blog - A Quick Tour of Variables and Data Types in Python

Storing information using variables

Computers are useful for two purposes: storing information and performing operations on stored information. While working with a programming language such as Python, informations is stored in variables. You can think of variables are containers for storing data. The data stored within a variable is called it’s value. It’s really easy to create variables in Python.

my_favorite_color = "black"

my_favorite_color

'black'

A variable is created using an assignment statement, which begins with the variable’s name, followed by the assignment operator = (different from the equality comparision operator ==), followed by the value to be stored within the variable.

You can also values to multiple variables in a single statement by separating the variable names and values with commas.

color1, color2, color3 = "red", "green", "blue"

color1

'red'

color2

'green'

color3

'blue'

You can assign the same value to multiple variables by chaining multiple assignment operations within a single statement.

color4 = color5 = color6 = "magenta"

color4

'magenta'

color5

'magenta'

color6

'magenta'

You can change the value stored within a variable simply by assigning a new value to it using another assignment statement. Be careful while reassgining variables: when you assign a new value to the variable, the old value is lost and no longer accessible.

my_favorite_color = "red"

my_favorite_color

'red'

While assigning a new value to a variable, you can also use the previous value of the variable to determine the new value.

counter = 10

counter = counter + 1

counter

The pattern var = var op something (where op is an arithmetic operator like +, -, *, /) is very commonly used, so Python provides a shorthand syntax for it.

counter = 10

# Same as `counter = counter + 4`
counter += 4

counter

Variable names can be short (a, x, y etc.) or descriptive ( my_favorite_color, profit_margin, the_3_musketeers etc.). Howerver, you must follow these rules while naming Python variables:

A variable’s name must start with a letter or the underscore character _. It cannot start with a number.
A variable name can only contain lowercase or uppercase letters, digits or underscores (a-z, A-Z, 0-9 and _).
Variable names are case-sensitive i.e. a_variable, A_Variable and A_VARIABLE are all different variables.

Here are some valid variable names:

a_variable = 23
is_today_Saturday = False
my_favorite_car = "Delorean"
the_3_musketeers = ["Athos", "Porthos", "Aramis"]

Let’s also try creating some variables with invalid names. Python prints a syntax error if your variable’s name is invalid.

Syntax: The syntax of a programming language refers to the rules which govern what a valid instruction or statement in the language should look like. If a statement does not follow these rules, Python stops execution and informs you that there is a syntax error. You can think of syntax as the rules of grammar for a programming language.

a variable = 23

SyntaxError: ignored

is_today_$aturday = False

SyntaxError: ignored

my-favorite-car = "Delorean"

SyntaxError: ignored

3_musketeers = ["Athos", "Porthos", "Aramis"]

SyntaxError: ignored

Built-in data types in Python

Any data or information stored within a Python variable has a type. The type of data stored within a variable can be checked using the type function.

a_variable

type(a_variable)

int

is_today_Saturday

False

type(is_today_Saturday)

bool

my_favorite_car

'Delorean'

type(my_favorite_car)

str

the_3_musketeers

['Athos', 'Porthos', 'Aramis']

type(the_3_musketeers)

list

Python has several built-in data types for storing different types of information in variables. Following are at some commonly used data types:

Integer
Float
Boolean
None
String
List
Tuple
Dictionary

Integer, float, boolean, None and string are primitive data types because they represent a single value. Other data types like list, tuple and dictionary are often called data structures or containers because they hold multiple pieces of data together.

Integer

Integers represent positive or negative whole numbers, from negative infinity to infinity. Note that integers should not include decimal points. Integers have the type int.

current_year = 2021

current_year

type(current_year)

int

Unlike some other programming languages, integers in Python can be arbirarily large (or small). There’s no lowest or highest value for integers, and there’s just one int type (as opposed to short, int, long, long long, unsigned int etc. in C/C++/Java).

a_large_negative_number = -23374038374832934334234317348343

a_large_negative_number

-23374038374832934334234317348343

type(a_large_negative_number)

int

Float

Floats (or floating point numbers) are numbers with a decimal point. There are no limits on the value of a float or the number of digits before or after the decimal point. Floating point numbers have the type float.

pi = 3.141592653589793238

pi

3.141592653589793

type(pi)

float

Note that a whole number is treated as a float if it is written with a decimal point, even though the decimal portion of the number is zero.

a_number = 3.0

a_number

3.0

type(a_number)

float

another_number = 4.

another_number

4.0

type(another_number)

float

Floating point numbers can also be written using the scientific notation with an “e” to indicate the power of 10.

one_hundredth = 1e-2

one_hundredth

0.01

type(one_hundredth)

float

avogadro_number = 6.02214076e23

avogadro_number

6.02214076e+23

type(avogadro_number)

float

Floats can be converted into integers and vice versa using the float and int functions. The operation of coverting one type of value into another is called casting.

float(current_year)

2021.0

float(a_large_negative_number)

-2.3374038374832935e+31

int(pi)

int(avogadro_number)

602214075999999987023872

While performing arithmetic operations, integers are automatically converted to floats if any of the operands is a float. Also, the division operator / always returns a float, even if both operands are integers. Use the // operator if you want the result of division to be an int.

type(45 * 3.0)

float

type(45 * 3)

int

type(10/3)

float

type(10/2)

float

type(10//2)

int

Boolean

Booleans represent one of 2 values: True and False. Booleans have the type bool.

is_today_Sunday = True

is_today_Sunday

True

type(is_today_Saturday)

bool

Booleans are generally returned as the result of a comparision operation (e.g. ==, >= etc.).

cost_of_ice_bag = 1.25
is_ice_bag_expensive = cost_of_ice_bag >= 10

is_ice_bag_expensive

False

type(is_ice_bag_expensive)

bool

Booleans are automatically converted to ints when used in arithmetic operations. True is converted to 1 and False is converted to 0.

5 + False

3. + True

4.0

Any value in Python can be converted to a Boolean using the bool function.

Only the following values evaluate to False (they are often called falsy values):

The value False itself
The integer 0
The float 0.0
The empty value None
The empty text ""
The empty list []
The empty tuple ()
The empty dictionary {}
The emtpy set set()
The empty range range(0)

Everything else evaluates to True (a value that evalutes to True is often called a truthy value).

bool(False)

False

bool(0)

False

bool(0.0)

False

bool(None)

False

bool("")

False

bool([])

False

bool(())

False

bool({})

False

bool(set())

False

bool(range(0))

False

bool(True), bool(1), bool(2.0), bool("hello"), bool([1,2]), bool((2,3)), bool(range(10))

(True, True, True, True, True, True, True)

None

The None type includes a single value None, used to indicate the absence of a value. None has the type NoneType. It is often used to declare a variable whose value may be assigned later.

nothing = None

type(nothing)

NoneType

String

A string is used to represent text (a string of characters) in Python. Strings must be surrounded using quotations (either the single quote ' or the double quote "). Strings have the type string.

today = "Friday"

today

'Friday'

type(today)

str

You can use single quotes inside a string written with double quotes, and vice versa.

my_favorite_movie = "One Flew over the Cuckoo's Nest"

my_favorite_movie

"One Flew over the Cuckoo's Nest"

my_favorite_pun = 'Thanks for explaining the word "many" to me, it means a lot.'

my_favorite_pun

'Thanks for explaining the word "many" to me, it means a lot.'

To use the a double quote within a string written with double quotes, escape the inner quotes by prefixing them with the \ character.

another_pun = "The first time I got a universal remote control, I thought to myself \"This changes everything\"."

another_pun

'The first time I got a universal remote control, I thought to myself "This changes everything".'

Strings created using single or double quotes must begin and end on the same line. To create multiline strings, use three single quotes ''' or three double quotes """ to begin and end the string. Line breaks are represented using the newline character \n.

yet_another_pun = '''Son: "Dad, can you tell me what a solar eclipse is?" 
Dad: "No sun."'''

yet_another_pun

'Son: "Dad, can you tell me what a solar eclipse is?" \nDad: "No sun."'

Multiline strings are best displayed using the print function.

print(yet_another_pun)

Son: "Dad, can you tell me what a solar eclipse is?" 
Dad: "No sun."

a_music_pun = """
Two windmills are standing in a field and one asks the other, 
"What kind of music do you like?"  

The other says, 
"I'm a big metal fan."
"""

print(a_music_pun)


Two windmills are standing in a field and one asks the other, 
"What kind of music do you like?"  

The other says, 
"I'm a big metal fan."

You can check the length of a string using the len function.

len(my_favorite_movie)

Note that special characters like \n and escaped characters like \" count as a single character, even though they are written and sometimes printed as 2 characters.

multiline_string = """a
b"""
multiline_string

'a\nb'

len(multiline_string)

A string can be converted into a list of characters using list function.

list(multiline_string)

['a', '\n', 'b']

Strings also support several list operations, which are discussed in the next section. We’ll look at a couple of examples here.

You can access individual characters within a string using the [] indexing notation. Note the character indices go from 0 to n-1, where n is the length of the string.

today = "Saturday"

today[0]

'S'

today[3]

'u'

today[7]

'y'

You can access a part of a string using by providing a start:end range instead of a single index in [].

today[5:8]

'day'

You can also check whether a string contains a some text using the in operator.

'day' in today

True

'Sun' in today

False

Two or more strings can be joined or concatenated using the + operator. Be careful while concatenating strings, sometimes you may need to add a space character " " between words.

full_name = "Derek O'Brien"

greeting = "Hello"

greeting + full_name

"HelloDerek O'Brien"

greeting + " " + full_name + "!" # additional space

"Hello Derek O'Brien!"

String in Python have many built-in methods that can be used to manipulate them. Let’s try out some common string methods.

Methods: Methods are functions associated with data types, and are accessed using the . notatation e.g. variable_name.method() or "a string".method(). Methods are a powerful technique for associating common operations with values of specific data types.

The .lower(), .upper() and .capitalize() methods are used to change the case of the characters.

today.lower()

'saturday'

"saturday".upper()

'SATURDAY'

"monday".capitalize() # changes first character to uppercase

'Monday'

The .replace method is used to replace a part of the string with another string. It takes the portion to be replaced and the replacement text as inputs or arguments.

another_day = today.replace("Satur", "Wednes")

another_day

'Wednesday'

Note that a new string is returned, and the original string is not modified.

today

'Saturday'

The .split method can be used to split a string into a list of strings based using the character(s) provided.

"Sun,Mon,Tue,Wed,Thu,Fri,Sat".split(",")

['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']

The .strip method is used to remove whitespace characters from the beginning and end of a string.

a_long_line = "       This is a long line with some space before, after,     and some space in the middle..    "

a_long_line_stripped = a_long_line.strip()

a_long_line_stripped

'This is a long line with some space before, after,     and some space in the middle..'

The .format method is used to combine values of other data types e.g. integers, floats, booleans, lists etc. with strings. It is often used to create output messages for display.

# Input variables
cost_of_ice_bag = 1.25
profit_margin = .2
number_of_bags = 500

# Template for output message
output_template = """If a grocery store sells ice bags at $ {} per bag, with a profit margin of {} %, 
then the total profit it makes by selling {} ice bags is $ {}."""

print(output_template)

If a grocery store sells ice bags at $ {} per bag, with a profit margin of {} %, 
then the total profit it makes by selling {} ice bags is $ {}.

# Inserting values into the string
total_profit = cost_of_ice_bag * profit_margin * number_of_bags
output_message = output_template.format(cost_of_ice_bag, profit_margin*100, number_of_bags, total_profit)

print(output_message)

If a grocery store sells ice bags at $ 1.25 per bag, with a profit margin of 20.0 %, 
then the total profit it makes by selling 500 ice bags is $ 125.0.

Notice how the placeholders {} in the output_template string are replaced with the arguments provided to the .format method.

It is also possible use the string concatenation operator + to combine strings with other values, however, those values must first be converted to strings using the str function.

"If a grocery store sells ice bags at $ " + cost_of_ice_bag + ", with a profit margin of " + profit_margin

TypeError: ignored

"If a grocery store sells ice bags at $ " + str(cost_of_ice_bag) + ", with a profit margin of " + str(profit_margin)

'If a grocery store sells ice bags at $ 1.25, with a profit margin of 0.2'

In fact, the str can be used to convert a value of any data type into a string.

str(23)

'23'

str(23.432)

'23.432'

str(True)

'True'

the_3_musketeers = ["Athos", "Porthos", "Aramis"]
str(the_3_musketeers)

"['Athos', 'Porthos', 'Aramis']"

Note that all string methods returns new values, and DO NOT change the existing string. You can find a full list of string methods here: https://www.w3schools.com/python/python_ref_string.asp.

Strings also support the comparision operators == and != for checking whether two strings are equal

first_name = "John"

first_name == "Doe"

False

first_name == "John"

True

first_name != "Jane"

True

We’ve looked at the primitive data types in Python, and we’re now ready to explore non-primitive data structures or containers.

List

A list in Python is an ordered collection of values. Lists can hold values of different data types, and support operations to add, remove and change values. Lists have the type list.

To create a list, enclose a list of values within square brackets [ and ], separated by commas.

fruits = ['apple', 'banana', 'cherry']

fruits

['apple', 'banana', 'cherry']

type(fruits)

list

Let’s try creating a list containing values of different data types, including another list.

a_list = [23, 'hello', None, 3.14, fruits, 3 <= 5]

a_list

[23, 'hello', None, 3.14, ['apple', 'banana', 'cherry'], True]

empty_list = []

empty_list

[]

To determine the number of values in a list, use the len function. In general, the len function can be used to determine of values in several other data types.

len(fruits)

print("Number of fruits:", len(fruits))

Number of fruits: 3

len(a_list)

len(empty_list)

You can access the elements of a list using the the index of the element, starting from the index 0.

fruits[0]

'apple'

fruits[1]

'banana'

fruits[2]

'cherry'

If you try to access an index equal to or higher than the length of the list, Python returns an IndexError.

fruits[3]

IndexError: ignored

fruits[4]

IndexError: ignored

fruits[-1]

'cherry'

fruits[-2]

'banana'

fruits[-3]

'apple'

fruits[-4]

IndexError: ignored

You can also access a range of values from the list. The result is itself a list. Let us look at some examples.

a_list = [23, 'hello', None, 3.14, fruits, 3 <= 5]

a_list

[23, 'hello', None, 3.14, ['apple', 'banana', 'cherry'], True]

len(a_list)

a_list[2:5]

[None, 3.14, ['apple', 'banana', 'cherry']]

Note that the start index (2 in the above example) of the range is included in the list, but the end index (5 in the above example) is not included. So, the result has 3 values (indices 2, 3 and 4).

Here are some experiments you should try out (use the empty cells below):

Try setting one or both indices of the range are larger than the size of the list e.g. a_list[2:10]
Try setting the start index of the range to be larger than the end index of the range e.g. list_a[2:10]
Try leaving out the start or end index of a range e.g. a_list[2:] or a_list[:5]
Try using negative indices for the range e.g. a_list[-2:-5] or a_list[-5:-2] (can you explain the results?)

The flexible and interactive nature of Jupyter notebooks makes them a great tool for learning and experimentation. Most questions that arise while you are learning Python for the first time can be resolved by simply typing the code into a cell and executing it. Let your curiosity run wild, and discover what Python is capable of, and what it isn’t!

You can also change the value at a specific index within a list using the assignment operation.

fruits

['apple', 'banana', 'cherry']

fruits[1] = 'blueberry'

fruits

['apple', 'blueberry', 'cherry']

A new value can be added to the end of a list using the append method.

fruits.append('dates')

fruits

['apple', 'blueberry', 'cherry', 'dates']

A new value can also be inserted a specific index using the insert method.

fruits.insert(1, 'banana')

fruits

['apple', 'banana', 'blueberry', 'cherry', 'dates']

You can remove a value from the list using the remove method.

fruits.remove('blueberry')

fruits

['apple', 'banana', 'cherry', 'dates']

What happens if a list has multiple instances of the value passed to .remove? Try it out.

To remove an element from a specific index, use the pop method. The method also returns the removed element.

fruits

['apple', 'banana', 'cherry', 'dates']

fruits.pop(1)

'banana'

fruits

['apple', 'cherry', 'dates']

If no index is provided, the pop method removes the last element of the list.

fruits.pop()

'dates'

fruits

['apple', 'cherry']

You can test whether a list contains a value using the in operator.

'pineapple' in fruits

False

'cherry' in fruits

True

To combine two or more lists, use the + operator. This operation is also called concatenation.

fruits

['apple', 'cherry']

more_fruits = fruits + ['pineapple', 'tomato', 'guava'] + ['dates', 'banana']

more_fruits

['apple', 'cherry', 'pineapple', 'tomato', 'guava', 'dates', 'banana']

To create a copy of a list, use the copy method. Modifying the copied list does not affect the original list.

more_fruits_copy = more_fruits.copy()

more_fruits_copy

['apple', 'cherry', 'pineapple', 'tomato', 'guava', 'dates', 'banana']

# Modify the copy
more_fruits_copy.remove('pineapple')
more_fruits_copy.pop()
more_fruits_copy

['apple', 'cherry', 'tomato', 'guava', 'dates']

# Original list remains unchanged
more_fruits

['apple', 'cherry', 'pineapple', 'tomato', 'guava', 'dates', 'banana']

Note that you cannot create a copy of a list by simply creating a new variable using the assignment operator =. The new variable will point to the same list, and any modifications performed using one variable will affect the other.

more_fruits

['apple', 'cherry', 'pineapple', 'tomato', 'guava', 'dates', 'banana']

more_fruits_not_a_copy = more_fruits

more_fruits_not_a_copy.remove('pineapple')
more_fruits_not_a_copy.pop()

'banana'

more_fruits_not_a_copy

['apple', 'cherry', 'tomato', 'guava', 'dates']

more_fruits

['apple', 'cherry', 'tomato', 'guava', 'dates']

Just like strings, there are several in-built methods to manipulate a list. Unlike strings, however, most list methods modify the original list, rather than returning a new one. Check out some common list operations here: https://www.w3schools.com/python/python_ref_list.asp

Following are some exercises you can try out with list methods (use the blank code cells below):

Reverse the order of elements in a list
Add the elements of one list to the end of another list
Sort a list of strings in alphabetical order
Sort a list of numbers in decreasing order

Tuple

A tuple is an ordered collection of values, similar to a list, however it is not possible to add, remove or modify values in a tuple. A tuple is created by enclosing values within parantheses ( and ), separated by commas.

Any data structure that cannot be modified after creation is called immutable. You can think of tuples as immutable lists.

Let’s try some experiments with tuples.

fruits = ('apple', 'cherry', 'dates')

# check no. of elements
len(fruits)

# get an element (positive index)
fruits[0]

'apple'

# get an element (negative index)
fruits[-2]

'cherry'

# check if it contains an element
'dates' in fruits

True

# try to change an element
fruits[0] = 'avocado'

TypeError: ignored

# try to append an element
fruits.append('blueberry')

AttributeError: ignored

# try to remove an element
fruits.remove('apple')

AttributeError: ignored

You can also skip the parantheses ( and ) while creating a tuple. Python automatically converts comma-separated values into a tuple.

the_3_musketeers = 'Athos', 'Porthos', 'Aramis'

the_3_musketeers

('Athos', 'Porthos', 'Aramis')

You can also create a tuple with just one element, if you include a comma after the element. Just wrapping it with parantheses ( and ) won’t create a tuple.

single_element_tuple = 4,

single_element_tuple

(4,)

another_single_element_tuple = (4,)

another_single_element_tuple

(4,)

not_a_tuple = (4)

not_a_tuple

Tuples are often used to create multiple variables with a single statement.

point = (3, 4)

point_x, point_y = point

point_x

point_y

You can convert a list into a tuple using the tuple function, and vice versa using the list function

tuple(['one', 'two', 'three'])

('one', 'two', 'three')

list(('Athos', 'Porthos', 'Aramis'))

['Athos', 'Porthos', 'Aramis']

Tuples have just 2 built-in methods: count and index. Can you figure out what they do? While look could look for documentation and examples online, there’s an easier way to check the documentation of a method, using the help function.

a_tuple = 23, "hello", False, None, 23, 37, "hello"

help(a_tuple.count)

Help on built-in function count:

count(value, /) method of builtins.tuple instance
    Return number of occurrences of value.

Within a Jupyter notebook, you can also start a code cell with ? and type the name of a function or method. When you execute this cell, you will see the documentation for the function/method in a pop-up window.

?a_tuple.index

Try using count and index with a_tuple in the code cells below.

Dictionary

A dictionary is an unordered collection of items. Each item stored in a dictionary has a key and value. Keys are used to retrieve values from the dictionary. Dictionaries have the type dict.

Dictionaries are often used to store many pieces of information e.g. details about a person, in a single variable. Dictionaries are created by enclosing key-value pairs within curly brackets { and }.

person1 = {
    'name': 'John Doe',
    'sex': 'Male',
    'age': 32,
    'married': True
}

person1

{'age': 32, 'married': True, 'name': 'John Doe', 'sex': 'Male'}

Dictionaries can also be created using the dict function.

person2 = dict(name='Jane Judy', sex='Female', age=28, married=False)

person2

{'age': 28, 'married': False, 'name': 'Jane Judy', 'sex': 'Female'}

type(person1)

dict

Keys can be used to access values using square brackets [ and ].

person1['name']

'John Doe'

person1['married']

True

person2['name']

'Jane Judy'

If a key isn’t present in the dictionary, then a KeyError is returned.

person1['address']

KeyError: ignored

The get method can also be used to access the value associated with a key.

person2.get("name")

'Jane Judy'

The get method also accepts a default value which is returned if the key is not present in the dictionary.

person2.get("address", "Unknown")

'Unknown'

You can check whether a key is present in a dictionary using the in operator.

'name' in person1

True

'address' in person1

False

You can change the value associated with a key using the assignment operator.

person2['married']

False

person2['married'] = True

person2['married']

True

The assignment operator can also be used to add new key-value pairs to the dictonary.

person1

{'age': 32, 'married': True, 'name': 'John Doe', 'sex': 'Male'}

person1['address'] = '1, Penny Lane'

person1

{'address': '1, Penny Lane',
 'age': 32,
 'married': True,
 'name': 'John Doe',
 'sex': 'Male'}

To remove a key and the associated value from a dictionary, use the pop method.

person1.pop('address')

'1, Penny Lane'

person1

{'age': 32, 'married': True, 'name': 'John Doe', 'sex': 'Male'}

Dictonaries also provide methods to view the list of keys, values or key-value pairs inside it.

person1.keys()

dict_keys(['name', 'sex', 'age', 'married'])

person1.values()

dict_values(['John Doe', 'Male', 32, True])

person1.items()

dict_items([('name', 'John Doe'), ('sex', 'Male'), ('age', 32), ('married', True)])

person1.items()[1]

TypeError: ignored

The result of the keys, values or items look like lists but don’t seem to support the indexing operator [] for retrieving elements.

Can you figure out how to access an element at a specific index from these results? Try it below. Hint: Use the list function

Dictionaries provide many other methods. You can learn more about them here: https://www.w3schools.com/python/python_ref_dictionary.asp

Here are some experiments you can try out with dictionaries (use the empty cells below): * What happens if you use the same key multiple times while creating a dictionary? * How can you create a copy of a dictionary (modifying the copy should not change the original)? * Can the value associated with a key itself be a dictionary? * How can you add the key value pairs from one dictionary into another dictionary? Hint: See the update method. * Can the keys of a dictionary be something other than a string e.g. a number, boolean, list etc.?