Introduction to Python - Lesson 2 - Basic Data Types

2.1. Data Types

Standard Data Types in Python are:

Simple Data Types

• int - integer numbers, like: 0, 1, 100, -23, …
• float - floating point numbers (decimals), like: 0.0, 1.0, 100.0, -23.0, …
• str - string, like: “hello”, “world”, “123”, …
• bool - boolean, like: True, False
• NoneType - None

Complex Data Types

• list - list of elements, like: [1, 2, 3], [“hello”, “world”], …
• tuple - immutable list, like: (1, 2, 3), (“hello”, “world”), …
• dict - dictionary, like: {“name”: “John”, “age”: 30}, …
• set - set of unique elements, like: {1, 2, 3}, {“hello”, “world”}, …

We have already seen some of these data types earlier, especially ints and floats.

2.2. Strings

• Strings are sequences of characters, using the syntax of either single quotes or double quotes.
• Strings are immutable, meaning they cannot be changed after they are created.
• Strings are ordered sequences, meaning each element in the sequence can be accessed using an index represented by the array of numbers.
• Strings can be sliced, concatenated, and formatted.
• Strings can be compared using comparison operators.
• Strings can be formatted using the format method or f-strings.
• Strings can be manipulated using methods like upper, lower, strip, replace, split, join, and many others.

# define a string variable
my_string = "Hello, World!"
# we can use single quotes as well
my_string2 = 'Hello, World!'
# print the strings
print(my_string)
print(my_string2)

Hello, World!
Hello, World!

# let us print the length of the string
# we can use len() function to find the length of the string
print(len("Tomislav"))  # prints 8
print(len(my_string))  # prints 13

8
13

• We can use the index operator [] to access an item in a string
• The index operator returns the character at the specified position
• The index must be an integer
• Negative indices count from the end of the string
• The index must be in the range 0 to len(string) - 1

""" 'Hello' is a string with 5 characters (length is 5)
The indices and characters are:
0: H
1: e
2: l
3: l
4: o
"""
word = 'Hello'
# print the first character of the string
print(word[0])  # prints H  -> we could also do it like: 'Hello'[0]
# print the last character of the string
print(word[-1])  # prints o
# print the character at index 3
print(word[3])  # prints l

H
o
l

2.2.1. Slicing Strings

When we take only a part of a string, we are slicing it. We can use the slice operator : to get a range of characters from the string. When slicing, the first index is included, while the last given index is excluded.

# slicing
s = 'hello world'
print(s[2:])  # prints llo world
print(s[:5])  # prints hello
print(s[2:5])  # prints llo
print(s[-5:])  # prints world
print(s[:-5])  # prints hello

# slicing with step
print(s[::2])  # prints hlowrd
print(s[1::2])  # prints el ol
print(s[::-1])  # prints dlrow olleh - this is reversing a string
# reversing a string with step -2
print(s[::-2])  # prints dlrow olleh

llo world
hello
llo
world
hello 
hlowrd
el ol
dlrow olleh
drwolh

2.2.2. Concatenate Strings

We can concatenate strings using the + operator. What it does is it joins the two strings together. We can also use the * operator to repeat a string a given number of times. Let us see some examples.

# define two string variables and concatenate them
f_name = 'Mark'
l_name = 'Twain'

full_name = f_name + l_name
print(full_name)  # prints MarkTwain
# is this what we wanted? No, we need a space between the first and last name
full_name = f_name + ' ' + l_name
print(full_name)  # prints Mark Twain

MarkTwain
Mark Twain

Multi Line Strings

• We can create multi-line strings using triple quotes ''' or """.
• We can also use the escape character \n to create multi-line strings.

# multi-line string using triple quotes
multiline_string = '''This is a
multiline string.
As you can see, it spans
multiple lines.
'''
print(multiline_string)

This is a
multiline string.
As you can see, it spans
multiple lines.

Escaping Characters

• We can use the escape character \ to escape characters that have a special meaning in Python, like " or \.
• We can also use the escape character \ to create new lines, tabs, and other special characters.
  • Examples: \\, \', \", \n, \t, …

<p style="color:red;">Attention:</p>

• When we are using single quotes ' to define a string, we can use double quotes " inside the string without escaping them.
• When we are using double quotes " to define a string, we can use single quotes ' inside the string without escaping them.
• When we are using triple quotes ''' or """ to define a string, we can use both single and double quotes inside the string without escaping them.
• We can also use the escape character \ to escape the quotes inside the string, e.g. \', \".

# escape characters
name = 'Hermione'
print('My name is \\', name )  # prints My name is \ Hermion
print('My name is \n', name)  # prints My name is and then goes to the next line and prints my name

print('My name is \'Tomislav\'')  # prints My name is 'Tomislav'
print('My name is "Tomislav"')  # prints My name is "Tomislav"
print('My name is \t', name)  # prints My name is <tab space> Hermione

My name is \ Hermione
My name is 
 Hermione
My name is 'Tomislav'
My name is "Tomislav"
My name is 	 Hermione

2.2.3. Printing Strings, numbers and string formatting

• We can use the print function to print strings, numbers, and other objects.
• We can use the + operator to concatenate strings and the * operator to repeat strings.
• But, we cannot concatenate strings with numbers or other objects.
• if we want to combine strings with numbers, we need first to convert the numbers to strings using the str function.

name = 'Charles Dickens'
age = 58
print(name + ' is ' + str(age) + ' years old')  # prints Charles Dickens is 58 years old

Charles Dickens is 58 years old

• if we use comma to separate the strings and numbers in the print function, it will automatically add a space between them. Therefore, we don’t need to add a space manually, and we don’t have to convert numbers to strings.

print(name, 'is', age, 'years old.')

Charles Dickens is 58 years old.

• we can use the formatting string or f-string to format strings and numbers in a more readable way.
• that way, we can use placeholders {} inside the string, and then pass the values to be formatted as arguments to the f string.

print(f'Hello, {name}. You are {age} years old.')

Hello, Charles Dickens. You are 58 years old.

2.2.4. String Methods

Strings have many methods that can be used to manipulate and format them:

• upper, lower, capitalize, title, swapcase
• strip, lstrip, rstrip
• replace, count
• find, index
• startswith, endswith
• split, join
• format
• isalnum, isalpha, isdigit, isnumeric, isdecimal, isspace, islower, isupper, istitle, isidentifier, isprintable
• encode, decode
• zfill
• center, ljust, rjust
• expandtabs

We will not go through all of them (there are too many), but we will see some of the most important ones.

title = 'this is a nice title of a book'
title2 = 'NOBODY SHOULD READ THIS BOOK'

# upper
print(title.upper())
# lower
print(title2.lower())
# capitalize
print(title.capitalize())
# title
print(title.title())

# startswith
print(title.startswith('this'))  # prints True
print(title.startswith('This'))  # prints False

# count
print(title.count('i'))  # prints 4

THIS IS A NICE TITLE OF A BOOK
nobody should read this book
This is a nice title of a book
This Is A Nice Title Of A Book
True
False
4

Some formating perks

# we can print a number with commas for thousands separator
print(f'{123456789:,}')  # prints 123,456,789
# we can print a number with commas for thousands separator and decimal separator
print(f'{123456789:,.2f}')  # prints 123,456,789.00

123,456,789
123,456,789.00

---

2.2.5. F-string

F-string is an advanced version of the format method. It allows us to format strings and numbers in a more readable way.

• We can use the f or F prefix before the string.
• Then, we can use placeholders {} inside the string, and then pass the values to be formatted as arguments to the f string.
• We can use the :=^10 specifier(here =) to align the string to the center and fill it with spaces to the left and right (here 10, but it can be any number).
• to center the string on the left we use :=<10 and to the right we use :=>10

name = 'Hermione'
age = 58
print(f'Hello, {name}. You are {age:-^10} years old.')
print(f'Hello, {name}. You are {age:=<10} years old.')
print(f'Hello, {name}. You are {age:*>10} years old.')

Hello, Hermione. You are ----58---- years old.
Hello, Hermione. You are 58======== years old.
Hello, Hermione. You are ********58 years old.

• We can use f-string to output the values of the expressions in curly brackets.

# f-string with expressions
name = 'Hermione'
age = 58
print(f'Hello, {name}. You are {age*2} years old.')

Hello, Hermione. You are 116 years old.

2.2.6. Multiline Strings

• We can use triple quotes to create multiline strings.
• F-strings can also be multiline.

# multiline strings

multiline_string = '''This is a
multiline string.
As you can see, it spans
multiple lines.
'''
print(multiline_string)

This is a
multiline string.
As you can see, it spans
multiple lines.

# multiline f-string
name = 'Hermione'
age = 58
print(f'''Hello, {name}.
You are {age} years old.''')

Hello, Hermione.
You are 58 years old.

2.2.7. Strings and Numbers

When we are combining strings and numbers in a print statemente, we have to convert the numbers to strings using the str function. We cannot concatenate strings with numbers.

# printing strings and numbers
print('Hello, World!')  # prints Hello, World!
print('Hello, World!', 123)  # prints Hello, World! 123
print('Hello, World!'+ 123)  # throws error

Hello, World!
Hello, World! 123



---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

Cell In[17], line 4
      2 print('Hello, World!')  # prints Hello, World!
      3 print('Hello, World!', 123)  # prints Hello, World! 123
----> 4 print('Hello, World!'+ 123)  # throws error


TypeError: can only concatenate str (not "int") to str

print('Hello, World!'+ str(123))  # prints Hello, World!123

Hello, World!123

But we can “multipy” strings and numbers by using operator *. It is not multiplication in mathematical sense. It is merly a concatenation of the same string by n times.

# print twice 'Hello world!'
print('Hello, World!')
print('Hello, World!')

# or
print('Hello, World!\n'*2)  # prints Hello, World! two times - here we added a new line character

Hello, World!
Hello, World!
Hello, World!
Hello, World!