Python 3 Cheat Sheet

Posted on May 14, 2024 in Computers

Python 3 Cheat sheet – CWK

Comments

A comment is a piece of text within a program that is not executed. It can be used to provide additional information to aid in understanding the code.

The # character is used to start a comment and it continues until the end of the line.

# Comment on a single lineuser “JDo”# Comment after code

Arithmetic Operations

Python supports different types of arithmetic operations that can be performed on literal numbers, variables, or some combination. The primary arithmetic operators are:

  • + for addition
  • - for subtraction
  • * for multiplication
  • / for division
  • % for modulus (returns the remainder)
  • ** for exponentiation

# Arithmetic operationsresult =10+30 result =40-10 result =50*5 result =16/4 result =25%2 result =5**3Plus-Equals Operator +=

The plus-equals operator += provides a convenient way to add a value to an existing variable and assign the new value back to the same variable. In the case where the variable and the value are strings, this operator performs string concatenation instead of addition.

The operation is performed in-place, meaning that any other variable which points to the variable being updated will also be updated.

# Plus-Equal Operatorcounter =0 counter +=10# This is equivalent tocounter =0 counter = counter +10# The operator will also perform string concatenationmessage “Part 1 of message” message +”Part 2 of message

Variables

A variable is used to store data that will be used by the program. This data can be a number, a string, a Boolean, a list or some other data type. Every variable has a name which can consist of letters, numbers, and the underscore character _.

The equal sign = is used to assign a value to a variable. After the initial assignment is made, the value of a variable can be updated to new values as needed.

# These are all valid variable names and assignmentuser_name “code” user_id =100 verified =False# A variable’s value can be changed after assignmentpoints =100 points =120

Modulo Operator %

A modulo calculation returns the remainder of a division between the first and second number. For example:

  • The result of the expression 4 % 2 would result in the value 0, because 4 is evenly divisible by 2 leaving no remainder.
  • The result of the expression 7 % 3 would return 1, because 7 is not evenly divisible by 3, leaving a remainder of 1.

Integers

An integer is a number that can be written without a fractional part (no decimal). An integer can be a positive number, a negative number or the number 0 so long as there is no decimal portion.

The number 0 represents an integer value but the same number written as 0.0 would represent a floating point number.

# Example integer numberschairs =4 tables =1 broken_chairs =-2 sofas =0# Non-integer numberslights =2.5 left_overs =0.0

# Modulo operationszero =8%4nonzero =12%5

 

String Concatenation

Python supports the joining (concatenation) of strings together using the + operator. The + operator is also used for mathematical addition operations. If the parameters passed to the + operator are strings, then concatenation will be performed. If the parameter passed to + have different types, then Python will report an error condition. Multiple variables or literal strings can be joined together using the + operator.

# String concatenationfirst =”Hello ” second =”World”result = first + secondlong_result = first + second +”!”

Strings

A string is a sequence of characters (letters, numbers, whitespace or punctuation) enclosed by quotation marks. It can be enclosed using either the double quotation mark " or the single quotation mark '.

If a string has to be broken into multiple lines, the backslash character \ can be used to indicate that the string continues on the next line.

user =”User Full Name” game =’Monopoly’longer = “This string is broken up \ over multiple lines”

print() Function

The print() function is used to output text, numbers, or other printable information to the console.

It takes one or more arguments and will output each of the arguments to the console separated by a space. If no arguments are provided, the print() function will output a blank line.

print(“Hello World!”)print(100)pi =3.14159 print(pi)



break Keyword

In a loop, the break keyword escapes the loop, regardless of the iteration number. Once break executes, the program will continue to execute after the loop.

In this example, the output would be:

  • 0
  • 254
  • 2
  • Negative number detected!
numbers =[0,254,2,-1,3]for num in numbers: if(num

Python List Comprehension

Python list comprehensions provide a concise way for creating lists. It consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses: [EXPRESSION for ITEM in LIST ].

The expressions can be anything – any kind of object can go into a list.

A list comprehension always returns a list.

# List comprehension for the squares of all even numbers between 0 and 9 result =[x**2for x inrange(10)if x %2==0]print(result) # [0, 4, 16, 36, 64]

Python For Loop

A Python for loop can be used to iterate over a list of items and perform a set of actions on each item. The syntax of a for loop consists of assigning a temporary value to a variable on each successive iteration.

When writing a for loop, remember to properly indent each action, otherwise an IndentationError will result.

forin:

#each num in nums will be printed below nums =[1,2,3,4,5] for num in nums: print(num)

The Python continue Keyword

In Python, the continue keyword is used inside a loop to skip the remaining code inside the loop code block and begin the next loop iteration.

big_number_list =[1,2,-1,4,-5,5,2,-9]# Print only positive numbers: for i in big_number_list: if i

Python Loops with range().

In Python, a for loop can be used to perform an action a specific number of times in a row.

The range() function can be used to create a list that can be used to specify the number of iterations in a for loop.

# Print the numbers 0, 1, 2: for i inrange(3): print(i)# Print “WARNING” 3 times: for i inrange(3): print(“WARNING”)

Infinite Loop

An infinite loop is a loop that never terminates. Infinite loops result when the conditions of the loop prevent it from terminating. This could be due to a typo in the conditional statement within the loop or incorrect logic. To interrupt a Python program that is running forever, press the Ctrl and C keys together on your keyboard.

Python while Loops

In Python, a while loop will repeatedly execute a code block as long as a condition evaluates to True.

The condition of a while loop is always checked first before the block of code runs. If the condition is not met initially, then the code block will never run.

# This loop will only run 1 time hungry =True while hungry: print(“Time to eat!”) hungry =False# This loop will run 5 times i =1 while i

Python Nested Loops

In Python, loops can be nested inside other loops. Nested loops can be used to access items of lists which are inside other lists. The item selected from the outer loop can be used as the list for the inner loop to iterate over.

groups =[[“Jobs”,”Gates”],[“Newton”,”Euclid”],[“Einstein”,”Feynman”]]# This outer loop will iterate over each list in the groups list for group in groups: # This inner loop will go through each name in each list for name in group: print(name)

if Statement

The Python if statement is used to determine the execution of code based on the evaluation of a Boolean expression.

  • If the if statement expression evaluates to True, then the indented code following the statement is executed.
  • If the expression evaluates to False then the indented code following the if statement is skipped and the program executes the next line of code which is indented at the same level as the if statement.
# if Statementtest_value =100if test_value >1: # Expression evaluates to True print(“This code is executed!”)if test_value >1000: # Expression evaluates to False print(“This code is NOT executed!”)print(“Program continues at this point.”)

else Statement

The Python else statement provides alternate code to execute if the expression in an if statement evaluates to False.

The indented code for the if statement is executed if the expression evaluates to True. The indented code immediately following the else is executed only if the expression evaluates to False. To mark the end of the else block, the code must be unindented to the same level as the starting if line.

# else Statementtest_value =50if test_value = 1″)test_string =”VALID”if test_string ==”NOT_VALID”: print(“String equals NOT_VALID”) else: print(“String equals something else!”)

elif Statement

The Python elif statement allows for continued checks to be performed after an initial if statement. An elif statement differs from the else statement because another expression is provided to be checked, just as with the initial if statement.

If the expression is True, the indented code following the elif is executed. If the expression evaluates to False, the code can continue to an optional else statement. Multiple elif statements can be used following an initial if to perform a series of checks. Once an elif expression evaluates to True, no further elif statements are executed.

# elif Statementpet_type =”fish”if pet_type ==”dog”: print(“You have a dog.”) elif pet_type ==”cat”: print(“You have a cat.”) elif pet_type ==”fish”: # this is performed print(“You have a fish”) else: print(“Not sure!”)

or Operator

The Python or operator combines two Boolean expressions and evaluates to True if at least one of the expressions returns True. Otherwise, if both expressions are False, then the entire expression evaluates to False.

TrueorTrue# Evaluates to True TrueorFalse# Evaluates to True FalseorFalse# Evaluates to False 16# Evaluates to False 1==1or1

Equal Operator ==

The equal operator, ==, is used to compare two values, variables or expressions to determine if they are the same.

If the values being compared are the same, the operator returns True, otherwise it returns False.

The operator takes the data type into account when making the comparison, so a string value of "2" is not considered the same as a numeric value of 2.

# Equal operatorif’Yes’==’Yes’: # evaluates to True print(‘They are equal’)if(2>1)==(5

Not Equals Operator !=

The Python not equals operator, !=, is used to compare two values, variables or expressions to determine if they are NOT the same. If they are NOT the same, the operator returns True. If they are the same, then it returns False.

The operator takes the data type into account when making the comparison so a value of 10 would NOT be equal to the string value "10" and the operator would return True. If expressions are used, then they are evaluated to a value of True or False before the comparison is made by the operator.

# Not Equals Operatorif”Yes”!=”No”: # evaluates to True print(“They are NOT equal”)val1 =10 val2 =20if val1 != val2: print(“They are NOT equal”)if(10>1)!=(10>1000): # True != False print(“They are NOT equal”)

Comparison Operators

In Python, relational operators compare two values or expressions. The most common ones are:

  •  less than
  • > greater than
  •  less than or equal to
  • >= greater than or equal too

If the relation is sound, then the entire expression will evaluate to True. If not, the expression evaluates to False.

a =2 b =3 a b # evaluates to False a >= b # evaluates to False a

Lists

In Python, lists are ordered collections of items that allow for easy use of a set of data.

List values are placed in between square brackets [ ], separated by commas. It is good practice to put a space between the comma and the next value. The values in a list do not need to be unique (the same value can be repeated).

Empty lists do not contain any values within the square brackets.

primes =[2,3,5,7,11] print(primes)empty_list =[]

Adding Lists Together

In Python, lists can be added to each other using the plus symbol +. As shown in the code block, this will result in a new list containing the same items in the same order with the first list’s items coming first.

Note: This will not work for adding one item at a time (use .append() method). In order to add one item, create a new list with a single value and then use the plus symbol to add the list.

items =[‘cake’,’cookie’,’bread’] total_items = items +[‘biscuit’,’tart’] print(total_items) # Result: [‘cake’, ‘cookie’, ‘bread’, ‘biscuit’, ‘tart’]

Python Lists: Data Types

In Python, lists are a versatile data type that can contain multiple different data types within the same square brackets. The possible data types within a list include numbers, strings, other objects, and even other lists.

numbers =[1,2,3,4,10] names =[‘Jenny’,’Sam’,’Alexis’] mixed =[‘Jenny’,1,2] list_of_lists =[[‘a’,1],[‘b’,2]]

List Method .append()

In Python, you can add values to the end of a list using the .append() method. This will place the object passed in as a new element at the very end of the list. Printing the list afterwards will visually show the appended value. This .append() method is not to be confused with returning an entirely new list with the passed object.

orders =[‘daisies’,’periwinkle’] orders.append(‘tulips’) print(orders) # Result: [‘daisies’, ‘periwinkle’, ‘tulips’]

Zero-Indexing

In Python, list index begins at zero and ends at the length of the list minus one. For example, in this list, 'Andy' is found at index 2.

names =[‘Roger’,’Rafael’,’Andy’,’Novak’]

List Indices

Python list elements are ordered by index, a number referring to their placement in the list. List indices start at 0 and increment by one.

To access a list element by index, square bracket notation is used: list[index].

berries =[“blueberry”,”cranberry”,”raspberry”]berries[0]# “blueberry” berries[2]# “raspberry”

Negative List Indices

Negative indices for lists in Python can be used to reference elements in relation to the end of a list. This can be used to access single list elements or as part of defining a list range. For instance:

  • To select the last element, my_list[-1].
  • To select the last three elements, my_list[-3:].
  • To select everything except the last two elements, my_list[:-2].
soups =[‘minestrone’,’lentil’,’pho’,’laksa’] soups[-1]# ‘laksa’ soups[-3:]# ‘lentil’, ‘pho’, ‘laksa’ soups[:-2]# ‘minestrone’, ‘lentil’

Modifying 2D Lists

In order to modify elements in a 2D list, an index for the sublist and the index for the element of the sublist need to be provided. The format for this is list[sublist_index][element_in_sublist_index] = new_value.

# A 2D list of names and hobbies class_name_hobbies =[[“Jenny”,”Breakdancing”],[“Alexus”,”Photography”],[“Grace”,”Soccer”]]# The sublist of Jenny is at index 0. The hobby is at index 1 of the sublist. class_name_hobbies[0][1]=”Meditation” print(class_name_hobbies)# Output # [[“Jenny”, “Meditation”], [“Alexus”, “Photography”], [“Grace”, “Soccer”]]

Accessing 2D Lists

In order to access elements in a 2D list, an index for the sublist and the index for the element of the sublist both need to be provided. The format for this is list[sublist_index][element_in_sublist_index].

# 2D list of people’s heights heights =[[“Noelle”,61],[“Ali”,70],[“Sam”,67]] # Access the sublist at index 0, and then access the 1st index of that sublist. noelles_height = heights[0][1] print(noelles_height)# Output # 61

List Method .remove()

The .remove() method in Python is used to remove an element from a list by passing in the value of the element to be removed as an argument. In the case where two or more elements in the list have the same value, the first occurrence of the element is removed.

# Create a list shopping_line =[“Cole”,”Kip”,”Chris”,”Sylvana”,”Chris”]# Removes the first occurance of “Chris” shopping_line.remove(“Chris”) print(shopping_line)# Output # [“Cole”, “Kip”, “Sylvana”, “Chris”]

List Method .count()

The .count() Python list method searches a list for whatever search term it receives as an argument, then returns the number of matching entries found.

backpack =[‘pencil’,’pen’,’notebook’,’textbook’,’pen’,’highlighter’,’pen’] numPen = backpack.count(‘pen’)print(numPen) # Output: 3

Determining List Length with len()

The Python len() function can be used to determine the number of items found in the list it accepts as an argument.

knapsack =[2,4,3,7,10] size =len(knapsack) print(size) # Output: 5

List Method .sort()

The .sort() Python list method will sort the contents of whatever list it is called on. Numerical lists will be sorted in ascending order, and lists of Strings will be sorted into alphabetical order. It modifies the original list, and has no return value.

exampleList =[4,2,1,3] exampleList.sort() print(exampleList) # Output: [1, 2, 3, 4]

List Slicing

A slice, or sub-list of Python list elements can be selected from a list using a colon-separated starting and ending point.

The syntax pattern is myList[START_NUMBER:END_NUMBER]. The slice will include the START_NUMBER index, and everything until but excluding the END_NUMBER item.

When slicing a list, a new list is returned, so if the slice is saved and then altered, the original list remains the same.

tools =[‘pen’,’hammer’,’lever’] tools_slice = tools[1:3]# [‘hammer’, ‘lever’] tools_slice[0]=’nail’# Original list is unaltered: print(tools)# [‘pen’, ‘hammer’, ‘lever’]

sorted() Function

The Python sorted() function accepts a list as an argument, and will return a new, sorted list containing the same elements as the original. Numerical lists will be sorted in ascending order, and lists of Strings will be sorted into alphabetical order. It does not modify the original, unsorted list.

unsortedList =[4,2,1,3] sortedList =sorted(unsortedList) print(sortedList) # Output: [1, 2, 3, 4]

List Method .insert()

The Python list method .insert() allows us to add an element to a specific index in a list.

It takes in two inputs:

  • The index that you want to insert into.
  • The element that you want to insert at the specified index.
# Here is a list representing a line of people at a store store_line =[“Karla”,”Maxium”,”Martim”,”Isabella”]# Here is how to insert “Vikor” after “Maxium” and before “Martim” store_line.insert(2,”Vikor”)print(store_line) # Output: [‘Karla’, ‘Maxium’, ‘Vikor’, ‘Martim’, ‘Isabella’]

List Method .pop()

The .pop() method allows us to remove an element from a list while also returning it. It accepts one optional input which is the index of the element to remove. If no index is provided, then the last element in the list will be removed and returned.

cs_topics =[“Python”,”Data Structures”,”Balloon Making”,”Algorithms”,”Clowns 101″]# Pop the last element removed_element = cs_topics.pop()print(cs_topics) print(removed_element)# Output: # [‘Python’, ‘Data Structures’, ‘Balloon Making’, ‘Algorithms’] # ‘Clowns 101’# Pop the element “Baloon Making” cs_topics.pop(2) print(cs_topics)# Output: # [‘Python’, ‘Data Structures’, ‘Algorithms’]