a generator expression can only be used once. Watch what happens if you do the for-loop again:

>>> for n in squares:
        print(n)

>>>

You can manually get the results one-at-a-time if you use the next() function. Try this:

>>> squares = (x*x for x in nums)
>>> next(squares)
1
>>> next(squares)
4
>>> next(squares)
9
>>>

Keeping typing next() to see what happens when there is no more data.

If the task you are performing is more complicated, you can still take advantage of generators by writing a generator function and using the yield statement instead. For example:

>>> def squares(nums):
        for x in nums:
            yield x*x

>>> for n in squares(nums):
        print(n)

1
4
9
16
25
>>>

We’ll return to generator functions a little later in the course–for now, just view such functions as having the interesting property of feeding values to the for-statement.

(e) Generator Expressions and Reduction Functions

Generator expressions are especially useful for feeding data into functions such as sum(), min(), max(), any(), etc. Try some examples using the portfolio data from earlier. Carefully observe that these examples are missing some extra square brackets ([]) that appeared when using list comprehensions.

>>> from readport import read_portfolio
>>> portfolio = read_portfolio('Data/portfolio.csv')
>>> sum(s['shares']*s['price'] for s in portfolio)
44671.15
>>> min(s['shares'] for s in portfolio)
50
>>> any(s['name'] == 'IBM' for s in portfolio)
True
>>> all(s['name'] == 'IBM' for s in portfolio)
False
>>> sum(s['shares'] for s in portfolio if s['name'] == 'IBM')
150
>>>

Here is a subtle use of a generator expression in making comma separated values:

>>> s = ('GOOG',100,490.10)
>>> ','.join(s)
... observe that it fails ...
>>> ','.join(str(x) for x in s)    # This works
'GOOG,100,490.1'
>>>

The syntax in the above examples takes some getting used to, but the critical point is that none of the operations ever create a fully populated list of results. This gives you a big memory savings. However, you do need to make sure you don’t go overboard with the syntax.

(f) Saving a lot of memory

In Exercise 2.1 you wrote a function read_rides_as_dicts() that read the CTA bus data into a list of dictionaries. Using it requires a lot of memory. For example, let’s find the day on which the route 22 bus had the greatest ridership:

>>> import tracemalloc
>>> tracemalloc.start()
>>> import readrides
>>> rows = readrides.read_rides_as_dicts('Data/ctabus.csv')
>>> rt22 = [row for row in rows if row['route'] == '22']
>>> max(rt22, key=lambda row: row['rides'])
{'date': '06/11/2008', 'route': '22', 'daytype': 'W', 'rides': 26896}
>>> tracemalloc.get_traced_memory()
... look at result. Should be around 220MB
>>>

Now, let’s try an example involving generators. Restart Python and try this:

>>> # RESTART
>>> import tracemalloc
>>> tracemalloc.start()
>>> import csv
>>> f = open('Data/ctabus.csv')
>>> f_csv = csv.reader(f)
>>> headers = next(f_csv)
>>> rows = (dict(zip(headers,row)) for row in f_csv)
>>> rt22 = (row for row in rows if row['route'] == '22')
>>> max(rt22, key=lambda row: int(row['rides']))
{'date': '06/11/2008', 'route': '22', 'daytype': 'W', 'rides': 26896}
>>> tracemalloc.get_traced_memory()
... look at result. Should be a LOT smaller than before
>>>

Keep in mind that you just processed the entire dataset as if it was stored as a sequence of dictionaries. Yet, nowhere did you actually create and store a list of dictionaries. Not all problems can be structured in this way, but if you can work with data in an iterative manner, generator expressions can save a huge amount of memory.

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