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.

More about print and other things

In this section you will learn about

More about the print function
Tips and tricks that make Python life easier

Multiple arguments

By writing multiple arguments to print we can print several things at once. For example, if we want to print the variables x and y, we can write print(x, y). They will then appear on the same line with a space between them. We have of course been able to do this before with print(str(x) + ' ' + str(y)) but you will surely agree that this is easier to write.

Example

Printing several things at once with multiple arguments to print.

print('One', 'Two')
print('One', 'Two', 'Three')

End with something other than a new line

So far, each print has resulted in a new line in the output. This is the default setting for print, but it can be changed with an argument named end. By default, end='\n' where \n is the new line character. Most commonly, end='' (nothing) or end=' ' (space) is used if we want to change the default behavior. This argument must be at the end of the arguments.

Example

Using the argument end when printing in a loop.

sequence = [0, 1, 1, 2, 3, 5]
for number in sequence:
    print(number, end=' ')

Separate with something other than space

As we have seen, the outputs are separated by spaces when we use multiple arguments to print. We can change this using the argument sep.

Example

The argument sep is used to set what should separate outputs.

print(2021,12,24, sep='-')
print('One', 'Two', 'Three', sep=' / ')

Unpack with the * operator

Printing the sequence in the example above can be done without a loop with the operator *. It "unpacks" the content of the list and the elements become arguments to the function instead.

Example

Unpack the elements in the list with the * operator.

sequence = [0, 1, 1, 2, 3, 5]
print(*sequence)
print(*sequence, sep='->') #sep and end can still be used

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?

Test 1 2 3.
End

Test, 1, 2, 3.
End

Test, 1, 2, 3.End

Test 1 2 3.End

Those were some good things to know about print. Now we will move on to other details in Python that can simplify life and are good to know.

Strings can be written with ' or "

At Pythonlab.dev, ' is used for strings, mostly because it only requires one keyboard press with a Swedish keyboard. In Python, however, it does not matter whether single or double quotes are used for strings.

Example

Two different ways to write a string.

a = 'A string'
#is the same as:
b = "A string"

It is not possible to mix single and double quotes for the same string.

New operators

Several times we have written lines like i = i + 1 to add one to the value of i. It can be written abbreviated i += 1. The operator += simply adds a value without us having to write the variable name twice. In the table below you can see more examples of similar operators.

Operator	Example	Same as
+=	i += 1	i = i + 1
−=	i -= 1	i = i - 1
*=	i = 10*	i = i 10*
/=	i /= 2	i = i / 2
//=	i //= 2	i = i // 2
%=	i %= 2	i = i % 2
**=	i = 2	i = i * 2*

The strip and split functions on strings

Sometimes a string contains unwanted characters at the beginning and/or end. An example is when we receive text from the user, then it is easy for a space to be included. We can use the string's built-in function strip() to remove these.

Just as append() and pop() are functions for lists, functions like strip() are only for strings. Such functions are called with a dot after the variable name, like this string.strip(). The function does not change the string, to save the result we need to write string = string.strip().

Example

Use strip() to clear spaces at the beginning and end of a string. Click test to see how many characters the string s has before and after.

s = '   pYtHoNlAbBeT    '
s = s.strip()

strip() also removes characters like \n (new line) and \t (tab).

Another function that comes with strings is split(). By default, it splits a string into smaller parts separated by whitespace (spaces, newlines, tabs). The function returns a list of the components. An example is a sentence, split() splits the sentence into words.

Example

Splits a sentence into words with split()

sentence = 'Hope it is fun to learn Python'
words = sentence.split()
#words = ['Hope','it','is','fun','to','learn','Python']

Since strip() returns a string, it is possible to use split() directly after strip() like this: string.strip().split(). However, it is not possible to use the functions in reverse order because split() returns a list and it is not possible to use strip() on a list.

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?

strange_string

strange string

strange

string

Create

The program should receive a line of integers separated by spaces as the only input.
For example 1 2 3. Print the sum of the numbers.

-- Output from your program will be here --

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