Manipulate Various Built-in Python Objects

Introduction

Objectives:

• Manipulate various built-in Python objects

Part 1 : Numbers

Numerical calculations work about like you would expect in Python. For example:

>>> 3 + 4*5
23
>>> 23.45 / 1e-02
2345.0
>>>

Be aware that integer division is different in Python 2 and Python 3.

>>> 7 / 4      ## In python 2, this truncates to 1
1.75
>>> 7 // 4     ## Truncating division
1
>>>

If you want Python 3 behavior in Python 2, do this:

>>> from __future__ import division
>>> 7 / 4
1.75
>>> 7 // 4      ## Truncating division
1
>>>

Numbers have a small set of methods, many of which are actually quite recent and overlooked by even experienced Python programmers. Try some of them.

>>> x = 1172.5
>>> x.as_integer_ratio()
(2345, 2)
>>> x.is_integer()
False
>>> y = 12345
>>> y.numerator
12345
>>> y.denominator
1
>>> y.bit_length()
14
>>>

Part 2 : String Manipulation

Define a string containing a series of stock ticker symbols like this:

>>> symbols = 'AAPL IBM MSFT YHOO SCO'

Now, let's experiment with different string operations:

Extracting individual characters and substrings

Strings are arrays of characters. Try extracting a few characters:

>>> symbols[0]
'A'
>>> symbols[1]
'A'
>>> symbols[2]
'P'
>>> symbols[-1]        ## Last character
'O'
>>> symbols[-2]        ## 2nd from last character
'C'
>>>

Try taking a few slices:

>>> symbols[:4]
'AAPL'
>>> symbols[-3:]
'SCO'
>>> symbols[5:8]
'IBM'
>>>

Strings as read-only objects

Strings are read-only. Verify this by trying to change the first character of symbols to a lower-case 'a'.

>>> symbols[0] = 'a'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>>

String concatenation

Although string data is read-only, you can always reassign a variable to a newly created string.
Try the following statement which concatenates a new symbol "GOOG" to the end of symbols:

>>> symbols += ' GOOG'
>>> symbols
... look at the result ...

Now, try adding "HPQ" to the beginning of symbols like this:

>>> symbols = 'HPQ ' + symbols
>>> symbols
... look at the result ...

It should be noted in both of these examples, the original string symbols is NOT being modified "in place." Instead, a completely new string is created. The variable name symbols is just bound to the result. Afterwards, the old string is destroyed since it's not being used anymore.

Membership testing (substring testing)

Experiment with the in operator to check for substrings. At the interactive prompt, try these operations:

>>> 'IBM' in symbols
True
>>> 'AA' in symbols
True
>>> 'CAT' in symbols
False
>>>

Make sure you understand why the check for "AA" returned True.

String Methods

At the Python interactive prompt, try experimenting with some of the string methods.

>>> symbols.lower()
'hpq aapl ibm msft yhoo sco goog'
>>> symbols
'HPQ AAPL IBM MSFT YHOO SCO GOOG'

Remember, strings are always read-only. If you want to save the result of an operation, you need to place it in a variable:

>>> lowersyms = symbols.lower()
>>> lowersyms
'hpq aapl ibm msft yhoo sco goog'
>>>

Try some more operations:

>>> symbols.find('MSFT')
13
>>> symbols[13:17]
'MSFT'
>>> symbols = symbols.replace('SCO','')
>>> symbols
'HPQ AAPL IBM MSFT YHOO  GOOG'
>>>

Part 3 : List Manipulation

In the first part, you worked with strings containing stock symbols. For example:

>>> symbols = 'HPQ AAPL IBM MSFT YHOO  GOOG'
>>>

Define the above variable and split it into a list of names using the split() operation of strings:

>>> symlist = symbols.split()
>>> symlist
['HPQ', 'AAPL', 'IBM', 'MSFT', 'YHOO', 'GOOG' ]
>>>

Extracting and reassigning list elements

Lists work like arrays where you can look up and modify elements by numerical index. Try a few lookups:

>>> symlist[0]
'HPQ'
>>> symlist[1]
'AAPL'
>>> symlist[-1]
'GOOG'
>>> symlist[-2]
'YHOO'
>>>

Try reassigning one of the items:

>>> symlist[2] = 'AIG'
>>> symlist
['HPQ', 'AAPL', 'AIG', 'MSFT', 'YHOO', 'GOOG' ]
>>>

Looping over list items

The for loop works by looping over data in a sequence such as a list. Check this out by typing the following loop and watching what happens:

>>> for s in symlist:
        print('s =', s)

... look at the output ...

Membership tests

Use the in operator to check if 'AIG','AA', and 'CAT' are in the list of symbols.

>>> 'AIG' in symlist
True
>>> 'AA' in symlist
False
>>>

Appending, inserting, and deleting items

Use the append() method to add the symbol 'RHT' to end of symlist.

>>> symlist.append('RHT')
>>> symlist
['HPQ', 'AAPL', 'AIG', 'MSFT', 'YHOO', 'GOOG', 'RHT']
>>>

Use the insert() method to insert the symbol 'AA' as the second item in the list.

>>> symlist.insert(1,'AA')
>>> symlist
['HPQ', 'AA', 'AAPL', 'AIG', 'MSFT', 'YHOO', 'GOOG', 'RHT']
>>>

Use the remove() method to remove 'MSFT' from the list.

>>> symlist.remove('MSFT')
>>> symlist
['HPQ', 'AA', 'AAPL', 'AIG', 'YHOO', 'GOOG', 'RHT']

Try calling remove() again to see what happens if the item can't be found.

>>> symlist.remove('MSFT')
... watch what happens ...
>>>

Use the index() method to find the position of 'YHOO' in the list.

>>> symlist.index('YHOO')
4
>>> symlist[4]
'YHOO'
>>>

List sorting

Want to sort a list? Use the sort() method. Try it out:

>>> symlist.sort()
>>> symlist
['AA', 'AAPL', 'AIG', 'GOOG', 'HPQ', 'RHT', 'YHOO']
>>>

Want to sort in reverse? Try this:

>>> symlist.sort(reverse=True)
>>> symlist
['YHOO', 'RHT', 'HPQ', 'GOOG', 'AIG', 'AAPL', 'AA']
>>>

Note: Sorting a list modifies its contents "in-place." That is, the elements of the list are shuffled around, but no new list is created as a result.

Lists of anything

Lists can contain any kind of object, including other lists (e.g., nested lists). Try this out:

>>> nums = [101,102,103]
>>> items = [symlist, nums]
>>> items
[['YHOO', 'RHT', 'HPQ', 'GOOG', 'AIG', 'AAPL', 'AA'], [101, 102, 103]]

Pay close attention to the above output. items is a list with two elements. Each element is list.

Try some nested list lookups:

>>> items[0]
['YHOO', 'RHT', 'HPQ', 'GOOG', 'AIG', 'AAPL', 'AA']
>>> items[0][1]
'RHT'
>>> items[0][1][2]
'T'
>>> items[1]
[101, 102, 103]
>>> items[1][1]
102
>>>

Part 4 : Dictionaries

In last few parts, you've simply worked with stock symbols. However, suppose you wanted to map stock symbols to other data such as the price? Use a dictionary:

>>> prices = { 'IBM': 91.1, 'GOOG': 490.1, 'AAPL':312.23 }
>>>

A dictionary maps keys to values. Here's how to access:

>>> prices['IBM']
91.1
>>> prices['IBM'] = 123.45
>>> prices['HPQ'] = 26.15
>>> prices
{'GOOG': 490.1, 'AAPL': 312.23, 'IBM': 123.45, 'HPQ': 26.15}
>>>

To get a list of keys, use this:

>>> list(prices)
['GOOG', 'AAPL', 'IBM', 'HPQ']
>>>

To delete a value, use del

>>> del prices['AAPL']
>>> prices
{'GOOG': 490.1, 'IBM': 123.45, 'HPQ': 26.15}
>>>

Summary

Congratulations! You have completed the Manipulate Various Built-in Python Objects lab. You can practice more labs in LabEx to improve your skills.