Getting Started with Data

In addition to the numeric and boolean classes, Python has a number of very powerful built-in collection classes. Lists, strings, and tuples are ordered collections that are very similar in general structure but have specific differences that must be understood for them to be used properly. Sets and dictionaries are unordered collections.

A list is an ordered collection of zero or more references to Python data objects. Lists are written as comma-delimited values enclosed in square brackets. The empty list is simply [ ]. Lists are heterogeneous, meaning that the data objects need not all be from the same class and the collection can be assigned to a variable as below. The following fragment shows a variety of Python data objects in a list.

>>> [1,3,True,6.5]
[1, 3, True, 6.5]
>>> myList = [1,3,True,6.5]
>>> myList
[1, 3, True, 6.5]

Note that when Python evaluates a list, the list itself is returned. However, in order to remember the list for later processing, its reference needs to be assigned to a variable.

Since lists are considered to be sequentially ordered, they support a number of operations that can be applied to any Python sequence. Table 2 reviews these operations and the following session gives examples of their use.

Table 2: Operations on Any Sequence in Python

Operation Name	Operator	Explanation
indexing	[ ]	Access an element of a sequence
concatenation	+	Combine sequences together
repetition	*	Concatenate a repeated number of times
membership	in	Ask whether an item is in a sequence
length	len	Ask the number of items in the sequence
slicing	[ : ]	Extract a part of a sequence

Note that the indices for lists (sequences) start counting with 0. The slice operation, myList[1:3], returns a list of items starting with the item indexed by 1 up to but not including the item indexed by 3.

Sometimes, you will want to initialize a list. This can quickly be accomplished by using repetition. For example,

>>> myList = [0] * 6
>>> myList
[0, 0, 0, 0, 0, 0]

One very important aside relating to the repetition operator is that the result is a repetition of references to the data objects in the sequence. This can best be seen by considering the following session:

The variable A holds a collection of three references to the original list called myList. Note that a change to one element of myList shows up in all three occurrences in A.
Lists support a number of methods that will be used to build data structures. Table 3 provides a summary. Examples of their use follow.

Table 3: Methods Provided by Lists in Python

Method Name	use	Explanation
append	alist.append(item)	Adds a new item to the end of a list
insert	alist.insert(i,item)	Inserts an item at the ith position in a list
pop	alist.pop()	Removes and returns the last item in a list
pop	alist.pop(i)	Removes and returns the ith item in a list
sort	alist.sort()	Modifies a list to be sorted
reverse	alist.reverse()	Modifies a list to be in reverse order
del	del alist[i]	Deletes the item in the ith position
index	alist.index(item)	Returns the index of the first occurrence of item
count	td>alist.count(item)	Returns the number of occurrences of item
remove	alist.remove(item)	Removes the first occurrence of item

You can see that some of the methods, such as pop , return a value and also modify the list. Others, such as reverse, simply modify the list with no return value. pop will default to the end of the list but can also remove and return a specific item. The index range starting from 0 is again used for these methods. You should also notice the familiar “dot” notation for asking an object to invoke a method. myList.append(False)can be read as “ask the object myList to perform its append method and send it the value False". Even simple data objects such as integers can invoke methods in this way.

>>> (54).__add__(21)
75
>>>

In this fragment we are asking the integer object 54 to execute its add method (called __add__ in Python) and passing it 21 as the value to add. The result is the sum, 75. Of course, we usually write this as 54+21. We will say much more about these methods later in this section.

One common Python function that is often discussed in conjunction with lists is the range function. range produces a range object that represents a sequence of values. By using the list function, it is possible to see the value of the range object as a list. This is illustrated below.

>>> range(10)
range(0, 10)
>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(5,10)
range(5, 10)
>>> list(range(5,10))
[5, 6, 7, 8, 9]
>>> list(range(5,10,2))
[5, 7, 9]
>>> list(range(10,1,-1))
[10, 9, 8, 7, 6, 5, 4, 3, 2]
>>>

The range object represents a sequence of integers. By default, it will start with 0. If you provide more parameters, it will start and end at particular points and can even skip items. In our first example, range(10), the sequence starts with 0 and goes up to but does not include 10. In our second example, range(5,10) starts at 5 and goes up to but not including 10. range(5,10,2) performs similarly but skips by twos (again, 10 is not included).

Strings are sequential collections of zero or more letters, numbers and other symbols. We call these letters, numbers and other symbols characters. Literal string values are differentiated from identifiers by using quotation marks (either single or double).

>>> "David"
'David'
>>> myName = "David"
>>> myName[3]
'i'
>>> myName*2
'DavidDavid'
>>> len(myName)
5
>>>

Since strings are sequences, all of the sequence operations described above work as you would expect. In addition, strings have a number of methods, some of which are shown in Table 4. For example,

>>> myName
'David'
>>> myName.upper()
'DAVID'
>>> myName.center(10)
'  David   '
>>> myName.find('v')
2
>>> myName.split('v')
['Da', 'id']

Of these, split will be very useful for processing data. split will take a string and return a list of strings using the split character as a division point. In the example, v is the division point. If no division is specified, the split method looks for whitespace characters such as tab, newline and space.

 Table 4: Methods Provided by Strings in Python

Method Name	Use	Explanation
center	astring.center(w)	Returns a string centered in a field of sizew
count	astring.count(item)	Returns the number of occurrences of item in the string
ljust	astring.ljust(w)	Returns a string left-justified in a field of sizew
lower	astring.lower()	Returns a string in all lowercase
rjust	astring.rjust(w)	Returns a string right-justified in a field of sizew
find	astring.find(item)	Returns the index of the first occurrence of item
split	astring.split(schar)	Splits a string into substrings at schar

A major difference between lists and strings is that lists can be modified while strings cannot. This is referred to as mutability. Lists are mutable; strings are immutable. For example, you can change an item in a list by using indexing and assignment. With a string that change is not allowed.

>>> myList
[1, 3, True, 6.5]
>>> myList[0]=2**10
>>> myList
[1024, 3, True, 6.5]
>>>
>>> myName
'David'
>>> myName[0]='X'

Traceback (most recent call last):
  File "", line 1, in -toplevel-
    myName[0]='X'
TypeError: object doesn't support item assignment
>>>

Tuples are very similar to lists in that they are heterogeneous sequences of data. The difference is that a tuple is immutable, like a string. A tuple cannot be changed. Tuples are written as comma-delimited values enclosed in parentheses. As sequences, they can use any operation described above. For example,

>>> myTuple = (2,True,4.96)
>>> myTuple
(2, True, 4.96)
>>> len(myTuple)
3
>>> myTuple[0]
2
>>> myTuple * 3
(2, True, 4.96, 2, True, 4.96, 2, True, 4.96)
>>> myTuple[0:2]
(2, True)
>>>

However, if you try to change an item in a tuple, you will get an error. Note that the error message provides location and reason for the problem.

>>> myTuple[1]=False

Traceback (most recent call last):
  File "", line 1, in -toplevel-
    myTuple[1]=False
TypeError: object doesn't support item assignment
>>>

A set is an unordered collection of zero or more immutable Python data objects. Sets do not allow duplicates and are written as comma-delimited values enclosed in curly braces. The empty set is represented by set(). Sets are heterogeneous, and the collection can be assigned to a variable as below.

>>> {3,6,"cat",4.5,False}
{False, 4.5, 3, 6, 'cat'}
>>> mySet = {3,6,"cat",4.5,False}
>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>>

Even though sets are not considered to be sequential, they do support a few of the familiar operations presented earlier. Table 5 reviews these operations and the following session gives examples of their use.

 Table 5: Operations on a Set in Python

Operation Name	Operator	Explanation
membership	in	Set membership
length	len	Returns the cardinality of the set
|	aset |  otherset	Returns a new set with all elements from both sets
&	aset & otherset	Returns a new set with only those elements common to both sets
-	aset - otherset	Returns a new set with all items from the first set not in second
<=	aset <= otherset	Asks whether all elements of the first set are in the second

>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>> len(mySet)
5
>>> False in mySet
True
>>> "dog" in mySet
False
>>>

Sets support a number of methods that should be familiar to those who have worked with them in a mathematics setting. Table 6 provides a summary. Examples of their use follow. Note that union, intersection, issubset, and differenceall have operators that can be used as well.

 Table 6: Methods Provided by Sets in Python

Method Name	Use	Explanation
union	aset.union(otherset)	Returns a new set with all elements from both sets
intersection	aset.intersection(otherset)	Returns a new set with only those elements common to both sets
difference	aset.difference(otherset)	Returns a new set with all items from first set not in second
issubset	aset.issubset(otherset)	Asks whether all elements of one set are in the other
add	aset.add(item)	Adds item to the set
remove	aset.remove(item)	Removes item from the set
pop	aset.pop()	Removes an arbitrary element from the set
clear	aset.clear()	Removes all elements from the set

>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>> yourSet = {99,3,100}
>>> mySet.union(yourSet)
{False, 4.5, 3, 100, 6, 'cat', 99}
>>> mySet | yourSet
{False, 4.5, 3, 100, 6, 'cat', 99}
>>> mySet.intersection(yourSet)
{3}
>>> mySet & yourSet
{3}
>>> mySet.difference(yourSet)
{False, 4.5, 6, 'cat'}
>>> mySet - yourSet
{False, 4.5, 6, 'cat'}
>>> {3,100}.issubset(yourSet)
True
>>> {3,100}<=yourSet
True
>>> mySet.add("house")
>>> mySet
{False, 4.5, 3, 6, 'house', 'cat'}
>>> mySet.remove(4.5)
>>> mySet
{False, 3, 6, 'house', 'cat'}
>>> mySet.pop()
False
>>> mySet
{3, 6, 'house', 'cat'}
>>> mySet.clear()
>>> mySet
set()
>>>

Our final Python collection is an unordered structure called a dictionary. Dictionaries are collections of associated pairs of items where each pair consists of a key and a value. This key-value pair is typically written as key:value. Dictionaries are written as comma-delimited key:value pairs enclosed in curly braces. For example,

>>> capitals = {'Iowa':'DesMoines','Wisconsin':'Madison'}
>>> capitals
{'Wisconsin': 'Madison', 'Iowa': 'DesMoines'}
>>>

We can manipulate a dictionary by accessing a value via its key or by adding another key-value pair. The syntax for access looks much like a sequence access except that instead of using the index of the item we use the key value. To add a new value is similar.

It is important to note that the dictionary is maintained in no particular order with respect to the keys. The first pair added ('Utah': 'SaltLakeCity')was placed first in the dictionary and the second pair added ('California': 'Sacramento')was placed last. The placement of a key is dependent on the idea of “hashing,” which will be explained in more detail in Chapter 4. We also show the length function performing the same role as with previous collections.

Dictionaries have both methods and operators. Table 7 and Table 8 describe them, and the session shows them in action. The keys, values, and items methods all return objects that contain the values of interest. You can use the listfunction to convert them to lists. You will also see that there are two variations on the get method. If the key is not present in the dictionary, get will return None. However, a second, optional parameter can specify a return value instead.

Table 7: Operators Provided by Dictionaries in Python

Operator	Use	Explanation
[]	myDict[k]	Returns the value associated withk, otherwise its an error
in	key in adict	ReturnsTrueif key is in the dictionary,Falseotherwise
del	del adict[key]	Removes the entry from the dictionary

>>> phoneext={'david':1410,'brad':1137}
>>> phoneext
{'brad': 1137, 'david': 1410}
>>> phoneext.keys()
dict_keys(['brad', 'david'])
>>> list(phoneext.keys())
['brad', 'david']
>>> phoneext.values()
dict_values([1137, 1410])
>>> list(phoneext.values())
[1137, 1410]
>>> phoneext.items()
dict_items([('brad', 1137), ('david', 1410)])
>>> list(phoneext.items())
[('brad', 1137), ('david', 1410)]
>>> phoneext.get("kent")
>>> phoneext.get("kent","NO ENTRY")
'NO ENTRY'
>>>

Table 8: Methods Provided by Dictionaries in Python

Method Name	Use	Explanation
keys	adict.keys()	Returns the keys of the dictionary in a dict_keys object
values	adict.values()	Returns the values of the dictionary in a dict_values object
items	adict.items()	Returns the key-value pairs in a dict_items object
get	adict.get(k)	Returns the value associated withk,Noneotherwise
get	adict.get(k,alt)	Returns the value associated with k,altotherwise