COMP7004 - Systems Scripting

Lecture 16: Dictionary and String Manipulations

Dr. Vincent Emeakaroha

vincent.emeakaroha@mtu.ie

Semester 2, 2026

The Dictionary Data Type

• Like a list, a dictionary is a collection of many values. But

unlike indexes for lists, indexes for dictionaries can use

many different data types, not just integers.

• Indexes for dictionaries are called keys. A key with its

associated value is called a key-value pair.

• A dictionary starts with a curly brace “{“ and ends with a

curly brace “}”.

• Example:

• The dictionary keys are ‘size’, ‘color’, and ‘disposition’. The

values for these keys are ‘fat’, ‘gray’, and ‘loud’,

respectively.

Accessing Dictionary

• The values in a dictionary can be accessed through their keys.

• For example:

• Dictionaries can still use integer values as keys, just like lists

use integer for indexes, but they do not have to start at 0 and

can be any number. For example:

Dictionaries vs Lists

• Unlike lists, items in dictionaries are unordered. The first item

in a list will be at index 0. But there is no first item in a

dictionary.

• While the order of items matters for determining whether two

lists are the same, it does not matter in which order the key-

value pairs are typed in a dictionary.

• For example:

Dictionaries vs Lists

• Because dictionaries are not ordered, they cannot be sliced like lists.

• Trying to access a key that does not exist in a dictionary will result in a

KeyError error message, just like accessing none existent index in a

list.

• Though dictionaries are not ordered, the fact that you can have

arbitrary data for keys allow you to organize your data in powerful

ways.

• For example:

• Write a program to store friends birthday

• We can use the names of our friends as key and their birthday as values.

• Solution: birthdays.py

Dictionary Example: birthday.py

birthday.py: Output

• Once the program terminates, all the data inputted are forgotten. We have

learnt about the shelve module that can be used to persist the information by

storing the data to a file.

The keys(), values() and items()

Methods

• These are three dictionary methods that will return

list-like values of dictionary’s keys, values or both

keys and values.

• The values returned by items() method are not true

list:

• They cannot be modified,

• They do not have an append() method.

• The data structures of these methods can be nicely

used in a for loop. See following examples:

keys(), values() and items(): Loop

Examples

• Note that the output of the

items() method is a tuple. To

get a true list equivalent, wrap

it around list() function.

Checking Key or Value Existence

• Recall from previous lecture that the in and not in

operators can check whether a value exist in a list.

You can use also these operators to see whether a key

or value exists in a dictionary.

The get() Method

• It can be tedious to check whether a key exist in a

dictionary before accessing the key value. Fortunately,

dictionaries have a get() method that takes two arguments:

• the key of the value to retrieve,

• a fall back value to return if the key does not exist.

• Example:

• Without the get() method, using the key ‘eggs’ could have

caused a KeyError error message.

The setdefault() Method

• If we want to set a value for a certain key in the

dictionary that does not have a value, we usually do:

• The setdefault() method offers a better way of doing

this in one line of code.

The setdefault() Method

• The setdefault() method is a nice shortcut to ensure that a

key exist in a dictionary in order to avoid error while

manipulating dynamically.

• For example:

Output:

Nested Dictionaries

• For modelling real world data structures, dictionaries

can be nested to associate keys with a series of

values.

• For example: - a nested dictionary to describe what

people brings to a picnic

Accessing Nested Dictionary

• To access elements of a nested dictionary, we use

indexing [] strategy.

• For example:

Manipulating Strings

Multiline Strings

• While you can use \n escape character to put a new

line into a string, it is often easier to use multiline

strings. A multiline string in Python begins and ends

with either:

• three single quotes or

• three double quotes.

• Any tabs, quotes or newline in between the ”triple

quotes” are considered part of the string.

Multiline String Example: catnapping.py

• Notice that the single quote character in Eve’s does not need to

be escaped. Escaping single and double quotes in multiline

string is optional.

Output

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.

• For example:

Useful String Methods

• Several string methods analyse strings or create

transformed string values. We will look at some of the

methods you might be using often.

• The upper() and lower() string methods return a new string

where all the letters in the original string have been

converted to uppercase or lowercase, respectively. Non-

letter characters in the string remains unchanged.

Example:

Useful String Methods

• The upper() and lower() are helpful if you need to make a

case-insensitive comparison.

• The string ‘great’ and ‘GREat’ are not equal to each other.

• But in the following program, it does not matter whether the

user type Great, GREAT, or greAT, because it is first converted

to lowercase.

Useful String Methods

• The title() and capitalize() methods convert the first

character of a string word to upper case and the rest of the

characters to lower case. However, they have subtle

differences.

• Example:

• Note that if a word contains an apostrophe or the separator

of the words is not white space, the behaviour of these

method varies.

• Example

Useful String Methods

• The isupper() and islower() methods will return a Boolean

True value if the string contain at least one letter and all the

letters are uppercase or lowercase respectively. Otherwise,

the method returns False.

• Example:

The isX Methods

• Along with islower() and isupper(), there are several string

methods that have names beginning with the word is.

These methods return a Boolean value that describes the

nature of the string. Here are some common ones:

• isalpha() returns True if the string consist of only letters

and is not blank.

• isalnum() returns True if the string consist of only letters

and numbers and is not blank.

• isdecimal() returns True if the string consist of only numeric

characters and is not blank.

The isX Methods

• isspace() returns True if string consists of only spaces,

tabs, or newlines and is not blank.

• istitle() returns True if the string consists of only words that

begin with an uppercase letter followed by only lowercase

letters.

• The isX methods are helpful when you want to validate

user input.

This program will repeatedly

ask users for their age until

they provide valid input

Removing Whitespace Around

Strings

• Sometimes you may want to strip off whitespace

characters (space, tabs and newline) from the left

side, right side or both sides of a string.

• The strip() string method will return a new string

without any whitespace character at the beginning or

end.

• The lstrip() and rstrip() methods will remove

whitespace characters from the left and right ends,

respectively.

Removing Whitespace Around

Strings

• The following code demonstrates the methods.

Removing Whitespace Around

Strings

• Optionally, a string argument will specify which characters

on the ends should be stripped. For example:

• Passing strip() the argument ‘ampS’ will tell it to strip

occurrences of a, m, p, and capital S from both ends of the

string stored in cat. The order of the characters in the string

passed to strip() does not matter: strip(‘ampS’) will do the

same thing as strip(‘mapS’) or strip(‘Spam’).

Thank You!