Operators

Operator	Operation	Example	Evaluates to …
**	Exponent	2 ** 3	8
%	Modulus / Remainder	22 % 8	6
//	Integer division / floored quotient	22 // 8	2
/	Division	22 / 8	2.75
*	Multiplication	3 * 5	15
-	Subtraction	5 - 2	3
+	Addition	2 + 2	4

Data types

Data type	Examples
Integers	-2, -1, 0, 1, 2, 3, 4, 5
Floating-point numbers	-1.25, -1.0, —0.5, 0.0, 0.5, 1.0, 1.25
Strings	’a’, ‘aa’, ‘aaa’, ‘Hello!’, ‘11 cats’

Note

The data type of None is NoneType.

Variables

A variable is like a box in the computer’s memory where you can store a single value. If you want to use the result of an evaluated expression later in your program, you can save it inside a variable.

It can be only one word.
It can use only letters,numbers, ans the underscore(_) character.
It can’t begin with a number.

Valid variable names	Invalid variable names
balance	current-balance (hyphens are not allowed)
currentBalance	current balance (spaces are not allowed)
current_balance	4account (can’t begin with a number)
_spam	42 (can’t begin with a number)
SPAM	`total_$um` (special character like $ are not allowed)
account4	’hello’ (special characters like ’ are not allowed)

Variable names are case-sensitive, meaning that spam, SPAM, Spam, and sPaM are four different variables.

Questions

Q. What is an expression made up of? What do all expressions do?
Ans. Expressions consist of values (such as 2) and operators (such as +), and they can always evaluate (that is, reduce) down to a single value.

Q. What tis the difference between an expression and statement?
Ans. An expression evaluates to a single value. A statement does not.

Comparison Operators

Operator	Meaning
==	Equal to
!=	Not equal to
<	Less than
>	Greater than
⇐	Less than or equal to
>=	Greater than or equal to

Tip

The Difference Between the == and = Operators.
You might have noticed that the == operator (equal to) has two equal signs, while the = operator (assignment) has just one equal sign. It’s easy to confuse these two operators with each other.

Boolean Values

True & False

Boolean Operators

and, or and not

The `and` Operator’s Truth Table

Expression	Evaluates to…
True and True	True
True and False	False
False and True	False
False and False	False

The `or` Operator’s Truth Table

Expression	Evaluates to…
True or True	True
True or False	True
False or True	True
False or False	False

The `not` Operator’s Truth Table

Expression	Evaluates to…
not True	False
not Flase	True

Gauss’s Clever Trick for Adding Numbers

total = 0
for num in range(101):
	total = total + num
print(total)

Imagine you have a list of numbers like this:
1, 2, 3, 4, 5, 6, ... all the way up to 100.

Gauss, a really smart kid, wanted to add them all up quickly without adding each number one by one. So he thought of a cool trick!

He looked at the first number (1) and the last number (100). When you add them together, you get 101 (1 + 100 = 101). Then he looked at the second number (2) and the second-to-last number (99). When you add them together, you also get 101 (2 + 99 = 101).

If you keep doing this, you’ll see that every time you add the first number with the last number, the second with the second-to-last, and so on, you always get 101. There are 50 pairs like this.

So instead of adding each number one by one, Gauss figured out that if you have 50 pairs, and each pair adds to 101, you just need to multiply:

50 pairs × 101 = 5050.

So the total of all the numbers from 1 to 100 is 5050. Isn’t that a cool shortcut?

`from` `import` Statements

An alternative form of the import statement is composed of the from keyword, followed by the module name, the import keyword, and a star; for example, from random import *.
With this form of import statement, calls to functions in random will not need the random. prefix. However, using the full name makes for more readable code, so it is better to use the normal form of the import statement.

The Augmented Assignment Operators

Augmented assignment statement	Equivalent assignment statement
spam = spam + 1	spam += 1
spam = spam - 1	spam -= 1
spam = spam * 1	spam *= 1
spam = spam / 1	spam /= 1
spam = spam % 1	spam %= 1

References

>>> spam = 42
>>> cheese = spam
>>> spam = 100
>>> spam
100
>>> cheese
42

You assign 42 to the spam variable, and then you copy the value in spam and assign it to the variable cheese . When you later change the value in spam to 100, this doesn’t affect the value in cheese . This is because spam and cheese are different variables that store different values.

But lists don’t work this way. When you assign a list to a variable, you are actually assigning a list reference to the variable. A reference is a value that points to some bit of data, and a list reference is a value that points to a list. Here is some code that will make this distinction easier to understand.

Enter this into the interactive shell:

>>> spam = [0, 1, 2, 3, 4, 5]
>>> cheese = spam
>>> cheese[1] = 'Hello!'
>>> spam
[0, 'Hello!', 2, 3, 4, 5]
>>> cheese
[0, 'Hello!', 2, 3, 4, 5]

The code changed only the cheese list, but it seems that both the cheese and spam lists have changed.

When you create the list 1 , you assign a reference to it in the spam variable. But the next line 2 copies only the list reference in spam to cheese , not the list value itself. This means the values stored in spam and cheese now both refer to the same list. There is only one underlying list because the list itself was never actually copied. So when you modify the first element of cheese 3 , you are modifying the same list that spam refers to.

Using boxes as a metaphor for variables, this Figures shows what happens when a list is assigned to the spam variable.

Question
Q. Name a few ways that list values are similar to string values.
Ans. Both lists and strings can be passed to len(), have indexes and slices, be used in for loops, be concatenated or replicated, and be used with the in and not in operators.

Q. How do you type the tuple value that has just the integer value 42 in it?
Ans. (42,) (The trailing comma is mandatory.)

Q. What is the difference between copy.copy() and copy.deepcopy()?
Ans. Python provides the copy module to create actual copies which offer functions for shallow (copy.copy()) and deep (copy. deepcopy ()) copies.

A shallow copy creates a new object but retains references to the objects contained within the original. It only copies the top-level structure without duplicating nested elements.

Changes made to a copy of an object do reflect in the original object. In python, this is implemented using the “copy.copy()” function.

Example:
>>> import copy
>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = copy.deepcopy(a)
>>> b[0][0]= 0
>>> a
`[[0, 2, 3], [4, 5, 6]]`
>>> b
`[[0, 2, 3], [4, 5, 6]]`
A deep copy creates a new compound object before inserting copies of the items found in the original into it in a recursive manner.

It will first construct a new collection object and then recursively populate it with copies of the child objects found in the original. It means that any changes made to a copy of the object do not reflect in the original object.

Example:
>>> import copy
>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = copy.deepcopy(a)
>>> b[0][0] = 0
>>> a
`[[1, 2, 3], [4, 5, 6]]`
>>> b
`[[0, 2, 3], [4, 5, 6]]`

Comma Code

Say you have a list value like this:
spam = ['apples', 'bananas', 'tofu', 'cats']
Write a function that takes a list value as an argument and returns a string with all the items separated by a comma and a space, with and inserted before the last item. For example, passing the previous spam list to the function would return 'apples, bananas, tofu, and cats'. But your function should be able to work with any list value passed to it.

Solution

spam = ['apples', 'bananas', 'tofu', 'cats']
def function(line):
    result = ''
    for i in line:
        if i == line[-1]:
            result += 'and '+i
        else:
            result += i+', '
    return result
print(function(spam))
 
#OUTPUT
apples, bananas, tofu, and cats

Character Picture Grid

Say you have a list of lists where each value in the inner lists is one-character string, like this:

grid = [['.', '.', '.', '.', '.', '.'],
		['.', 'O', 'O', '.', '.', '.'],
		['O', 'O', 'O', 'O', '.', '.'],
		['O', 'O', 'O', 'O', 'O', '.'],
		['.', 'O', 'O', 'O', 'O', 'O'],
		['O', 'O', 'O', 'O', 'O', '.'],
		['O', 'O', 'O', 'O', '.', '.'],
		['.', 'O', 'O', '.', '.', '.'],
		['.', '.', '.', '.', '.', '.']]

You can think of grid[x][y] as being the character at the x- and y-coordinates of a picture” drawn with text characters. The (0, 0) origin will be in the upper-left corner, the x-coordinates increase going right, and w the y-coordinates increase going down.
Copy the previous grid value, and write code that uses it to print the image.

..OO.OO..
.OOOOOOO.
.OOOOOOO.
..OOOOO..
...OOO...
....O....

Solution

grid = [['.', '.', '.', '.', '.', '.'],
['.', 'O', 'O', '.', '.', '.'],
['O', 'O', 'O', 'O', '.', '.'],
['O', 'O', 'O', 'O', 'O', '.'],
['.', 'O', 'O', 'O', 'O', 'O'],
['O', 'O', 'O', 'O', 'O', '.'],
['O', 'O', 'O', 'O', '.', '.'],
['.', 'O', 'O', '.', '.', '.'],
['.', '.', '.', '.', '.', '.']]
 
for i in range(6):
    for j in range(9):
        print(grid[j][i], end='')
    print()

Dictionary

Dictionaries vs. Lists

Unlike lists, items in dictionaries are unordered.

>>> spam = ['cats', 'dogs', 'moose']
>>> bacon = ['dogs', 'moose', 'cats']
>>> spam == bacon
False
>>> eggs = {'name': 'Zophie', 'species': 'cat', 'age': '8'}
>>> ham = {'species': 'cat', 'age': '8', 'name': 'Zophie'}
>>> eggs == ham
True

The keys(), values(), and items() Methods

>>> spam = {'color': 'red', 'age': 42}
>>> for v in spam.values():
		print(v)
red
42
 
>>> for k in spam.keys():
print(k)
color
age
 
>>> for i in spam.items():
print(i)
('color', 'red')
('age', 42)

Note

The values in the dict_items value returned by the items() method are
tuples of the key and value.

>>> spam = {'color': 'red', 'age': 42}
>>> spam.keys()
dict_keys(['color', 'age'])
>>> list(spam.keys())
['color', 'age']

The list(spam.keys()) line takes the dict_keys value returned from keys() and passes it to list() , which then returns a list value of ['color', 'age'].

The get() Method

It’s tedious to check whether a key exists in a dictionary before accessing that key’s value. Fortunately, dictionaries have a get() method that takes two arguments: the key of the value to retrieve and a fallback value to return if that key does not exist.

>>> picnicItems = {'apples': 5, 'cups': 2}
>>> 'I am bringing ' + str(picnicItems.get('cups', 0)) + ' cups.'
'I am bringing 2 cups.'
>>> 'I am bringing ' + str(picnicItems.get('eggs', 0)) + ' eggs.'
'I am bringing 0 eggs.'

The setdefault() Method

You’ll often have to set a value in a dictionary for a certain key only if that key does not already have a value. The code looks something like this:

spam = {'name': 'Pooka', 'age': 5}
if 'color' not in spam:
	spam['color'] = 'black'

The setdefault() method offers a way to do this in one line of code. The first argument passed to the method is the key to check for, and the second argument is the value to set at that key if the key does not exist. If the key does exist, the setdefault() method returns the key’s value. Enter the following into the interactive shell:

>>> spam = {'name': 'Pooka', 'age': 5}
>>> spam.setdefault('color', 'black')
'black'
>>> spam
{'color': 'black', 'age': 5, 'name': 'Pooka'}
>>> spam.setdefault('color', 'white')
'black'
>>> spam
{'color': 'black', 'age': 5, 'name': 'Pooka'}

characterCount.py

message = 'It was a bright cold day in April, and the clocks were striking thirteen.'
count = {}
for character in message:
	count.setdefault(character, 0)
	count[character] = count[character] + 1
print(count)

Output

{' ': 13, ',': 1, '.': 1, 'A': 1, 'I': 1, 'a': 4, 'c': 3, 'b': 1, 'e': 5, 'd': 3, 'g': 2, 'i': 6, 'h': 3, 'k': 2, 'l': 3, 'o': 2, 'n': 4, 'p': 1, 's': 3, 'r': 5, 't': 6, 'w': 2, 'y': 1}

Pretty Printing

import pprint
message = 'It was a bright cold day in April, and the clocks were striking thirteen.'
count = {}
for i in message:
	count.setdefault(i, 0)
	count[i] = count[i] + 1
pprint.pprint(count)
 
# OUTPUT
{' ': 13,
 ',': 1,
 '.': 1,
 'A': 1,
 'I': 1,
 'a': 4,
 'b': 1,
 'c': 3,
 'd': 3,
 'e': 5,
 'g': 2,
 'h': 3,
 'i': 6,
 'k': 2,
 'l': 3,
 'n': 4,
 'o': 2,
 'p': 1,
 'r': 5,
 's': 3,
 't': 6,
 'w': 2,
 'y': 1}

A Tic-Tac-Toe Board

# Board Initialization
theBoard = {'top-L': ' ', 'top-M': ' ', 'top-R': ' ',
			'mid-L': ' ', 'mid-M': ' ', 'mid-R': ' ',
			'low-L': ' ', 'low-M': ' ', 'low-R': ' '}
 
# Printing the Board
def printBoard(board):
	print(board['top-L'] + '|' + board['top-M'] + '|' + board['top-R'])
	print('-+-+-')
	print(board['mid-L'] + '|' + board['mid-M'] + '|' + board['mid-R'])
	print('-+-+-')
	print(board['low-L'] + '|' + board['low-M'] + '|' + board['low-R'])
 
# Checking for a Win
def checkWin(board):
	winning_combinations = [
		['top-L', 'top-M', 'top-R'], 
		['mid-L', 'mid-M', 'mid-R'], 
		['low-L', 'low-M', 'low-R'], 
		['top-L', 'mid-L', 'low-L'], 
		['top-M', 'mid-M', 'low-M'], 
		['top-R', 'mid-R', 'low-R'], 
		['top-L', 'mid-M', 'low-R'], 
		['top-R', 'mid-M', 'low-L']
	]
	for i in winning_combinations:
		if board[i[0]] == board[i[1]] == board[i[2]] and board[i[0]] != ' ':
			return board[i[0]]
	return None
 
# Checking if the Board is Full
def isBoardFull(board):
	for value in Board.values():
		if value == '':
			return False
		return True
 
# Getting a Vaild Move
def getValidMove():
	move = input()
	while move not in theBoard or theBoard[move] != ' ':
		print("Invallid mmove. Please choose an empty spot.")
		move = input()
	return move
 
# Game Loop
turn = 'X'
while True:
	printBoard(theBoard)
	print('Turn for ' + turn + '. Move on which space?')
	move = getValidMove()
	theBoard[move] = turn
	
	winner = checkWin(theBoard)
	if winner:
		print(f"Player {winner} wins!")
		break
	
	if isBoardFull(theBoard):
		printBoard(theBoard)
		print("It's a tie!")
		break
		
	if turn == 'X':
		turn = 'O'
	else:
		turn = 'X'

Questions

Q. If a dictionary is stored in spam, what is the difference between the expressions 'cat' in spam and 'cat' in spam.keys()?
Ans. There is no difference. The in operator checks whether a value exists as a key in the dictionary.

Q. If a dictionary is stored in spam, what is the difference between the expressions 'cat' in spam and 'cat' in spam.values()?
Ans. 'cat' in spam checks whether there is a ‘cat’ key in the dictionary, while 'cat' in spam.values() checks whether there is a value ‘cat’ for one of the keys in spam.

String

Escape Characters

Escape character	Prints as
`\'`	Single quote
`\''`	Double quote
`\t`	tab
`\n`	Newline (line break)
`\\`	Backslash

Raw Strings

>>> print(r'That is Carol\'s cat.')
That is Carol\'s cat.

The isX String Methods

Here are some common isX string methods:

isalpha() returns True if the string consists only of letters and is not blank.
isalnum() returns True if the string consists only of letters and numbers and is not blank.
isdecimal() returns True if the string consists only of numeric characters and is not blank.
isspace() returns True if the string consists only of spaces, tabs, and newlines and is not blank.
istitle() returns True if the string consists only of words that begin with an uppercase letter followed by only lowercase letters.

>>> 'hello'.isalpha()
True
>>> 'hello123'.isalpha()
False
>>> 'hello123'.isalnum()
True
>>> 'hello'.isalnum()
True
>>> '123'.isdecimal()
True
>>> '
'.isspace()
True
>>> 'This Is Title Case'.istitle()
True
>>> 'This Is Title Case 123'.istitle()
True
>>> 'This Is not Title Case'.istitle()
False
>>> 'This Is NOT Title Case Either'.istitle()
False

The startswith() and endswith() String Methods

>>> 'Hello world!'.startswith('Hello')
True
>>> 'Hello world!'.endswith('world!')
True
>>> 'abc123'.startswith('abcdef')
False
>>> 'abc123'.endswith('12')
False
>>> 'Hello world!'.startswith('Hello world!')
True
>>> 'Hello world!'.endswith('Hello world!')
True

Tip

There methods are useful alternatives to the == equals operator if you need to check only whether the first or last part of the string, rather than the whole thing, is equal to another string.

The join() and split() String Methods

>>> ', '.join(['cats', 'rats', 'bats'])
'cats, rats, bats'
>>> ' '.join(['My', 'name', 'is', 'Simon'])
'My name is Simon'
>>> 'ABC'.join(['My', 'name', 'is', 'Simon'])
'MyABCnameABCisABCSimon'
 
 
>>> 'My name is Simon'.split()
['My', 'name', 'is', 'Simon']
 
 
>>> 'MyABCnameABCisABCSimon'.split('ABC')
['My', 'name', 'is', 'Simon']
>>> 'My name is Simon'.split('m')
['My na', 'e is Si', 'on']
 
 
>>> spam = '''Dear Alice,
How have you been? I am fine.
There is a container in the fridge
that is labeled "Milk Experiment".
Please do not drink it.
Sincerely,
Bob'''
>>> spam.split('\n')
['Dear Alice,', 'How have you been? I am fine.', 'There is a container in the
fridge', 'that is labeled "Milk Experiment".', '', 'Please do not drink it.',
'Sincerely,', 'Bob']

Justifying Text with rjust(), ljust, and center()

>>> 'Hello'.rjust(10)
'     Hello'
>>> 'Hello'.rjust(20)
'               Hello'
>>> 'Hello World'.rjust(20)
'         Hello World'
 
>>> 'Hello'.ljust(10)
'Hello     '
 
>>> 'Hello'.rjust(20,'*')
'***************Hello'
>>> 'Hello'.ljust(20,'-')
'Hello---------------'
 
>>> 'Hello'.center(20)
'       Hello        '
>>> 'Hello'.center(20,'=')
'=======Hello========'

Example

def printPicnic(itemsDict, leftWidth, rightWidth):
  print('PICNIC ITEMS'.center(leftWidth + rightWidth, '-'))
  for k, v in itemsDict.items():
  	print(k.ljust(leftWidth, '.') + str(v).rjust(rightWidth))
picnicItems = {'sandwiches': 4, 'apples': 12, 'cups': 4, 'cookies': 8000}
printPicnic(picnicItems, 12, 5)
printPicnic(picnicItems, 20, 6)
 
# OUTPUT
---PICNIC ITEMS--
sandwiches..    4
apples......   12
cups........    4
cookies..... 8000
-------PICNIC ITEMS-------
sandwiches..........     4
apples..............    12
cups................     4
cookies.............  8000

Removing Whitespace with strip(), rstrip(), and lstrip()

>>> spam = '     Hello World     '
>>> spam.strip()
'Hello World'
>>> spam.lstrip()
'Hello World     '
>>> spam.rstrip()
'     Hello World'
 
>>> spam = 'SpamSpamBaconSpamEggsSpamSpam'
>>> spam.strip('ampS')
'BaconSpamEggs'

Copying and Pasting Strings with the pyperclip Module

>>> import pyperclip
>>> pyperclip.copy('Hello world!')
>>> pyperclip.paste()
'Hello world!'
 
>>> pyperclip.paste()
'For example, if I copied this sentence to the clipboard and then called
paste(), it would look like this:'

Project : Password Locker

#! python
# pw.py - An insecure password locker program.
 
PASSWORDS = {'email' : 'F7minlBDDuvMJuxESSKHFhTxFtjVB6',
			'blog': 'VmALvQyKAxiVH5G8v01if1MLZF3sdt',
			'luggage': '12345'}
 
import sys,pyperclip
 
if len(sys.argv) < 2:
	print('Usage: python pw.py [account] - copy account password')
	sys.exit()
 
account = sys.argv[1]
 
if account in PASSWORDS:
	pyperclip.copy(PASSWORDS[account])
	print('Password for ' + account + ' copied to clipboard.')
else:
	print('There is no account named ' + account)

OUTPUT

┌──(venv3)(voldemort🔥IdeaPad)-[~]
└─$ python pw.py email
Password for emailcopied to clipboard.
 
┌──(venv3)(voldemort🔥IdeaPad)-[~]
└─$ python pw.py blog
Password for blogcopied to clipboard.

Project : Adding Bullets to Wiki Markup

#! python3
# bulletPointAdder.py - Adds Wikipedia bullet points to the start of each line of text on the clipboard
 
import pyperclip
text = pyperclip.paste()
 
#Seperate lines and add stars.
lines = text.split('\n')
for i in range(len(lines)): # loop through all indexes for "lines" list
	lines[i] = '* ' + lines[i] # add start to each string in "lines" list
 
text = '\n'.join(lines)
pyperclip.copy(text)

OUTPUT

#COPIED
In the C wing,
there are 24 rooms, a narrow hallway,
a kitchen, a solarium and a locked entrance.
(It’s not rocket science).
 
┌──(venv3)(voldemort🔥IdeaPad)-[~]
└─$ python bulletPointAdder.py 
 
#PASTED
* In the C wing,
* there are 24 rooms, a narrow hallway,
* a kitchen, a solarium and a locked entrance.
* (It’s not rocket science).

Project : Table Printer

tableData = [['apples', 'oranges', 'cherries', 'banana'],
			['Alice', 'Bob', 'Carol', 'David'],
			['dogs', 'cats', 'moose', 'goose']]
 
column = []
for m in tableData:
	for n in m:
		lc = 0
		if len(n) > lc:
			lc = len(n)
	colum.append(lc+2)
 
for i in range(len(tableData[0])):
	for j in range(len(tableData)):
		print(tableData[j][i].rjust(colum[j]),end=' ')
	print('\n')

OUTPUT

┌──(venv3)(voldemort🔥IdeaPad)-[~]
└─$ python TablePrinter.py 
  apples   Alice    dogs 
 oranges     Bob    cats 
cherries   Carol   moose 
  banana   David   goose

Character Classes

Shorthand character class	Represents
\d	Any numeric digit from 0 to 9.
\D	And character that is not a numeric digit from 0 to 9.
\w	Any letter, numeric digit, or the underscore character.
\W	Any character that is not a letter, numeric digit, or the underscore character.
\s	Any space, tab, or newline character.
\S	Any character that is not a space, tab, or newline.

Explorer

Python Basic

Table of Contents

Operators

Data types

Variables

Comparison Operators

Boolean Values

Boolean Operators

The and Operator’s Truth Table

The or Operator’s Truth Table

The not Operator’s Truth Table

Gauss’s Clever Trick for Adding Numbers

from import Statements

The Augmented Assignment Operators

References

Comma Code

Solution

Character Picture Grid

Solution

Dictionary

Dictionaries vs. Lists

The keys(), values(), and items() Methods

The get() Method

The setdefault() Method

characterCount.py

Output

Pretty Printing

A Tic-Tac-Toe Board

String

Escape Characters

Raw Strings

The isX String Methods

The startswith() and endswith() String Methods

The join() and split() String Methods

Justifying Text with rjust(), ljust, and center()

Removing Whitespace with strip(), rstrip(), and lstrip()

Copying and Pasting Strings with the pyperclip Module

Project : Password Locker

OUTPUT

Project : Adding Bullets to Wiki Markup

OUTPUT

Project : Table Printer

OUTPUT

Character Classes

Explorer

Graph View

Table of Contents

Recently Added

Tip & Tricks

Headers, DLLs and Functions

Code Injection

The `and` Operator’s Truth Table

The `or` Operator’s Truth Table

The `not` Operator’s Truth Table

`from` `import` Statements