Introduction to Python

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Containers: More List Operations

using containers to collect data

Containers are Python objects that can contain other objects.

• a container is an object that can contain other objects
• we collect values (numbers and strings) from a data source and store them in a container to manipulate and analyze the data
• the four Python containers are list, tuple, set and dict
• each one stores data in a different way that makes it convenient for us to manipulate and analyze

containers allow for manipulation and analysis

Once collected, values in a container can be sorted or filtered (i.e. selected) according to whatever rules we choose. A collection of integer or floating-point values offers many opportunities for analysis. We can calculate:

• the median (the "middle" value in a sorted list)
• the standard deviation (the average distance of the values from an average value)
• the top 5 or bottom 3, and the average of those
• the dividing of values into "quartiles" or "percentiles"

A collection of string values allows us to perform text analysis:

• the frequency of a word
• the position of a word within a text
• whether a word in one collection is present in another collection

container object summary : list, set, tuple

Compare and contrast the characteristics of each container.

mylist =  ['a', 'b', 'c', 'd', 1, 2, 3]

mytuple = ('a', 'b', 'c', 'd', 1, 2, 3)

myset =   {'a', 'b', 'c', 'd', 1, 2, 3}

mydict =  {'a': 1, 'b': 2, 'c': 3, 'd': 4}

list: ordered, mutable sequence of objects tuple: ordered, immutable sequence of objects set: unordered, mutable, unique collection of objects dict: unordered, mutable collection of object key-value pairs, with unique keys (discussed upcoming)

• Note the different brackets used for each container (dicts are a little more elaborate).
• This is just an overview, to show you where we are going -- no need to learn these characteristics now.

review: the list container object

A list is an ordered sequence of values.

var = []                     # initialize an empty list

var2 = [1, 2, 3, 'a', 'b']   # initialize a list of values

• the list is the most commonly used container
• it stores values (items) in a sequence
• it allows us to access an item by position
• lists can be sorted, sliced and looped through ('for' loop, coming up)

review: subscripting a list

Subscripting allows us to read individual items from a list.

mylist = [1, 2, 3, 'a', 'b']       # list

xx = mylist[2]                     # 3

yy = mylist[-1]                    # 'b'

• indexing starts at 0
• so index 1 is the 2nd item, index 2 is the 3rd item, etc.
• indexing can also be counted from the end, at -1
• so index -1 is the last item, index -2 is the 2nd to last item, etc.

review: slicing a list

Slicing a list returns a new list.

var2 = [1, 2, 3, 'a', 'b']            # list

sublist1 = var2[0:3]                  # [1, 2, 3]

sublist2 = var2[2:4]                  # [3, 'a']

sublist3 = var2[3:]                   # ['a', 'b']

Remember the rules of slicing:

• indexing begins at 0, so 0 is the first item
• the "upper bound" (2nd integer) is an index one greater than the item that will be returned (non-inclusive)
• to slice off the end, leave off the upper bound

in operator: finding an item within a list

The in operator returns True if an item is in the list.

mylist = [1, 2, 3, 'a', 'b']             # list

if 'b' in mylist:                        # this is True for mylist
    print("'b' can be found in mylist")

print('b' in mylist)                     # True:  the 'in' operator
                                         # actually returns True or False

Ex. 5.1

summary functions: len(), sum(), max(), min()

Summary functions offer a speedy answer to basic analysis questions: how many? How much? Highest value? Lowest value?

mylist = [1, 3, 5, 7, 9]        # list

print(len(mylist))               # 5 (count of items)
print(sum(mylist))               # 25 (sum of values)
print(min(mylist))               # 1 (smallest value)
print(max(mylist))               # 9 (largest value)

sorting a list

sorted() returns a new list of sorted values.

mylist = [4, 9, 1.2, -5, 200, 20]

smyl = sorted(mylist)              # list, [-5, 1.2, 4, 9, 20, 200]

Ex. 5.2

concatenating two lists with +

List concatenation works in the same way as it does with strings.

var = ['a', 'b', 'c']
var2 = ['d', 'e', 'f']

var3 = var + var2            # list, ['a', 'b', 'c', 'd', 'e', 'f']

adding (appending) an item to a list

var = []

var.append(4)                # Note well! call is not assigned
var.append(5.5)              # list is changed in-place

print(var)                    # [4, 5.5]

• It is the nature of a list to hold these items in order as they were added.
• Note that the list is changing in-place
• The call to .append() is not assigned back to var.

5.11

the AttributeError exception

An AttributeError exception occurs when calling a method on an object type that doesn't support that method.

mylines = ['line1\n', 'line2\n', 'line3\n']

mylines = mylines.rstrip()         # AttributeError:
                                   # 'list' object has no attribute 'rstrip'

Debugging:

• the problem with the last line here is that .rstrip() is a string method, but it's being called on a string
• most AttributeError exceptions are raised when we call a method on the wrong object
• when you get an AttributeError, check the object type and whether that type supports the method

Understanding the name AttributeError:

• although .rstrip() is a method, we can refer to it more generally as an attribute
• an attribute is any name that appears after an object and a dot (object.attribute).
• the attribute is often a method, though it may point at any type of object (str, int, list, etc.)

the AttributeError when using .append()

This exception may sometimes result from a misuse of the append() method -- it should not be assigned to any variable.

mylist = ['a', 'b', 'c']

# oops:  returns None -- call to append() should not be assigned
mylist = mylist.append('d')

mylist = mylist.append('e')        # AttributeError:  'NoneType'
                                   # object has no attribute 'append'

• since .append() isn't designed to return an object, it returns the default value None
• (None is a value that means 'null' or 'empty')
• .append() returns None because it wasn't designed to return a value -- None is its way of saying 'nothing here')
• if we assign to the list variable, it replaces the list with None
• the next time we try to use .append() we are attempting to call it on None
• Python's error message is actually saying "the None object doesn't have an .append() method"

the correct use of .append()

Just remember that we don't assign from .append().

mylist = ['a', 'b', 'c']

mylist.append('d')                 # now mylist equals ['a', 'b', 'c', 'd']

• because .append() does not return a useable value, we should not assign from the call
• simply call the method and understand that the list will change in-place
• the list is known to be mutable, meaning that it can be changed

sidebar: removing a container item

There are a number of additional list methods to manipulate a list, though they are less often used.

mylist = ['a', 'hello', 5, 9]

popped = mylist.pop(0)      # str, 'a'
                            # (argument specifies the index  of the item to remove)

mylist.remove(5)            # remove an item by value
print(mylist)               # ['hello', 9]

mylist.insert(0, 10)
print(mylist)               # [10, 'hello', 9]