Reference

Click on a function to read more.

Basics part 1

Syntax

Short description

break

break interrupts code that is repeated

Example

while True:
    print('You are nice!')
    answer = input('Should I stop saying that? Write yes in that case.')
    if answer == 'yes':
        break

Explanation

break interrupts code that is repeated. For example when while is used.

continue

continue interrupts a repetition and starts the next

Example

i = 0
while i < 100: 
    if i % 2 == 0: # continue with the next repetition if i is divisible by 2
        continue
    print(i,'is an odd number')

Explanation

continue interrupts the execution in the current repetition/iteration and continues with the next repetition. Used in while-statements and for-statements.

elif

elif is a combination of else and if

Example #1

temp = int(input('What is the temperature?'))
if temp > 20:
    print('Warm and nice!')
elif temp > 0: #if temp <= 20 and temp > 0
    print('Not very cold, but not so warm either.')

Example #2

number = int(input('Enter an integer'))
if number == 0:
    print('You entered the number 0.')
elif number > 0:
    print('You entered a positive number.')
else:
    print('You entered a negative number.')

Explanation

elif is only used after if. It is a combination of else and if, meaning ELSE (if the above is false) and IF (if the new condition is true). The condition for elif is only tested if the condition for if is false.

Syntax

elif condition:

condition

Required. Something that is True or False.

else

else can be used last in an if-statement

Example

age = int(input('How old are you?'))
if age >= 18:
    print('You are an adult!')
else:
    print('You are not an adult.')

Explanation

else is only used at the end of a block that starts with if, and the code under else is only executed if the condition for if is false.

float()

Converts to type float (decimal number)

Example

float('0.01')  #converts the string '0.01' to 0.01
float(10)  #converts the integer 10 to 10.0

Explanation

The function float() converts the argument to type float (decimal number). Usually, the argument is a string or an integer.

Syntax

float(number)

Argument

number

A number in the form of a string or an integer.

if controls the code to do different things depending on a condition

Example #1

number = 10
if number == 10:
    print('The number is equal to 10.') #this will be printed

Example #2

number = int(input('Enter an integer'))
if number == 0:
    print('You entered the number 0.')
elif number > 0:
    print('You entered a positive number.')
else:
    print('You entered a negative number.')

Explanation

if is used to control what the program does depending on a condition. If the condition is true, the code in the if-block is executed. To control what happens if the condition is not true, elif and else can be used.

Syntax

if condition:

condition

Required. Something that is True or False.

input()

Receives input from the user

Example

answer = input('What is your name?')  #The user's answer is saved in the variable answer

Explanation

The function input() allows the user to enter input into the program. It is possible to include text that describes what should be entered.

Syntax

input(prompt)

Argument

prompt

An optional text the user sees.

int()

Converts to type int (integer)

Example

int(42)  #converts the string '42' to 42
int(3.94)  #converts the decimal number 3.94 to 3

Explanation

The function int() converts the argument to type int (integer). Usually, the argument is a string or a decimal number.

Syntax

int(number)

Argument

number

A number in the form of a string or a decimal number.

print()

Prints text or the content of a variable to the screen

Example #1

print('Hello World')  #prints "Hello World"
print(1+1) #prints the number 2
print(x)  #prints the content of variable x

Example #2

print('Hello World', end='')  #prints "Hello World" without a new line

Example #3

print('Hello','to','you')  #Prints "Hello to you"
print('Hello','to','you',sep=' | ',end='!')  #prints "Hello | to | you!" without a new line at the end.

Explanation

The function print() prints a string to the screen. Automatically tries to convert what is to be printed to text.

Syntax

print(*object, sep=' ', end='\n')

Argument

*object

Required. One or more objects to be printed. Separate objects with commas.

sep

Character to separate the objects being printed. Default is space.

end

Character that ends the text being printed. Default is "\n" - which means new line.

randint()

Generates a random integer

Example

from random import randint
roll = randint(1,6)  #a random number 1-6

Explanation

The function randint() generates a random integer between two integer values a and b such that a <= N <= b. randint is in the random module.

Syntax

randint(a,b)

Argument

The smallest random number that can be generated.

The largest random number that can be generated.

round()

Rounds a number

Example #1

round(7.89)  #rounds to 8
round(1.23)  #rounds to 1

Example #2

round(1.23456,2)  #rounds 1.23
round(1.23456,4)  #rounds to 1.2346

Explanation

The function round() rounds a decimal number. By default to an integer, it is possible to set round() to round to an arbitrary number of decimals.

Syntax

round(number, decimalplaces = 0)

Argument

number

Required. The number to be rounded.

decimalplaces

Number of decimals the number should be rounded to. Default is 0.

str()

Converts to type string

Example #1

str(2.71)  #converts the decimal number 2.71 to the string '2.71'
str(10)  #converts the integer 10 to '10'

Example #2

age = 15
print('You are ' + str(age) + ' years old.')

Explanation

The function str() converts the argument to a string. Often the argument is an integer or a decimal number. It can also be significantly more complex data types that can be converted to strings.

Syntax

str(object)

Argument

object

An object that can be converted to a string.

while

while is used to repeat code

Example

n = int(input('How many numbers do you want to print?'))
i = 1
while i <= n:
    print(i)
    i = i + 1

Explanation

while repeats the code in the block as long as the condition is true.

Syntax

while condition:

condition

Required. Something that is True or False. Must become false at some point for the while-block to be exited.

Basics part 2

Syntax

Short description

abs

Absolute value

Example

a = abs(5) # a = 5
b = abs(-5.2) # b = 5.2

Explanation

abs takes the absolute value of a number and thus always returns a positive number.

Syntax

abs(number)

Argument

number

Required. A valid number, integer, float, or complex number.

append

Adds an element to a list

Example #1

my_list = [1,2,3]
my_list.append(4) # now [1,2,3,4]

Example #2

my_list = ["one",2,3.0] # in Python, a list can consist of different types
my_list.append([1,2,3,4]) # now ["one",2,3.0,[1,2,3,4]]

Explanation

append adds an element to the end of a list. The list must already exist. It can be empty.

Syntax

my_list.append(element)

Argument

element

Required. The element to be added to the end of the list.

def

Defines a function

Example #1

def print_hello():
    print('Hello!')

Example #2

def hello_name(name):
    print('Hello ' + name)

hello_name('Tanja') #Prints "Hello Tanja"
hello_name('Emil') #Prints "Hello Emil"

Example #3

def perimeter_rectangle(width, height):
    perimeter = 2*width + 2*height
    return perimeter

Explanation

def defines a new function. A function can have arguments that you can use to pass data to the function. The function can also return data using return.

for

for is used to repeat code

Example #1

for i in range(10):  # i goes from 0 to 9
    print(i) # runs ten times

Example #2

for character in 'Pythonlab.dev':
    print(character)

Example #3

for element in [1,3,3,7]:
    print(element)

Explanation

for repeats the code in the block for all elements in an object. The object is for example a range, list or string.

Syntax

for variable in object:

variable

Required. The variable changes content to the current element for each iteration.

object

Required. The object to iterate over. For example, a list.

len

Returns the number of elements in an object

Example #1

my_list = [1,2,3]
length = len(my_list) # length = 3

Example #2

country = 'Sweden'
length = len(country) # length = 7

Example #3

tuple = ('Sweden', 22472, 'Lund')
length = len(tuple) # length = 3

Explanation

len returns the length or number of elements in an object such as a string, list, tuple, dictionary or a range.

Syntax

len(object)

Argument

object

Required. The object whose length is to be returned.

max

Returns the largest element

Example #1

lista = [1,2,3,4]
largest = max(lista) # largest = 4

Example #2

max(a,0) # 0 if a is negative, otherwise a

Explanation

max finds the largest element.

Syntax

max(object)

Argument

object

Required. An iterable object.

min

Returns the smallest element

Example #1

lista = [1,2,3,4]
smallest = min(lista) # smallest = 1

Example #2

min(a,0) # 0 if a is positive, otherwise a

Explanation

min finds the smallest element.

Syntax

min(object)

Argument

object

Required. An iterable object.

pop

Removes an element from a list

Example #1

lista = [1,2,3]
lista.pop() # now [1,2]

Example #2

lista = [1,2,3]
lista.pop(0) # now [2,3]

Explanation

pop removes an element at a given position from a list.

Syntax

lista.pop(element)

Argument

element

Optional. Index of the element to be removed from the list. Default value is -1 (last element).

return

Returns data from a function

Example

def square_and_add(x,y):
    return x**2 + y**2

result = square_and_add(5,7)
print('Squaring and adding 5 and 7 gives', result)

Explanation

return sends data back from a function to the place where it was called.

sum

Sums the elements in an object

Example #1

lista = [1,2,3,4]
sum_val = sum(lista) # sum_val = 10

Example #2

tupel = (1,2,3)
sum_val = sum(tupel) # sum_val = 6

Explanation

sum adds all elements in an object to a sum.

Syntax

sum(object)

Argument

object

Required. An iterable object.

Sets

In this section you will learn to

Understand what a set is and what it is used for
Add, remove and check if an element exists in the set

What is a set?

Sets are a common concept in mathematics. A set is a collection of elements. Duplicates are not allowed. If we try to add an element that already exists in the set, nothing happens, the set remains the same.

In Python, a set is called set, which is also the English word for set. The most common functions for sets are to add elements, remove elements and check if an element exists in the set.

If only the most common functions are used, the similarities between sets and dictionaries are great. For sets in Python, however, there are specialized functions that are useful specifically for sets. On two sets, for example, we can use the union (combine) and the intersection (see which values are in both sets). For readability, it is recommended to use a set if it is sufficient.

Common functions

Just like for dictionaries, there are two common ways to create a set. See example below.

Example

Create a set with curly braces.

my_set = {1, 2, 3}

Example

Create an empty set and then add elements one by one.

my_set = set()
my_set.add(1)
my_set.add(2)
my_set.add(3)
my_set.add(3) #nothing happens, 3 already exists.

To remove an element from a set, there are among others set.pop() and set.remove(element). The function pop returns the element that is removed and can be used if it does not matter which element is picked out of the list.

Use remove to remove a specific element. Note that remove gives Keyerror if the element you are trying to remove does not exist. If you don't want that, there is set.discard(element) which works even if the element is not in the set.

Example

Remove an element with pop() and remove(element).

pets = {'Cat', 'Dog', 'Goat', 'Zebra'}
pets.remove('Zebra')
removed = pets.pop()

To see if an element exists in the set, in is used just like for keys in a dictionary.

Example

See if an element exists in the set with in.

pets = {'Cat', 'Dog', 'Goat', 'Zebra'}
if 'Dog' in pets:
    print('Dog is in the pet set.')

What does the code do?

Read the code below and try to figure out what the program prints. Run the program after you answer and see if you were correct.

-- Output from your program will be here --

Question: What will the program print?

-61

123

Duplicates and set()

There is a simple trick to count the number of unique elements in a list or a tuple. If the list or tuple is converted to a set, all duplicates are automatically removed. Just remember that the order of the elements is not preserved.

Example

We use set() on a list to remove all duplicates.

my_list = [3, 7, 2, 3, 6, 8, 3]
my_set = set(my_list)
number_of_unique_elements = len(my_set) # 5
number_of_duplicates = len(my_list) - len(my_set) # 2

Iterate over a set

If we want to iterate through the entire set element by element and at the same time keep the set intact, we can easily do so with for.

Example

Iterate over a set. The set looks the same afterwards.

racketlon = {'bordtennis', 'badminton', 'squash', 'tennis'}
for sport in racketlon:
    print(sport)

If we instead want to extract one element at a time until the set is empty, we can use while set: and set.pop() as the example below shows. The while-statement while set: continues until the set is empty. It is therefore important that the set is guaranteed to eventually become empty, otherwise it will be an infinite loop.

Example

Extract one element at a time from a set until the set is empty.

my_set = {'Carina', 'Per-Goran', 'Elisabeth', 'Ola'}
while my_set:
    name = my_set.pop()
    print('Extracted name', name)

Change the code

The program below receives a number of strings from the user and puts them in a list. Use what you have learned about sets to calculate how many unique elements the user has entered. Save the number in the variable unique_elements.

-- Output from your program will be here --

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