Module 2
Module 2
Module 2
&
Prepared By:
Dr. A. Ajina
Assistant Professor,
Department of Information Science and Engineering
RVITM, Bengaluru – 560076
Email: ajinaa.rvitm@rvei.edu.in
2.1 3
LISTS
• Methods
• References
2.2 18
DICTIONARIES AND STRUCTURING DATA
• Pretty Printing
➢ The spam variable ❶ is still assigned only one value: the list value (contains multiple values).
➢ The value [] is an empty list that contains no values, similar to '', the empty string.
➢ Say you have the list ['cat', 'bat', 'rat', 'elephant'] stored in a variable named spam.
➢ The Python code spam[0] would evaluate to 'cat', and spam[1] would evaluate to 'bat', and so
on.
➢ The first value in the list is at index 0, the second value is at index 1, and the third value is at
index2, and so on.
➢ For example, type the following expressions into the interactive shell.
➢ The expression 'Hello ' + spam[0] evaluates to 'Hello ' + 'cat' because spam[0] evaluates
to thestring 'cat'. This expression in turn evaluates to the string value 'Hello cat'.
➢ If we use an index that exceeds the number of values in the list value then, python gives
IndexError
➢ Lists can also contain other list values. The values in these lists of lists can be accessed using
multiple indexes.
➢ The first index dictates which list value to use, and the second indicates the value within the list
value. Ex, spam[0][1] prints 'bat', the second value in the first list.
Negative Indexes
➢ We can also use negative integers for the index. The integer value -1 refers to the last index in a
list,the value -2 refers to the second-to-last index in a list, and so on.
➢ An index will get a single value from a list, a slice can get several values from a list, in the form
of anew list.
➢ A slice is typed between square brackets, like an index, but it has two integers separated by a
colon.
➢ Difference between indexes and slices.
o spam[2] is a list with an index (one integer).
o spam[1:4] is a list with a slice (two integers).
➢ In a slice, the first integer is the index where the slice starts. The second integer is the index
wherethe slice ends (but will not include the value at the second index).
➢ As a shortcut, we can leave out one or both of the indexes on either side of the colon in the slice.
• Leaving out the first index is the same as using 0, or the beginning of the list.
• Leaving out the second index is the same as using the length of the list, which will slice to
the end of the list.
➢ The len() function will return the number of values that are in a list value.
➢ We can also use an index of a list to change the value at that index. Ex: spam[1] = 'aardvark'
means“Assign the value at index 1 in the list spam to the string 'aardvark'.”
➢ The + operator can combine two lists to create a new list value in the same way it combines
twostrings into a new string value.
➢ The * operator can also be used with a list and an integer value to replicate the list.
➢ The del statement can also be used to delete a variable After deleting if we try to use the variable,
we will get a NameError error because the variable no longer exists.
➢ In practice, you almost never need to delete simple variables.
➢ The del statement is mostly used to delete values from lists.
➢ When we first begin writing programs, it’s tempting to create many individual variables to store a group
of similar values.
➢ Instead of using multiple, repetitive variables, we can use a single variable that contains a list value.
➢ For Ex: The following program uses a single list and it can store any number of cats that the user
typesin.
➢ Program:
➢ Output:
➢ The benefit of using a list is that our data is now in a structure, so our program is much more
flexible inprocessing the data than it would be with several repetitive variables.
➢ A for loop repeats the code block once for each value in a list or list-like value.
Program
Output:
➢ A common Python technique is to use range (len(someList)) with a for loop to iterate over
theindexes of a list.
➢ The code in the loop will access the index (as the variable i), the value at that index (as supplies[i])
and range(len(supplies)) will iterate through all the indexes of supplies, no matter how many items
it contains.
➢ We can determine whether a value is or isn’t in a list with the in and not in operators.
➢ in and not in are used in expressions and connect two values: a value to look for in a list and the
listwhere it may be found and these expressions will evaluate to a Boolean value.
➢ The following program lets the user type in a pet name and then checks to see whether the
name isin a list of pets.
Program
➢ The multiple assignment trick is a shortcut that lets you assign multiple variables with the values in
a list in one line of code.
➢ Instead of left-side program we could type the right-side program to assignment multiple variables
but the number of variables and the length of the list must be exactly equal, or Python will give you
a ValueError:
➢ When assigning a value to a variable, we will frequently use the variable itself.
➢ Instead of left-side program we could use right-side program i.e., with the augmented assignment
operator += to do the same thing as a shortcut.
➢ The Augmented Assignment Operators are listed in the below table:
➢ The += operator can also do string and list concatenation, and the *= operator can do string and list
replication.
➢ List values have an index() method that can be passed a value, and if that value exists in the list,
theindex of the value is returned. If the value isn’t in the list, then Python produces a ValueError
error.
➢ When there are duplicates of the value in the list, the index of its first appearance is returned.
➢ To add new values to a list, use the append() and insert() methods.
➢ The append() method call adds the argument to the end of the list.
➢ The insert() method can insert a value at any index in the list. The first argument to insert() is the
index for the new value, and the second argument is the new value to be inserted.
➢ The remove() method is passed the value to be removed from the list it is called on.
➢ Attempting to delete a value that does not exist in the list will result in a ValueError error.
➢ If the value appears multiple times in the list, only the first instance of the value will be removed.
➢ The del statement is good to use when you know the index of the value you want to remove from
the list. The remove() method is good when you know the value you want to remove from the list.
➢ You can also pass True for the reverse keyword argument to have sort() sort the values in
reverseorder.
➢ There are three things you should note about the sort() method.
• First, the sort() method sorts the list in place; don’t try to return value by writing code like
spam = spam.sort().
• Second, we cannot sort lists that have both number values and string values in them.
• Third, sort() uses “ASCIIbetical order(upper case)” rather than actual alphabetical
order(lower case) for sorting strings.
➢ If we need to sort the values in regular alphabetical order, pass str.lower for the key keyword
argument in the sort() method call.
➢ We can write a much more elegant version of the Magic 8 Ball program. Instead of several lines
of nearly identical elif statements, we can create a single list.
➢ The amount of indentation for a line of code tells Python what block it is in.
➢ lists can actually span several lines in the source code file. The indentation of these lines do not
matter; Python knows that until it sees the ending square bracket, the list is not finished.
➢ We can also split up a single instruction across multiple lines using the \ line continuation character
at the end.
➢ Lists aren’t the only data types that represent ordered sequences of values.
➢ Ex, we can also do these with strings: indexing; slicing; and using them with for loops, with len(),
and with the in and not in operators.
➢ The proper way to “mutate” a string is to use slicing and concatenation to build a new
string bycopying from parts of the old string.
➢ We used [0:7] and [8:12] to refer to the characters that we don’t wish to replace. Notice that
theoriginal 'Zophie a cat' string is not modified because strings are immutable.
❖ List
➢ A list value is a mutable data type: It can have values added, removed, or changed.
➢ The list value in eggs isn’t being changed here; rather, an entirely new and different list value
([4, 5,6]) is overwriting the old list value ([1, 2, 3]). Fig.2.1 shows the pictorial representation.
Fig.2.1: When eggs = [4, 5, 6] is executed, the contents of eggs are replaced with a new list value.
➢ If we want to modify the original list in eggs to contain [4, 5, 6], you would have to delete the
itemsin that and then add items to it. Fig.2.2 shows the pictorial representation the del statement
and the append() method modify the same list value in place.
➢ Second, benefit of using tuples instead of lists is that, because they are immutable and their
contentsdon’t change. Tuples cannot have their values modified, appended, or removed.
➢ If you have only one value in your tuple, you can indicate this by placing a trailing comma
after thevalue inside the parentheses.
➢ We assign 42 to the spam variable, and then we copy the value in spam and assign it to the
variablecheese. When we later change the value in spam to 100, this doesn’t affect the value
in cheese. Thisis because spam and cheese are different variables that store different values.
➢ But lists works differently. When we assign a list to a variable, we are actually assigning a
list reference to the variable. A reference is a value that points to some bit of data, and a list
reference isa value that points to a list.
➢ When we create the list ❶, we assign a reference to it in the spam variable. But the next line
copiesonly the list reference in spam to cheese, not the list value itself. This means the values
stored in spam and cheese now both refer to the same list.
➢ There is only one underlying list because the list itself was never actually copied. So when
wemodify the first element of cheese, we are modifying the same list that spam refers to.
➢ List variables don’t actually contain lists—they contain references to lists. Fig.2.3 shows
spam = [0, 1, 2, 3, 4, 5] stores a reference to a list, not the actual list
Fig.2.3: spam = [0, 1, 2, 3, 4, 5] stores a reference to a list, not the actual list.
Fig.2.5: cheese [1] = 'Hello!' modifies the list that both variables refer to
➢ Variables will contain references to list values rather than list values themselves.
➢ But for strings and integer values, variables will contain the string or integer value.
➢ Python uses references whenever variables must store values of mutable data types, such as
lists or dictionaries. For values of immutable data types such as strings, integers, or tuples,
Python variableswill store the value itself.
Passing References
➢ References are particularly important for understanding how arguments get passed to functions.
➢ When a function is called, the values of the arguments are copied to the parameter variables.
Program Output
➢ when eggs() is called, a return value is not used to assign a new value to spam.
➢ Even though spam and someParameter contain separate references, they both refer to the same
list.This is why the append('Hello') method call inside the function affects the list even after the
function call has returned.
The copy Module’s copy() and deepcopy() Functions
➢ If the function modifies the list or dictionary that is passed, we may not want these changes
in theoriginal list or dictionary value.
➢ For this, Python provides a module named copy that provides both the copy() and
deepcopy()functions.
Fig.2.6: cheese = copy.copy(spam) creates a second list that can be modified independently of the first.
➢ If the list you need to copy contains lists, then use the copy. deepcopy() function instead of
copy.copy(). The deepcopy() function will copy these inner lists as well.
➢ This assigns a dictionary to the myCat variable. This dictionary’s keys are 'size', 'color',
and'disposition'. The values for these keys are 'fat', 'gray', and 'loud', respectively. You can
access these values through their keys:
➢ Dictionaries can still use integer values as keys, but they do not have to start at 0 and can
be any number.
➢ We can have arbitrary values for the keys that allows us to organize our data in powerful ways.
➢ Ex: we want to store data about our friends’ birthdays. We can use a dictionary with the names as
keys and the birthdays as values.
Program Output
➢ We create an initial dictionary and store it in birthdays 1.
➢ We can see if the entered name exists as a key in the dictionary with the in keyword 2.
➢ If the name is in the dictionary, we access the associated value using square brackets 3; if not, we
can add it using the same square bracket syntax combined with the assignment operator 4.
➢ A for loop can iterate over the keys, values, or key-value pairs in a dictionary by using keys(),
values(), and items() methods.
➢ The values in the dict_items value returned by the items() method are tuples of the key and value.
➢ If we want a true list from one of these methods, pass its list-like return value to the list() function.
➢ The list(spam.keys()) line takes the dict_keys value returned from keys() and passes it to list(),
which then returns a list value of ['color', 'age'].
➢ We can also use the multiple assignment trick in a for loop to assign the key and value to separate
variables.
➢ The first time setdefault() is called, the dictionary in spam changes to {'color': 'black', 'age': 5, 'name':
'Pooka'}. The method returns the value 'black' because this is now the value set for the key 'color'.
Whenspam.setdefault('color', 'white') is called next, the value for that key is not changed to 'white'
because spam already has a key named 'color'.
➢ The program loops over each character in the message variable’s string, counting how often
eachcharacter appears.
➢ The setdefault() method call ensures that the key is in the count dictionary (with a default value of 0),
sothe program doesn’t throw a KeyError error when count[character] = count[character] + 1 is
executed.
➢ Output:
➢ Importing pprint module will provide access to the pprint() and pformat() functions that will “pretty
print” a dictionary’s values.
➢ This is helpful when we want a cleaner display of the items in a dictionary than what print() provides
and also it is helpful when the dictionary itself contains nested lists or dictionaries..
Program:
counts the number of occurrences of each letter in a string.
➢ If we want to obtain the prettified text as a string value instead of displaying it on the screen,
call pprint.pformat().
A Tic-Tac-Toe Board
➢ A tic-tac-toe board looks like a large hash symbol (#) with nine slots that can each contain an X, an
O,or a blank. To represent the board with a dictionary, we can assign each slot a string-value key as
shownin below figure.
➢ The data structure stored in the theBoard variable represents the tic-tactoe board in the below Figure.
➢ The data structure in theBoard now represents the tic-tac-toe board in the below Figure.
Output:
➢ The printBoard() function can handle any tic-tac-toe data structure you pass it.
Program
Output:
➢ Now we created a data structure to represent a tic-tac-toe board and wrote code in printBoard()
tointerpret that data structure, we now have a program that “models” the tic-tac-toe board.
Program: allows the players to enter their moves.
➢ The new code prints out the board at the start of each new turn 1, gets the active player’s move 2,
updates the game board accordingly 3, and then swaps the active player 4 before moving on to the
next turn.
Nested Dictionaries and Lists
➢ We can have program that contains dictionaries and lists which in turn contain other dictionaries
and lists.
➢ Lists are useful to contain an ordered series of values, and dictionaries are useful for associating
keys with values.
Program: which contains nested dictionaries in order to see who is bringing what to a picnic.
➢ Inside the totalBrought() function, the for loop iterates over the keyvalue pairs in guests 1.
➢ Inside the loop, the string of the guest’s name is assigned to k, and the dictionary of picnic items
they’rebringing is assigned to v.
➢ If the item parameter exists as a key in this dictionary, it’s value (the quantity) is added to
numBrought 2.
[Type here] [Type here] [Type here]
➢ If it does not exist as a key, the get() method returns 0 to be added to numBrought.
Output:
String Literals
➢ String values begin and end with a single quote.
➢ But we want to use either double or single quotes within a string then we have a multiple way to
doit as shown below.
Double Quotes
➢ One benefit of using double quotes is that the string can have a single quote character in it.
➢ Since the string begins with a double quote, Python knows that the single quote is part of the
string and not marking the end of the string.
Escape Characters
➢ If you need to use both single quotes and double quotes in the string, you’ll need to use
escapecharacters.
➢ An escape character consists of a backslash (\) followed by the character you want to add to the
string.
➢ Python knows that the single quote in Bob\'s has a backslash, it is not a single quote meant to
end the string value. The escape characters \' and \" allows to put single quotes and double quotes
inside your strings, respectively.
➢ Ex:
➢ The different special escape characters can be used in a program as listed below in a table.
Output
➢ The following print() call would print identical text but doesn’t use a multiline string.
Multiline Comments
➢ While the hash character (#) marks the beginning of a comment for the rest of the line.
➢ A multiline string is often used for comments that span multiple lines.
➢ The space and exclamation point are included in the character count, so 'Hello world!' is 12
characters long.
➢ If we specify an index, you’ll get the character at that position in the string.
➢ If we specify a range from one index to another, the starting index is included and the ending index
is not.
➢ The substring we get from spam[0:5] will include everything from spam[0] to spam[4], leaving
out the space at index 5.
Note: slicing a string does not modify the original string.
The in and not in Operators with Strings
➢ The in and not in operators can be used with strings just like with list values.
➢ An expression with two strings joined using in or not in will evaluate to a Boolean True or False.
➢ These methods do not change the string itself but return new string values.
➢ If we want to change the original string, we have to call upper() or lower() on the string and
thenassign the new string to the variable where the original was stored.
➢ The upper() and lower() methods are helpful if we need to make a case-insensitive comparison.
➢ In the following small program, it does not matter whether the user types Great, GREAT,
or grEAT, because the string is first converted to lowercase.
Program Output
➢ The isupper() and islower() methods will return a Boolean True value if the string has at least one
letter and all the letters are uppercase or lowercase, respectively. Otherwise, the method returns
False.
➢ Since the upper() and lower() string methods themselves return strings, you can call string
methods on those returned string values as well. Expressions that do this will look like a chain of
method calls.
➢ The isX string methods are helpful when you need to validate user input.
➢ For example, the following program repeatedly asks users for their age and a password until
theyprovide valid input.
Program output
➢ These methods are useful alternatives to the == equals operator if we need to check only whether
the first or last part of the string, rather than the whole thing, is equal to another string.
➢ string join() calls on is inserted between each string of the list argument.
o Ex: when join(['cats', 'rats', 'bats']) is called on the ', ' string, the returned string is 'cats, rats,
bats'.
o join() is called on a string value and is passed a list value.
Split()
➢ The split() method is called on a string value and returns a list of strings.
➢ We can pass a delimiter string to the split() method to specify a different string to split upon.
➢ A common use of split() is to split a multiline string along the newline characters.
➢ 'Hello'.rjust(10) says that we want to right-justify 'Hello' in a string of total length 10. 'Hello' is five
characters, so five spaces will be added to its left, giving us a string of 10 characters with 'Hello'
justified right.
➢ An optional second argument to rjust() and ljust() will specify a fill character other than a space
character.
➢ The center() string method works like ljust() and rjust() but centers the text rather than justifying
it to the left or right.
➢ These methods are especially useful when you need to print tabular data that has the correct
spacing.
➢ In the below program, we define a printPicnic() method that will take in a dictionary of
informationand use center(), ljust(), and rjust() to display that information in a neatly aligned table-
like format.
➢ Optionally, a string argument will specify which characters on the ends should be stripped.
➢ Passing strip() the argument 'ampS' will tell it to strip occurences of a, m, p, and capital S from
theends of the string stored in spam.
➢ The order of the characters in the string passed to strip() does not matter: strip('ampS') will do
thesame thing as strip('mapS') or strip('Spam').
Copying and Pasting Strings with the pyperclip Module
➢ The pyperclip module has copy() and paste() functions that can send text to and receive text
from your computer’s clipboard.
➢ Of course, if something outside of your program changes the clipboard contents, the paste()
function will return it.
➢ With this batch file created, running the password-safe program on Windows is just a matter of
pressing win-R and typing pw <account name>.
Program output
Step 1: Copy and Paste from the Clipboard
➢ You want the bulletPointAdder.py program to do the following:
1. Paste text from the clipboard
2. Do something to it
➢ We split the text along its newlines to get a list in which each item is one line of the text. For
each line, we add a star and a space to the start of the line. Now each string in lines begins with a
star.
Step 3: Join the Modified Lines
➢ The lines list now contains modified lines that start with stars.
➢ pyperclip.copy() is expecting a single string value, not a list of string values. To make this
single string value, pass lines into the join() method to get a single string joined from the list’s
strings.
➢ When this program is run, it replaces the text on the clipboard with text that has stars at the start
of each line.