Python is a programming language that lets you work quickly and integrate systems more effectively.
Python is a popular programming language. It was created by Guido van Rossum, and released in 1991.
It is used for:
- Web Development (Server-side)
- Software Development
- Mathematics
- System Scripting
- Python can be used on a server to create web applications.
- Python can be used alongside software to create workflows.
- Python can connect to database systems. It can also read and modify files.
- Python can be used to handle big data and perform complex mathematics.
- Python can be used for rapid prototyping, or for production-ready software development.
- Python works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc).
- Python has a simple syntax similar to the English language.
- Python has syntax that allows developers to write programs with fewer lines than some other programming languages.
- Python runs on an interpreter system, meaning that code can be executed as soon as it is written. This means that prototyping can be very quick.
- Python can be treated in a procedural way, an object-oriented way or a functional way.
- The most recent major version of Python is Python 3, which we shall be using in this tutorial. However, Python 2, although not being updated with anything other than security updates, is still quite popular.
- In this tutorial Python will be written in a text editor. It is possible to write Python in an Integrated Development Environment, such as Thonny, Pycharm, Netbeans or Eclipse which are particularly useful when managing larger collections of Python files.
- Python was designed for readability, and has some similarities to the English language with influence from mathematics.
- Python uses new lines to complete a command, as opposed to other programming languages which often use semicolons or parentheses.
- Python relies on indentation, using whitespace, to define scope; such as the scope of loops, functions and classes. Other programming languages often use curly-brackets for this purpose.
For this guide, you can checkout this blog from Real Python.
Comments can be used to explain Python code.
Comments can be used to make the code more readable.
Comments can be used to prevent execution when testing code.
Comments starts with a #, and Python will ignore them:
#This is a comment
print("Hello, World!")Comments can be placed at the end of a line, and Python will ignore the rest of the line:
print("Hello, World!") #This is a commentA comment does not have to be text that explains the code, it can also be used to prevent Python from executing code:
#print("Hello, World!")
print("Cheers, Mate!")Python does not really have a syntax for multiline comments.
To add a multiline comment you could insert a # for each line:
#This is a comment
#written in
#more than just one line
print("Hello, World!")Or, not quite as intended, you can use a multiline string.
Since Python will ignore string literals that are not assigned to a variable, you can add a multiline string (triple quotes) in your code, and place your comment inside it:
"""
This is a comment
written in
more than just one line
"""
print("Hello, World!")As long as the string is not assigned to a variable, Python will read the code, but then ignore it, and you have made a multiline comment.
Variables are containers for storing data values.
Python has no command for declaring a variable.
A variable is created the moment you first assign a value to it.
x = 5
y = "John"
print(x)
print(y)Variables do not need to be declared with any particular type, and can even change type after they have been set.
x = 4 # x is of type int
x = "Sally" # x is now of type str
print(x)If you want to specify the data type of a variable, this can be done with casting.
x = str(3) # x will be '3'
y = int(3) # y will be 3
z = float(3) # z will be 3.0You can get the data type of a variable with the type() function.
x = 5
y = "John"
print(type(x))
print(type(y))String variables can be declared either by using single or double quotes:
x = "John"
# is the same as
x = 'John'Variable names are case-sensitive.
# This will create two variables:
a = 4
A = "Sally"
#A will not overwrite aA variable can have a short name (like x and y) or a more descriptive name (age, carname, total_volume). Rules for Python variables:
- A variable name must start with a letter or the underscore character
- A variable name cannot start with a number
- A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
- Variable names are case-sensitive (age, Age and AGE are three different variables)
- A variable name cannot be any of the Python keywords.
# Legal variable names:
myvar = "John"
my_var = "John"
_my_var = "John"
myVar = "John"
MYVAR = "John"
myvar2 = "John"# Illegal variable names:
2myvar = "John"
my-var = "John"
my var = "John"Remember that variable names are case-sensitive
Variable names with more than one word can be difficult to read.
There are several techniques you can use to make them more readable:
Each word, except the first, starts with a capital letter:
myVariableName = "John"Each word starts with a capital letter:
MyVariableName = "John"Each word is separated by an underscore character:
my_variable_name = "John"Python allows you to assign values to multiple variables in one line:
x, y, z = "Orange", "Banana", "Cherry"
print(x)
print(y)
print(z)Note: Make sure the number of variables matches the number of values, or else you will get an error.
And you can assign the same value to multiple variables in one line:
x = y = z = "Orange"
print(x)
print(y)
print(z)If you have a collection of values in a list, tuple etc. Python allows you to extract the values into variables. This is called unpacking.
# Unpack a list:
fruits = ["apple", "banana", "cherry"]
x, y, z = fruits
print(x)
print(y)
print(z)The Python print() function is often used to output variables.
x = "Python is awesome"
print(x)In the print() function, you output multiple variables, separated by a comma:
x = "Python"
y = "is"
z = "awesome"
print(x, y, z)You can also use the + operator to output multiple variables:
x = "Python "
y = "is "
z = "awesome"
print(x + y + z)Notice the space character after
"Python "and"is ", without them the result would be "Pythonisawesome".
For numbers, the + character works as a mathematical operator:
x = 5
y = 10
print(x + y)In the print() function, when you try to combine a string and a number with the + operator, Python will give you an error:
x = 5
y = "John"
print(x + y)The best way to output multiple variables in the print() function is to separate them with commas, which even support different data types:
x = 5
y = "John"
print(x, y)Variables that are created outside of a function (as in all of the examples above) are known as global variables.
Global variables can be used by everyone, both inside of functions and outside.
# Create a variable outside of a function, and use it inside the function
x = "awesome"
def myfunc():
print("Python is " + x)
myfunc()If you create a variable with the same name inside a function, this variable will be local, and can only be used inside the function. The global variable with the same name will remain as it was, global and with the original value.
# Create a variable inside a function, with the same name as the global variables
x = "awesome"
def myfunc():
x = "fantastic"
print("Python is " + x)
myfunc()
print("Python is " + x)Normally, when you create a variable inside a function, that variable is local, and can only be used inside that function.
To create a global variable inside a function, you can use the global keyword.
# If you use the global keyword, the variable belongs to the global scope:
def myfunc():
global x
x = "fantastic"
myfunc()
print("Python is " + x)Also, use the global keyword if you want to change a global variable inside a function.
# To change the value of a global variable inside a function, refer to the variable by using the global keyword:
x = "awesome"
def myfunc():
global x
x = "fantastic"
myfunc()
print("Python is " + x)In programming, data type is an important concept.
Variables can store data of different types, and different types can do different things.
Python has the following data types built-in by default, in these categories:
- Text Type:
str - Numeric Types:
int,float,complex - Sequence Types:
list,tuple,range - Mapping Type:
dict - Set Types: set,
frozenset - Boolean Type:
bool - Binary Types:
bytes,bytearray,memoryview - None Type:
NoneType
You can get the data type of any object by using the type() function:
# Print the data type of the variable x:
x = 5
print(type(x))In Python, the data type is set when you assign a value to a variable:
| Example | Data Type |
|---|---|
| x = "Hello World" | str |
| x = 20 | int |
| x = 20.5 | float |
| x = 1j | complex |
| x = ["apple", "banana", "cherry"] | list |
| x = ("apple", "banana", "cherry") | tuple |
| x = range(6) | range |
| x = {"name" : "John", "age" : 36} | dict |
| x = {"apple", "banana", "cherry"} | set |
| x = frozenset({"apple", "banana", "cherry"}) | frozenset |
| x = True | bool |
| x = b"Hello" | bytes |
| x = bytearray(5) | bytearray |
| x = memoryview(bytes(5)) | memoryview |
| x = None | NoneType |
If you want to specify the data type, you can use the following constructor functions:
| Example | Data Type |
|---|---|
| x = str("Hello World") | str |
| x = int(20) | int |
| x = float(20.5) | float |
| x = complex(1j) | complex |
| x = list(("apple", "banana", "cherry")) | list |
| x = tuple(("apple", "banana", "cherry")) | tuple |
| x = range(6) | range |
| x = dict(name="John", age=36) | dict |
| x = set(("apple", "banana", "cherry")) | set |
| x = frozenset(("apple", "banana", "cherry")) | frozenset |
| x = bool(5) | bool |
| x = bytes(5) | bytes |
| x = bytearray(5) | bytearray |
| x = memoryview(bytes(5)) | memoryview |
There are three numeric types in Python:
intfloatcomplex
Variables of numeric types are created when you assign a value to them:
x = 1 # int
y = 2.8 # float
z = 1j # complexTo verify the type of any object in Python, use the type() function:
print(type(x))
print(type(y))
print(type(z))Int, or integer, is a whole number, positive or negative, without decimals, of unlimited length.
# Integers:
x = 1
y = 35656222554887711
z = -3255522
print(type(x))
print(type(y))
print(type(z))Float, or "floating point number" is a number, positive or negative, containing one or more decimals.
# Floats:
x = 1.10
y = 1.0
z = -35.59
print(type(x))
print(type(y))
print(type(z))Float can also be scientific numbers with an "e" to indicate the power of 10.
# Floats:
x = 35e3
y = 12E4
z = -87.7e100
print(type(x))
print(type(y))
print(type(z))Complex numbers are written with a "j" as the imaginary part:
# Complex:
x = 3+5j
y = 5j
z = -5j
print(type(x))
print(type(y))
print(type(z))You can convert from one type to another with the int(), float(), and complex() methods:
# Convert from one type to another:
x = 1 # int
y = 2.8 # float
z = 1j # complex
#convert from int to float:
a = float(x)
#convert from float to int:
b = int(y)
#convert from int to complex:
c = complex(x)
print(a)
print(b)
print(c)
print(type(a))
print(type(b))
print(type(c))Note: You cannot convert complex numbers into another number type.
Python does not have a random() function to make a random number, but Python has a built-in module called random that can be used to make random numbers:
# Import the random module, and display a random number between 1 and 9:
import random
print(random.randrange(1, 10))There may be times when you want to specify a type on to a variable. This can be done with casting. Python is an object-orientated language, and as such it uses classes to define data types, including its primitive types.
Casting in python is therefore done using constructor functions:
int()- constructs an integer number from an integer literal, a float literal (by removing all decimals), or a string literal (providing the string represents a whole number)float()- constructs a float number from an integer literal, a float literal or a string literal (providing the string represents a float or an integer)str()- constructs a string from a wide variety of data types, including strings, integer literals and float literals
# Integers:
x = int(1) # x will be 1
y = int(2.8) # y will be 2
z = int("3") # z will be 3# Floats:
x = float(1) # x will be 1.0
y = float(2.8) # y will be 2.8
z = float("3") # z will be 3.0
w = float("4.2") # w will be 4.2# Strings:
x = str("s1") # x will be 's1'
y = str(2) # y will be '2'
z = str(3.0) # z will be '3.0'Strings in python are surrounded by either single quotation marks, or double quotation marks.
'hello' is the same as "hello".
You can display a string literal with the print() function:
print("Hello")
print('Hello')Assigning a string to a variable is done with the variable name followed by an equal sign and the string:
a = "Hello"
print(a)You can assign a multiline string to a variable by using three quotes:
# You can use three double quotes:
a = """Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua."""
print(a)Or three single quotes:
a = '''Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua.'''
print(a)Note: in the result, the line breaks are inserted at the same position as in the code.
Like many other popular programming languages, strings in Python are arrays of bytes representing unicode characters.
However, Python does not have a character data type, a single character is simply a string with a length of 1.
Square brackets can be used to access elements of the string.
# Get the character at position 1 (remember that the first character has the position 0):
a = "Hello, World!"
print(a[1])Since strings are arrays, we can loop through the characters in a string, with a for loop.
# Loop through the letters in the word "banana":
for x in "banana":
print(x)To get the length of a string, use the len() function.
# The len() function returns the length of a string:
a = "Hello, World!"
print(len(a))To check if a certain phrase or character is present in a string, we can use the keyword in.
# Check if "free" is present in the following text:
txt = "The best things in life are free!"
print("free" in txt)Use it in an if statement:
# Print only if "free" is present:
txt = "The best things in life are free!"
if "free" in txt:
print("Yes, 'free' is present.")To check if a certain phrase or character is NOT present in a string, we can use the keyword not in.
# Check if "expensive" is NOT present in the following text:
txt = "The best things in life are free!"
print("expensive" not in txt)Use it in an if statement:
# print only if "expensive" is NOT present:
txt = "The best things in life are free!"
if "expensive" not in txt:
print("No, 'expensive' is NOT present.")You can return a range of characters by using the slice syntax.
Specify the start index and the end index, separated by a colon, to return a part of the string.
# Get the characters from position 2 to position 5 (not included):
b = "Hello, World!"
print(b[2:5])Note: The first character has index 0.
By leaving out the start index, the range will start at the first character:
# Get the characters from the start to position 5 (not included):
b = "Hello, World!"
print(b[:5])By leaving out the end index, the range will go to the end:
# Get the characters from position 2, and all the way to the end:
b = "Hello, World!"
print(b[2:])Use negative indexes to start the slice from the end of the string:
Example
# Get the characters:
# From: "o" in "World!" (position -5)
# To, but not included: "d" in "World!" (position -2):
b = "Hello, World!"
print(b[-5:-2])Python has a set of built-in methods that you can use on strings.
# The upper() method returns the string in upper case:
a = "Hello, World!"
print(a.upper())# The lower() method returns the string in lower case:
a = "Hello, World!"
print(a.lower())Whitespace is the space before and/or after the actual text, and very often you want to remove this space.
# The strip() method removes any whitespace from the beginning or the end:
a = " Hello, World! "
print(a.strip()) # returns "Hello, World!"# The replace() method replaces a string with another string:
a = "Hello, World!"
print(a.replace("H", "J"))The split() method returns a list where the text between the specified separator becomes the list items.
# The split() method splits the string into substrings if it finds instances of the separator:
a = "Hello, World!"
print(a.split(",")) # returns ['Hello', ' World!']Learn more about String Methods with our String Methods Reference
To concatenate, or combine, two strings you can use the + operator.
# Merge variable a with variable b into variable c:
a = "Hello"
b = "World"
c = a + b
print(c)# To add a space between them, add a " ":
a = "Hello"
b = "World"
c = a + " " + b
print(c)As we learned in the Python Variables chapter, we cannot combine strings and numbers like this:
age = 36
txt = "My name is John, I am " + age
print(txt)But we can combine strings and numbers by using the format() method!
The format() method takes the passed arguments, formats them, and places them in the string where the placeholders {} are:
# Use the format() method to insert numbers into strings:
age = 36
txt = "My name is John, and I am {}"
print(txt.format(age))The format() method takes unlimited number of arguments, and are placed into the respective placeholders:
quantity = 3
itemno = 567
price = 49.95
myorder = "I want {} pieces of item {} for {} dollars."
print(myorder.format(quantity, itemno, price))You can use index numbers {0} to be sure the arguments are placed in the correct placeholders:
quantity = 3
itemno = 567
price = 49.95
myorder = "I want to pay {2} dollars for {0} pieces of item {1}."
print(myorder.format(quantity, itemno, price))To insert characters that are illegal in a string, use an escape character.
An escape character is a backslash \ followed by the character you want to insert.
An example of an illegal character is a double quote inside a string that is surrounded by double quotes:
# You will get an error if you use double quotes inside a string that is surrounded by double quotes:
txt = "We are the so-called "Vikings" from the north."To fix this problem, use the escape character \":
# The escape character allows you to use double quotes when you normally would not be allowed:
txt = "We are the so-called \"Vikings\" from the north."Other escape characters used in Python:
| Code | Result |
|---|---|
| \' | Single Quote |
| \\ | Backslash |
| \n | New Line |
| \r | Carriage Return |
| \t | Tab |
| \b | Backspace |
| \f | Form Feed |
| \ooo | Octal value |
| \xhh | Hex value |
Python has a set of built-in methods that you can use on strings.
Note: All string methods return new values. They do not change the original string.
| Method | Description |
|---|---|
| capitalize() | Converts the first character to upper case |
| casefold() | Converts string into lower case |
| center() | Returns a centered string |
| count() | Returns the number of times a specified value occurs in a string |
| encode() | Returns an encoded version of the string |
| endswith() | Returns true if the string ends with the specified value |
| expandtabs() | Sets the tab size of the string |
| find() | Searches the string for a specified value and returns the position of where it was found |
| format() | Formats specified values in a string |
| format_map() | Formats specified values in a string |
| index() | Searches the string for a specified value and returns the position of where it was found |
| isalnum() | Returns True if all characters in the string are alphanumeric |
| isalpha() | Returns True if all characters in the string are in the alphabet |
| isdecimal() | Returns True if all characters in the string are decimals |
| isdigit() | Returns True if all characters in the string are digits |
| isidentifier() | Returns True if the string is an identifier |
| islower() | Returns True if all characters in the string are lower case |
| isnumeric() | Returns True if all characters in the string are numeric |
| isprintable() | Returns True if all characters in the string are printable |
| isspace() | Returns True if all characters in the string are whitespaces |
| istitle() | Returns True if the string follows the rules of a title |
| isupper() | Returns True if all characters in the string are upper case |
| join() | Joins the elements of an iterable to the end of the string |
| ljust() | Returns a left justified version of the string |
| lower() | Converts a string into lower case |
| lstrip() | Returns a left trim version of the string |
| maketrans() | Returns a translation table to be used in translations |
| partition() | Returns a tuple where the string is parted into three parts |
| replace() | Returns a string where a specified value is replaced with a specified value |
| rfind() | Searches the string for a specified value and returns the last position of where it was found |
| rindex() | Searches the string for a specified value and returns the last position of where it was found |
| rjust() | Returns a right justified version of the string |
| rpartition() | Returns a tuple where the string is parted into three parts |
| rsplit() | Splits the string at the specified separator, and returns a list |
| rstrip() | Returns a right trim version of the string |
| split() | Splits the string at the specified separator, and returns a list |
| splitlines() | Splits the string at line breaks and returns a list |
| startswith() | Returns true if the string starts with the specified value |
| strip() | Returns a trimmed version of the string |
| swapcase() | Swaps cases, lower case becomes upper case and vice versa |
| title() | Converts the first character of each word to upper case |
| translate() | Returns a translated string |
| upper() | Converts a string into upper case |
| zfill() | Fills the string with a specified number of 0 values at the beginning |
Booleans represent one of two values: True or False.
In programming you often need to know if an expression is True or False.
You can evaluate any expression in Python, and get one of two answers, True or False.
When you compare two values, the expression is evaluated and Python returns the Boolean answer:
print(10 > 9)
print(10 == 9)
print(10 < 9)When you run a condition in an if statement, Python returns True or False:
# Print a message based on whether the condition is True or False:
a = 200
b = 33
if b > a:
print("b is greater than a")
else:
print("b is not greater than a")The bool() function allows you to evaluate any value, and give you True or False in return.
# Evaluate a string and a number:
print(bool("Hello"))
print(bool(15))# Evaluate two variables:
x = "Hello"
y = 15
print(bool(x))
print(bool(y))Almost any value is evaluated to True if it has some sort of content.
Any string is True, except empty strings.
Any number is True, except 0.
Any list, tuple, set, and dictionary are True, except empty ones.
# The following will return True:
bool("abc")
bool(123)
bool(["apple", "cherry", "banana"])In fact, there are not many values that evaluate to False, except empty values, such as (), [], {}, "", the number 0, and the value None. And of course the value False evaluates to False.
# The following will return False:
bool(False)
bool(None)
bool(0)
bool("")
bool(())
bool([])
bool({})One more value, or object in this case, evaluates to False, and that is if you have an object that is made from a class with a __len__ function that returns 0 or False:
class myclass():
def __len__(self):
return 0
myobj = myclass()
print(bool(myobj))You can create functions that returns a Boolean Value:
# Print the answer of a function:
def myFunction() :
return True
print(myFunction())You can execute code based on the Boolean answer of a function:
# Print "YES!" if the function returns True, otherwise print "NO!":
def myFunction() :
return True
if myFunction():
print("YES!")
else:
print("NO!")Python also has many built-in functions that return a boolean value, like the isinstance() function, which can be used to determine if an object is of a certain data type:
# Check if an object is an integer or not:
x = 200
print(isinstance(x, int))Operators are used to perform operations on variables and values.
In the example below, we use the + operator to add together two values:
print(10 + 5)Python divides the operators in the following groups:
- Arithmetic operators
- Assignment operators
- Comparison operators
- Logical operators
- Identity operators
- Membership operators
- Bitwise operators
Arithmetic operators are used with numeric values to perform common mathematical operations:
| Operator | Name | Example |
|---|---|---|
| + | Addition | x + y |
| - | Subtraction | x - y |
| * | Multiplication | x * y |
| / | Division | x / y |
| % | Modulus | x % y |
| ** | Exponentiation | x ** y |
| // | Floor division | x // y |
Assignment operators are used to assign values to variables:
| Operator | Example | Same As |
|---|---|---|
| = | x = 5 | x = 5 |
| += | x += 3 | x = x + 3 |
| -= | x -= 3 | x = x - 3 |
| *= | x *= 3 | x = x * 3 |
| /= | x /= 3 | x = x / 3 |
| %= | x %= 3 | x = x % 3 |
| //= | x //= 3 | x = x // 3 |
| **= | x **= 3 | x = x ** 3 |
| &= | x &= 3 | x = x & 3 |
| |= | x |= 3 | x = x | 3 |
| ^= | x ^= 3 | x = x ^ 3 |
| >>= | x >>= 3 | x = x >> 3 |
| <<= | x <<= 3 | x = x << 3 |
Comparison operators are used to compare two values:
| Operator | Name | Example |
|---|---|---|
| == | Equal | x == y |
| != | Not equal | x != y |
| > | Greater than | x > y |
| < | Less than | x < y |
| >= | Greater than or equal to | x >= y |
| <= | Less than or equal to | x <= y |
Logical operators are used to combine conditional statements:
| Operator | Description | Example |
|---|---|---|
| and | Returns True if both statements are true | x < 5 and x < 10 |
| or | Returns True if one of the statements is true | x < 5 or x < 4 |
| not | Reverse the result, returns False if the result is true | not(x < 5 and x < 10) |
Identity operators are used to compare the objects, not if they are equal, but if they are actually the same object, with the same memory location:
| Operator | Description | Example |
|---|---|---|
| is | Returns True if both variables are the same object | x is y |
| is not | Returns True if both variables are not the same object | x is not y |
Membership operators are used to test if a sequence is presented in an object:
| Operator | Description | Example |
|---|---|---|
| in | Returns True if a sequence with the specified value is present in the object | x in y |
| not in | Returns True if a sequence with the specified value is not present in the object | x not in y |
Bitwise operators are used to compare (binary) numbers:
| Operator | Name | Description | Example |
|---|---|---|---|
| & | AND | Sets each bit to 1 if both bits are 1 | x & y |
| | | OR | Sets each bit to 1 if one of two bits is 1 | x | y |
| ^ | XOR | Sets each bit to 1 if only one of two bits is 1 | x ^ y |
| ~ | NOT | Inverts all the bits | ~x |
| << | Zero fill left shift | Shift left by pushing zeros in from the right and let the leftmost bits fall off | x << 2 |
| >> | Signed right shift | Shift right by pushing copies of the leftmost bit in from the left, and let the rightmost bits fall off | x >> 2 |
Operator precedence describes the order in which operations are performed.
# Parentheses has the highest precedence, meaning that expressions inside parentheses must be evaluated first:
print((6 + 3) - (6 + 3))# Multiplication * has higher precedence than addition +, and therefor multiplications are evaluated before additions:
print(100 + 5 * 3)The precedence order is described in the table below, starting with the highest precedence at the top:
| Operator | Description |
|---|---|
() |
Parentheses |
** |
Exponentiation |
+x -x ~x |
Unary plus, unary minus, and bitwise NOT |
* / // % |
Multiplication, division, floor division, and modulus |
+ - |
Addition and subtraction |
<< >> |
Bitwise left and right shifts |
& |
Bitwise AND |
^ |
Bitwise XOR |
| | | Bitwise OR |
== != > >= < <= is is not in not in |
Comparisons, identity, and membership operators |
not |
Logical NOT |
and |
AND |
or |
OR |
If two operators have the same precedence, the expression is evaluated from left to right.
# Addition + and subtraction - has the same precedence, and therefor we evaluate the expression from left to right:
print(5 + 4 - 7 + 3)Lists are used to store multiple items in a single variable.
Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple, Set, and Dictionary, all with different qualities and usage.
Lists are created using square brackets:
# Create a List:
thislist = ["apple", "banana", "cherry"]
print(thislist)List items are ordered, changeable, and allow duplicate values.
List items are indexed, the first item has index [0], the second item has index [1] etc.
When we say that lists are ordered, it means that the items have a defined order, and that order will not change.
If you add new items to a list, the new items will be placed at the end of the list.
Note: There are some list methods that will change the order, but in general: the order of the items will not change.
The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.
Since lists are indexed, lists can have items with the same value:
# Lists allow duplicate values:
thislist = ["apple", "banana", "cherry", "apple", "cherry"]
print(thislist)To determine how many items a list has, use the len() function:
# Print the number of items in the list:
thislist = ["apple", "banana", "cherry"]
print(len(thislist))List items can be of any data type:
# String, int and boolean data types:
list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]A list can contain different data types:
# A list with strings, integers and boolean values:
list1 = ["abc", 34, True, 40, "male"]From Python's perspective, lists are defined as objects with the data type 'list':
<class 'list'>#What is the data type of a list?
mylist = ["apple", "banana", "cherry"]
print(type(mylist))It is also possible to use the list() constructor when creating a new list.
# Using the list() constructor to make a List:
thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
print(thislist)List items are indexed and you can access them by referring to the index number:
#Print the second item of the list:
thislist = ["apple", "banana", "cherry"]
print(thislist[1])Note: The first item has index 0.
Negative indexing means start from the end
-1 refers to the last item, -2 refers to the second last item etc.
# Print the last item of the list:
thislist = ["apple", "banana", "cherry"]
print(thislist[-1])You can specify a range of indexes by specifying where to start and where to end the range.
When specifying a range, the return value will be a new list with the specified items.
# Return the third, fourth, and fifth item:
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[2:5])Note: The search will start at index 2 (included) and end at index 5 (not included).
Remember that the first item has index 0.
By leaving out the start value, the range will start at the first item:
# This example returns the items from the beginning to, but NOT including, "kiwi":
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[:4])By leaving out the end value, the range will go on to the end of the list:
# This example returns the items from "cherry" to the end:
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[2:])Specify negative indexes if you want to start the search from the end of the list:
# This example returns the items from "orange" (-4) to, but NOT including "mango" (-1):
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[-4:-1])To determine if a specified item is present in a list use the in keyword:
# Check if "apple" is present in the list:
thislist = ["apple", "banana", "cherry"]
if "apple" in thislist:
print("Yes, 'apple' is in the fruits list")To change the value of a specific item, refer to the index number:
# Change the second item:
thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print(thislist)To change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values:
# Change the values "banana" and "cherry" with the values "blackcurrant" and "watermelon":
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
thislist[1:3] = ["blackcurrant", "watermelon"]
print(thislist)If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:
#Change the second value by replacing it with two new values:
thislist = ["apple", "banana", "cherry"]
thislist[1:2] = ["blackcurrant", "watermelon"]
print(thislist)Note: The length of the list will change when the number of items inserted does not match the number of items replaced.
If you insert less items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:
# Change the second and third value by replacing it with one value:
thislist = ["apple", "banana", "cherry"]
thislist[1:3] = ["watermelon"]
print(thislist)To insert a new list item, without replacing any of the existing values, we can use the insert() method.
The insert() method inserts an item at the specified index:
# Insert "watermelon" as the third item:
thislist = ["apple", "banana", "cherry"]
thislist.insert(2, "watermelon")
print(thislist)Note: As a result of the example above, the list will now contain 4 items.
To add an item to the end of the list, use the append() method:
# Using the append() method to append an item:
thislist = ["apple", "banana", "cherry"]
thislist.append("orange")
print(thislist)To insert a list item at a specified index, use the insert() method.
The insert() method inserts an item at the specified index:
# Insert an item as the second position:
thislist = ["apple", "banana", "cherry"]
thislist.insert(1, "orange")
print(thislist)Note: As a result of the examples above, the lists will now contain 4 items.
To append elements from another list to the current list, use the extend() method.
# Add the elements of tropical to thislist:
thislist = ["apple", "banana", "cherry"]
tropical = ["mango", "pineapple", "papaya"]
thislist.extend(tropical)
print(thislist)The elements will be added to the end of the list.
The extend() method does not have to append lists, you can add any iterable object (tuples, sets, dictionaries etc.).
# Add elements of a tuple to a list:
thislist = ["apple", "banana", "cherry"]
thistuple = ("kiwi", "orange")
thislist.extend(thistuple)
print(thislist)The remove() method removes the specified item.
# Remove "banana":
thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
print(thislist)The pop() method removes the specified index.
# Remove the second item:
thislist = ["apple", "banana", "cherry"]
thislist.pop(1)
print(thislist)If you do not specify the index, the pop() method removes the last item.
# Remove the last item:
thislist = ["apple", "banana", "cherry"]
thislist.pop()
print(thislist)The del keyword also removes the specified index:
# Remove the first item:
thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist)The del keyword can also delete the list completely.
# Delete the entire list:
thislist = ["apple", "banana", "cherry"]
del thislistThe clear() method empties the list.
The list still remains, but it has no content.
# Clear the list content:
thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist)You can loop through the list items by using a for loop:
# Print all items in the list, one by one:
thislist = ["apple", "banana", "cherry"]
for x in thislist:
print(x)Learn more about for loops in our Python For Loops Chapter.
You can also loop through the list items by referring to their index number.
Use the range() and len() functions to create a suitable iterable.
# Print all items by referring to their index number:
thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)):
print(thislist[i])The iterable created in the example above is [0, 1, 2].
You can loop through the list items by using a while loop.
Use the len() function to determine the length of the list, then start at 0 and loop your way through the list items by referring to their indexes.
Remember to increase the index by 1 after each iteration.
# Print all items, using a while loop to go through all the index numbers
thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
print(thislist[i])
i = i + 1Learn more about while loops in our Python While Loops Chapter.
List Comprehension offers the shortest syntax for looping through lists:
# A short hand for loop that will print all items in a list:
thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]Learn more about list comprehension in the next chapter: List Comprehension.
List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.
Example:
Based on a list of fruits, you want a new list, containing only the fruits with the letter "a" in the name.
Without list comprehension you will have to write a for statement with a conditional test inside:
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []
for x in fruits:
if "a" in x:
newlist.append(x)
print(newlist)With list comprehension you can do all that with only one line of code:
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]
print(newlist)newlist = [expression for item in iterable if condition == True]The return value is a new list, leaving the old list unchanged.
The condition is like a filter that only accepts the items that valuate to True.
# Only accept items that are not "apple":
newlist = [x for x in fruits if x != "apple"]The condition if x != "apple" will return True for all elements other than "apple", making the new list contain all fruits except "apple".
The condition is optional and can be omitted:
# With no if statement:
newlist = [x for x in fruits]The iterable can be any iterable object, like a list, tuple, set etc.
# You can use the range() function to create an iterable:
newlist = [x for x in range(10)]Same example, but with a condition:
# Accept only numbers lower than 5:
newlist = [x for x in range(10) if x < 5]The expression is the current item in the iteration, but it is also the outcome, which you can manipulate before it ends up like a list item in the new list:
# Set the values in the new list to upper case:
newlist = [x.upper() for x in fruits]You can set the outcome to whatever you like:
# Set all values in the new list to 'hello':
newlist = ['hello' for x in fruits]The expression can also contain conditions, not like a filter, but as a way to manipulate the outcome:
# Return "orange" instead of "banana":
newlist = [x if x != "banana" else "orange" for x in fruits]The expression in the example above says:
"Return the item if it is not banana, if it is banana return orange".
List objects have a sort() method that will sort the list alphanumerically, ascending, by default:
# Sort the list alphabetically:
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
print(thislist)# Sort the list numerically:
thislist = [100, 50, 65, 82, 23]
thislist.sort()
print(thislist)To sort descending, use the keyword argument reverse = True:
# Sort the list descending:
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
print(thislist)# Sort the list descending:
thislist = [100, 50, 65, 82, 23]
thislist.sort(reverse = True)
print(thislist)You can also customize your own function by using the keyword argument key = function.
The function will return a number that will be used to sort the list (the lowest number first):
# Sort the list based on how close the number is to 50:
def myfunc(n):
return abs(n - 50)
thislist = [100, 50, 65, 82, 23]
thislist.sort(key = myfunc)
print(thislist)By default the sort() method is case sensitive, resulting in all capital letters being sorted before lower case letters:
# Case sensitive sorting can give an unexpected result:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort()
print(thislist)Luckily we can use built-in functions as key functions when sorting a list.
So if you want a case-insensitive sort function, use str.lower as a key function:
# Perform a case-insensitive sort of the list:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
print(thislist)What if you want to reverse the order of a list, regardless of the alphabet?
The reverse() method reverses the current sorting order of the elements.
# Reverse the order of the list items:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
print(thislist)You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.
There are ways to make a copy, one way is to use the built-in List method copy().
# Make a copy of a list with the copy() method:
thislist = ["apple", "banana", "cherry"]
mylist = thislist.copy()
print(mylist)Another way to make a copy is to use the built-in method list().
# Make a copy of a list with the list() method:
thislist = ["apple", "banana", "cherry"]
mylist = list(thislist)
print(mylist)There are several ways to join, or concatenate, two or more lists in Python.
One of the easiest ways are by using the + operator.
# Join two list:
list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)Another way to join two lists is by appending all the items from list2 into list1, one by one:
# Append list2 into list1:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
for x in list2:
list1.append(x)
print(list1)Or you can use the extend() method, which purpose is to add elements from one list to another list:
# Use the extend() method to add list2 at the end of list1:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1)Python has a set of built-in methods that you can use on lists.
| Method | Description |
|---|---|
| append() | Adds an element at the end of the list |
| clear() | Removes all the elements from the list |
| copy() | Returns a copy of the list |
| count() | Returns the number of elements with the specified value |
| extend() | Add the elements of a list (or any iterable), to the end of the current list |
| index() | Returns the index of the first element with the specified value |
| insert() | Adds an element at the specified position |
| pop() | Removes the element at the specified position |
| remove() | Removes the item with the specified value |
| reverse() | Reverses the order of the list |
| sort() | Sorts the list |
Tuples are used to store multiple items in a single variable.
Tuple is one of 4 built-in data types in Python used to store collections of data, the other 3 are List, Set, and Dictionary, all with different qualities and usage.
A tuple is a collection which is ordered and unchangeable.
Tuples are written with round brackets.
# Create a Tuple:
thistuple = ("apple", "banana", "cherry")
print(thistuple)Tuple items are ordered, unchangeable, and allow duplicate values.
Tuple items are indexed, the first item has index [0], the second item has index [1] etc.
When we say that tuples are ordered, it means that the items have a defined order, and that order will not change.
Tuples are unchangeable, meaning that we cannot change, add or remove items after the tuple has been created.
Since tuples are indexed, they can have items with the same value:
# Tuples allow duplicate values:
thistuple = ("apple", "banana", "cherry", "apple", "cherry")
print(thistuple)To determine how many items a tuple has, use the len() function:
# Print the number of items in the tuple:
thistuple = ("apple", "banana", "cherry")
print(len(thistuple))To create a tuple with only one item, you have to add a comma after the item, otherwise Python will not recognize it as a tuple.
# One item tuple, remember the comma:
thistuple = ("apple",)
print(type(thistuple))
#NOT a tuple
thistuple = ("apple")
print(type(thistuple))Tuple items can be of any data type:
# String, int and boolean data types:
tuple1 = ("apple", "banana", "cherry")
tuple2 = (1, 5, 7, 9, 3)
tuple3 = (True, False, False)A tuple can contain different data types:
# A tuple with strings, integers and boolean values:
tuple1 = ("abc", 34, True, 40, "male")From Python's perspective, tuples are defined as objects with the data type 'tuple':
<class 'tuple'># What is the data type of a tuple?
mytuple = ("apple", "banana", "cherry")
print(type(mytuple))It is also possible to use the tuple() constructor to make a tuple.
# Using the tuple() method to make a tuple:
thistuple = tuple(("apple", "banana", "cherry")) # note the double round-brackets
print(thistuple)You can access tuple items by referring to the index number, inside square brackets:
# Print the second item in the tuple:
thistuple = ("apple", "banana", "cherry")
print(thistuple[1])Note: The first item has index 0.
Negative indexing means start from the end.
-1 refers to the last item, -2 refers to the second last item etc.
# Print the last item of the tuple:
thistuple = ("apple", "banana", "cherry")
print(thistuple[-1])You can specify a range of indexes by specifying where to start and where to end the range.
When specifying a range, the return value will be a new tuple with the specified items.
# Return the third, fourth, and fifth item:
thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")
print(thistuple[2:5])Note: The search will start at index 2 (included) and end at index 5 (not included).
Remember that the first item has index 0.
By leaving out the start value, the range will start at the first item:
# This example returns the items from the beginning to, but NOT included, "kiwi":
thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")
print(thistuple[:4])By leaving out the end value, the range will go on to the end of the list:
# This example returns the items from "cherry" and to the end:
thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")
print(thistuple[2:])Specify negative indexes if you want to start the search from the end of the tuple:
# This example returns the items from index -4 (included) to index -1 (excluded)
thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")
print(thistuple[-4:-1])To determine if a specified item is present in a tuple use the in keyword:
# Check if "apple" is present in the tuple:
thistuple = ("apple", "banana", "cherry")
if "apple" in thistuple:
print("Yes, 'apple' is in the fruits tuple")Tuples are unchangeable, meaning that you cannot change, add, or remove items once the tuple is created.
But there are some workarounds.
Once a tuple is created, you cannot change its values. Tuples are unchangeable, or immutable as it also is called.
But there is a workaround. You can convert the tuple into a list, change the list, and convert the list back into a tuple.
# Convert the tuple into a list to be able to change it:
x = ("apple", "banana", "cherry")
y = list(x)
y[1] = "kiwi"
x = tuple(y)
print(x)Since tuples are immutable, they do not have a built-in append() method, but there are other ways to add items to a tuple.
-
Convert into a list: Just like the workaround for changing a tuple, you can convert it into a list, add your item(s), and convert it back into a tuple.
# Convert the tuple into a list, add "orange", and convert it back into a tuple: thistuple = ("apple", "banana", "cherry") y = list(thistuple) y.append("orange") thistuple = tuple(y)
-
Add tuple to a tuple. You are allowed to add tuples to tuples, so if you want to add one item, (or many), create a new tuple with the item(s), and add it to the existing tuple:
# Create a new tuple with the value "orange", and add that tuple: thistuple = ("apple", "banana", "cherry") y = ("orange",) thistuple += y print(thistuple)
Note: When creating a tuple with only one item, remember to include a comma after the item, otherwise it will not be identified as a tuple.
Note: You cannot remove items in a tuple.
Tuples are unchangeable, so you cannot remove items from it, but you can use the same workaround as we used for changing and adding tuple items:
# Convert the tuple into a list, remove "apple", and convert it back into a tuple:
thistuple = ("apple", "banana", "cherry")
y = list(thistuple)
y.remove("apple")
thistuple = tuple(y)Or you can delete the tuple completely:
# The del keyword can delete the tuple completely:
thistuple = ("apple", "banana", "cherry")
del thistuple
print(thistuple) #this will raise an error because the tuple no longer existsWhen we create a tuple, we normally assign values to it. This is called "packing" a tuple:
# Packing a tuple:
fruits = ("apple", "banana", "cherry")But, in Python, we are also allowed to extract the values back into variables. This is called "unpacking":
# Unpacking a tuple:
fruits = ("apple", "banana", "cherry")
(green, yellow, red) = fruits
print(green)
print(yellow)
print(red)Note: The number of variables must match the number of values in the tuple, if not, you must use an asterisk to collect the remaining values as a list.
If the number of variables is less than the number of values, you can add an * to the variable name and the values will be assigned to the variable as a list:
# Assign the rest of the values as a list called "red":
fruits = ("apple", "banana", "cherry", "strawberry", "raspberry")
(green, yellow, *red) = fruits
print(green)
print(yellow)
print(red)If the asterisk is added to another variable name than the last, Python will assign values to the variable until the number of values left matches the number of variables left.
# Add a list of values the "tropic" variable:
fruits = ("apple", "mango", "papaya", "pineapple", "cherry")
(green, *tropic, red) = fruits
print(green)
print(tropic)
print(red)- Mengsreang-Chhoeung @mengsreang_dev