Types and Operators

Last updated on 2026-06-27 | Edit this page

Estimated time: 25 minutes

Overview

Questions

“Python has types?”
“How do you check the type of a variable?”
“What are the basic data types in Python?”
“How do you convert between different data types?”
“What are the rules for naming variables in Python?”

Objectives

Understand the concept of a type system in programming languages
Identify and use basic data types in Python
Learn how to check the type of a variable in Python
Convert between different data types in Python
Use string slicing and indexing to manipulate strings in Python
Use lists to group data in Python
Understand the rules and conventions for naming variables in Python

In Python, almost everything is an object, and every object has a type. In the context of Python, a type can be thought of as a description of what a value is and what you can do with it. That is, the type of an Python object determines what operations can be performed on it and how it behaves. Python has a rich set of built-in data types, and understanding these types is fundamental to programming in Python.

In contrast with a strongly typed language that assures type safety (e.g., 🦀Rust), Python is dynamically typed, meaning that the type of a variable is determined at runtime and can change as the program executes. In practice, this makes Python easier to write and run, but it also means that type-related errors (and many other runtime errors) may only be discovered during execution.

Basically, Python does not care about your types, but your types help Python understand how to interpret your code.

Types in Python

Instructor Note

Floats are so called because they represent numbers that can “float” around the decimal point.

Python has several built-in data types, including but not limited to:

int: Integer numbers (e.g., 1, 42, -5)
float: Floating-point numbers (e.g., 3.14, -0.001)
str: Strings, which are sequences of characters (e.g., "Hello, World!")
bool: Boolean values, which can be either True or False
list: Lists, ordered, mutable collections of items (e.g., [1, 2, 3])
tuple: Tuples, ordered, immutable collections of items (e.g., (1, 2, 3))
dict: Dictionaries, unordered collections of key-value pairs (e.g., {"name": "Alice", "age": 30})
set: Sets, unordered collections of unique items (e.g., {1, 2, 3})

Python provides the built-in function type() that can be used to check the type of a variable or value,

>>> type(42)
<class 'int'>

>>> type(3.14)
<class 'float'>

>>> type("Hello, World!")
<class 'str'>

>>> type(True)
<class 'bool'>

>>> type([1, 2, 3])
<class 'list'>

>>> type((1, 2, 3))
<class 'tuple'>

>>> type({"name": "Alice", "age": 30})
<class 'dict'>

>>> type({1, 2, 3})
<class 'set'>

As is the case with most coding languages, integers (-2, -1, 0, 1, etc.) are of the type int and rational numbers (-2.0, -1.3, 0.2, 1.4, etc.) are of the type float. Mixing float and int numbers typically results in converting everything to a float. Using the / division operator to divide integers also results in a float, unless you use the // “floor” division operator

BASH

>>> type(1.0)
<class 'float'>
>>> type(1)
<class 'int'>
>>> 4.2 * 2
8.4
>>> 17 / 3
5.666666666666667
>>> 17 // 3
5

A string in Python is a sequence of characters that usually represent a form of text. In other coding languages, such as Fortran and C, text is usually stored in some form of “character” or “char” datatype. Python, however, does not have a character datatype. Instead, a single character in Python is simply a “string” datatype called str with length 1 (a string with two characters would be of length 2, etc.). Like with numbers, Python also has the ability to manipulate strings.

We have already used a few strings in this lesson when we used the print function, but you can also assign strings to variables using =. To create a string in Python, all you have to do is enclose it in a pair of quotation marks, either a pair of single quotes '...' or double quotes "...". The choice between the two is usually personal preference; however, " is helpful when the string you are dealing with already has a ' in it, and vice versa. For example, consider the following code,

BASH

>>> x = "Hello"
>>> y = 'Hello'
>>> x==y
True
>>> no_error_1 = "You're cool"
>>> error_1 = 'You're cool'

  File "<stdin>", line 1
    error_1 = 'You're cool'
                    ^
SyntaxError: invalid syntax
>>> no_error_2 = ' No "i" in "Team" '
>>> error_2 = " No "i" in "Team" "

  File "<stdin>", line 1
    error_2 = " No "i" in "Team" "
                    ^
SyntaxError: invalid syntax

For a more advanced approach, you can explicitly “escape” the error messages seen above by using either \' or \" in problematic areas, as in

BASH

>>> error_1 = 'You\'re cool'
>>> error_2 = " No \"i\" in \"Team\" "

Similar to numbers, strings also can be manipulated using the + and * operators. More specifically, the + operator combines two strings, while the * operator can be used to repeat a string if used with a number, as in

BASH

>>> "gg" + "ez"
'ggez'
>>> 3 * "yes"
'yesyesyes'
>>> ( 3 * "yes" ) + "no"
'yesyesyesno'

You may recall from the first episode that Python is dynamically typed, which means that the type of a variable is determined at runtime. This allows for flexibility in how you use variables, but it also means that you need to be careful when performing operations on variables of different types, as it may lead to unexpected results and/or errors.

BASH

>>> 42 + 3
45
>>> "foo" + "bar"
'foobar'
>>> "foo" + 42
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str

Type Conversion

Python provides built-in functions for converting between different data types. For example, you can convert a string to an integer using the int() function, and you can convert an integer to a string using the str() function.

BASH

>>> int("42")
42
>>> str(42)
'42'

Challenge

The Cast Awakens

What does str(True) evaluate to?

Show me the solution

PYTHON

'True'

Challenge

The Matrix: Re-casted

What does int(True) evaluate to?

Show me the solution

Boolean values can be converted to integers, where True is converted to 1 and False is converted to 0. We call values such as 1 and True “truthy” values, while values such as 0 and False are called “falsy” values.

PYTHON

Challenge

The Return of the String

What does int("Gandalf") evaluate to?

Show me the solution

PYTHON

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'Gandalf'

Operators in Python

One of the most common things Python is used for is being a fancy numerical calculator.

Python behaves similarly to other languages when performing arithmetic, using the operators +, -, *, / for addition, subtraction, multiplication, and division, respectively.

BASH

>>> 2 + 3
5
>>> 5 - 2
3
>>> 4 * 3
12
>>> 10 / 2
5.0

Callout

Python also supports exponentiation with the ** operator, modular division with the % operator, and “floor” division with the // operator.

BASH

>>> 2 ** 3
8
>>> 7 % 3
1
>>> 7 // 3
2

In Python (and many other programming languages), = is called the “assignment operator” and is used to assign a value to a variable. The == operator is called the “equality operator” and is used to compare two values for equality, returning True if they are equal and False otherwise. Here are some examples of how = and == are used in Python,

BASH

>>> x = 5
>>> y = 10
>>> x == y
False
>>> x == 5
True

The equality operator, ==, will be covered in the next episode, but it is important to understand that = and == are not interchangeable. Using = when you meant to use == can lead to unexpected results and/or errors.

Variables

Variables are used in every aspect of modern life, from the nickname you have for your cat to how we store and analyze data in science and engineering. As it is in real life, variable names in Python can be almost anything, but there are rules and conventions to follow. Rules impose restrictions on how you can name your variables, while conventions are more like guidelines that help make your code more readable and maintainable (e.g., “Pythonic”).

✅ Allowed

Variable names must start with a letter (a-z, A-Z) or an underscore (_)
Variable names can only contain letters, numbers, and underscores

Examples

PYTHON

cake = "chocolate"
_cake = "vanilla"
i_love_cake = True

❌ Not Allowed

Variable names cannot be a reserved keyword in Python (e.g., if, else, for, while, etc.)
Variable names cannot start with a number
Variable names cannot contain spaces (use underscores instead)

Examples

PYTHON

c@ke = "carrot"
0cake = "bettercream"
i love cake = False

Instructor Note

Not all programming languages have the same rules and conventions for variable names.

For example, in R, variable names can contain dots (.) and dashes (-), but in Python, these characters are not allowed in variable names. In JavaScript, variable names can contain dollar signs ($), but in Python, this character is not allowed in variable names. In PowerShell, variable names are not case-sensitive, but in Python, variable names are case-sensitive.

Caution

Python variable names are case-sensitive, meaning that cake, Cake, and CAKE are all different variables.

String Slicing and Indexing

One cool thing about strings is that you can “slice” them to extract specific portions of the string. Slicing is very useful in Python programming, especially when analyzing data, plotting, and sorting. “Indexing” is a subset of slicing that only extracts one character from a string, while “slicing” can extract multiple characters at once (i.e., multiple indices).

Python is “zero-indexed”, meaning indexing starts at 0 instead of 1. The first character in a string has index 0, the second character has index 1, and so on and so forth. To find the full length of a string you can use the len() function. To start, you must put these indices in square brackets [ ], as in

BASH

>>> alphabet = "abcdefghijklmnopqrstuvwxyz"
>>> len(alphabet)
26
>>> alphabet[0]
'a'
>>> alphabet[1]
'b'
>>> alphabet[25]
'z'

Note that in our example we had 26 letters (a to z), but since indexing starts from 0, the final letter in the string is index 25. You can also use negative indices to start at the end of the string. Therefore, you can think of using negative indices as reading from “right to left” instead of “left to right”, as in

BASH

>>> alphabet[-1]
'z'
>>> alphabet[-2]
'y'

Negative indices can be helpful when you don’t know how long a string is, but you know that you want specific characters located at the end of the string, however long the string may be.

you can use the : operator inside of [ ] to pick a range of indices (inserted on either side of the :). Note that when using :, the left side is inclusive while the right side is exclusive. Therefore, for a desired range, this results in a syntax of [desired start index : desired end index + 1 ]. For example, using our alphabet string variable of length 26, we have

BASH

>>> alphabet[0:10]
'abcdefghij'
>>> alphabet[0:5]
'abcde'
>>> alphabet[1:5]
'bcde'
>>> alphabet[1 : (4+1)]  # slices indices 1-4 (same as above, but with math)
'bcde'

Remember that when performing math inside of [ ] on indices, the numbers must be integers (i.e., int type objects) and/or the math that should result in an integer.

Instead of explicitly indicating the start/end of a slice with indices, you can choose to leave a side of the : blank. Leaving the left side blank means that you want the slice to begin at the start of the string, while leaving the right side blank means you want the slice to stop at the end of the string. Therefore, leaving both sides blank (i.e., [ : ]) indicates that you just want the entire string returned. Using our alphabet string variable again, we have

BASH

>>> alphabet[:]
'abcdefghijklmnopqrstuvwxyz'
>>> alphabet[2:]
'cdefghijklmnopqrstuvwxyz'
>>> alphabet[:8]
'abcdefgh'
>>> alphabet[:-1]
'abcdefghijklmnopqrstuvwxy'
>>> alphabet[:-2]
'abcdefghijklmnopqrstuvwx'

Because of the left side of : being included and the right side being excluded, this makes sure that alphabet[ : index ] + alphabet[ index : ] is always equal to alphabet

BASH

>>> alphabet[:2]
'ab'
>>> alphabet[2:]
'cdefghijklmnopqrstuvwxyz'
>>> alphabet[:2] + alphabet[2:]
'abcdefghijklmnopqrstuvwxyz'
>>> alphabet[:4] + alphabet[4:]
'abcdefghijklmnopqrstuvwxyz'

The List Frontier

The next thing to cover is how to group data – specifically by using something in Python called a “list”. Lists are defined as a “compound” datatype and are able to group together both numbers and strings, either individually or in a mix. The data contained in a list are comma separated and enclosed by square brackets.

Here is a short lists of examples of lists in Python,

PYTHON

# empty list
blank_list = []

# list of length 1
some_list = ["hola"]

# list of numbers of length 5
numbers = [1, 2, 3, 4, 5]

# list of mixed types
mixed = ["foo", 3.14, "bar", True]

Unlike with strings, you can modify a specific entry in a list with slicing/indexing, as in

PYTHON

squares = [0, 1, 4, 9, 16, 25, 'temp'] 
print(squares[6])
# 'temp'

squares[6] = 36
print(squares)
# [0, 1, 4, 9, 16, 25, 36]

This slicing ability can also be used to modify sections of lists, or be used to completely remove sections, as in

PYTHON

letters = ['a', 'b', 'c', 'd', 'e']
print(letters[2:4])
# ['c', 'd']

letters[2:4] = ['C', 'D']
print(letters)
# ['a', 'b', 'C', 'D', 'e']

letters[2:4] = []
print(letters)
# ['a', 'b', 'e']

letters[:] = []
print(letters)
# []

Lists are a powerful data structure in Python (and almost every other programming language) and are used extensively in data analysis, machine learning, and many other applications.

Challenge

Knowledge Check

What does (alphabet[13] + alphabet[:1]) * int("8") evaluate to?

Show me the solution

PYTHON

'nananananananana'

We can break this down step by step.

alphabet[13] gives us the character at index 13, which is 'n'.
alphabet[:1] gives us the slice of the string from the start up to (but not including) index 1, which is 'a'.
Adding these together with + gives us 'n' + 'a', which results in the string 'na'.
int("8") converts the string "8" to the integer 8.
Finally, multiplying the string 'na' by 8 with * gives us 'na' * 8, which results in the string 'nananananananana'.

A`dict`-ed to Python

We cannot cover every data type in Python in this lesson, but we will briefly mention one more important data type — the “dictionary” (or dict for short). A dictionary is a collection of key-value pairs, where each key is unique and maps to a specific value. Dictionaries are defined using curly braces {} and the key-value pairs are separated by colons, as in

PYTHON

lookup = {'name': 'Batman', 'city': 'Gotham'}

Values in a dictionary can be accessed using their corresponding keys, as in

PYTHON

name = lookup['name']
print(name)

OUTPUT

'Batman'

Values can also be modified or added to a dictionary using their keys, as in

PYTHON

lookup['name'] = 'Superman'
lookup['city'] = 'Metropolis'
print(lookup['name'])

OUTPUT

'Superman'

Key Points

Python has built-in types that describe the nature of values and how they can be manipulated.
The type() function can be used to check the type of a variable or value.
Python is dynamically typed, meaning that the type of a variable can change at runtime.
Python provides built-in functions for converting between different data types.
Python supports various operators for performing arithmetic and other operations on values.
String slicing and indexing allow you to extract specific portions of a string based on their position.
Lists are a compound data type that can hold multiple values of different types and can be modified using slicing and indexing.

Types and Operators

Overview

Questions

Objectives

Types in Python

Instructor Note

BASH

BASH

BASH

BASH

BASH

Type Conversion

BASH

The Cast Awakens

Show me the solution

PYTHON

The Matrix: Re-casted

Show me the solution

PYTHON

The Return of the String

Show me the solution

PYTHON

Operators in Python

BASH

BASH

BASH

Variables

✅ Allowed

Examples

PYTHON

❌ Not Allowed

Examples

PYTHON

Instructor Note

String Slicing and Indexing

BASH

BASH

BASH

BASH

BASH

The List Frontier

PYTHON

PYTHON

PYTHON

Knowledge Check

Show me the solution

PYTHON

Adict-ed to Python

PYTHON

PYTHON

OUTPUT

PYTHON

OUTPUT

A`dict`-ed to Python