W3 python

Question 1

What is Python?

Answer 1

Python is a popular programming language. It was created by Guido van Rossum, and released in 1991.

Question 2

It is used for:

Answer 2

web development (server-side),
software development,
mathematics,
system scripting.

Question 3

What can Python do?

Answer 3

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.

Question 4

Why Python?

Answer 4

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.

Question 5

Python Syntax compared to other programming languages

Answer 5

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.

Question 6

Python Indentation

Answer 6

Indentation refers to the spaces at the beginning of a code line.

Where in other programming languages the indentation in code is for readability only, the indentation in Python is very important.

Python uses indentation to indicate a block of code

Python will give you an error if you skip the indentation

The number of spaces is up to you as a programmer, the most common use is four, but it has to be at least one.

You have to use the same number of spaces in the same block of code, otherwise Python will give you an error

Question 7

Python Variables

Answer 7

Python has no command for declaring a variable.

Question 8

Comments

Answer 8

Python has commenting capability for the purpose of in-code documentation.

Comments start with a #, and Python will render the rest of the line as a comment

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

Question 9

Multiline Comments

Answer 9

Python does not really have a syntax for multiline comments.

To add a multiline comment you could insert a # for each line

Question 10

multiline string

Answer 10

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
“””

As long as the string is not assigned to a variable, Python will read the code, but then ignore it

Question 11

Variables

Answer 11

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.
Variables do not need to be declared with any particular type, and can even change type after they have been set.

Question 12

Casting

Answer 12

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 3.0

Question 13

Get the Type

Answer 13

You can get the data type of a variable with the type() function.

Example
x = 5
y = “John”
print(type(x))
print(type(y))

Question 14

Single or Double Quotes?

Answer 14

String variables can be declared either by using single or double quotes:

Example
x = “John”
# is the same as
x = ‘John’

Question 15

Case-Sensitive

Answer 15

Variable names are case-sensitive.

Example
This will create two variables:

a = 4
A = “Sally”
#A will not overwrite a

Question 16

Variable Names

Answer 16

A 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.

Illegal variable names:

2myvar = “John”
my-var = “John”
my var = “John”

Question 17

Multi Words Variable Names

Answer 17

Variable names with more than one word can be difficult to read.

There are several techniques you can use to make them more readable:

Camel Case
Each word, except the first, starts with a capital letter:

myVariableName = “John”
Pascal Case
Each word starts with a capital letter:

MyVariableName = “John”
Snake Case
Each word is separated by an underscore character:

my_variable_name = “John”

Question 18

Many Values to Multiple Variables

Answer 18

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.

Question 19

One Value to Multiple Variables

Answer 19

And you can assign the same value to multiple variables in one line:

Example
x = y = z = “Orange”
print(x)
print(y)
print(z)

Question 20

Unpack a Collection

Answer 20

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.

Example
Unpack a list:

fruits = [“apple”, “banana”, “cherry”]
x, y, z = fruits
print(x)
print(y)
print(z)

Question 21

Output Variables

Answer 21

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:

Example
x = “Python”
y = “is”
z = “awesome”
print(x, y, z)

You can also use the + operator to output multiple variables:

Example
x = “Python “
y = “is “
z = “awesome”
print(x + y + z)

In the print() function, when you try to combine a string and a number with the + operator, Python will give you an error.

The best way to output multiple variables in the print() function is to separate them with commas, which even support different data types:

Example
x = 5
y = “John”
print(x, y)

Question 22

Global Variables

Answer 22

Variables that are created outside of a function.
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 variable

x = “awesome”

def myfunc():
x = “fantastic”
print(“Python is “ + x)

myfunc()

print(“Python is “ + x)

Question 23

The global Keyword

Answer 23

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.

Example
If you use the global keyword, the variable belongs to the global scope:

def myfunc():
global x
x = “fantastic”

myfunc()

print(“Python is “ + x)

lso, use the global keyword if you want to change a global variable inside a function.

Example
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)

Question 24

Built-in Data Types

Answer 24

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

Question 25

Getting the Data Type

Answer 25

You can get the data type of any object by using the type() function: Example Print the data type of the variable x: x = 5 print(type(x))

Question 26

Setting the Data Type

Answer 26

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

Question 27

Python Numbers

Answer 27

There are three numeric types in Python: int float complex Variables of numeric types are created when you assign a value to them: Example x = 1 # int y = 2.8 # float z = 1j # complex

Question 28

Int

Answer 28

Int, or integer, is a whole number, positive or negative, without decimals, of unlimited length. Example Integers: x = 1 y = 35656222554887711 z = -3255522 print(type(x)) print(type(y)) print(type(z))

Question 29

Float

Answer 29

Float, or "floating point number" is a number, positive or negative, containing one or more decimals. Example 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. Example Floats: x = 35e3 y = 12E4 z = -87.7e100 print(type(x)) print(type(y)) print(type(z))

Question 30

Complex

Answer 30

Complex numbers are written with a "j" as the imaginary part: Example Complex: x = 3+5j y = 5j z = -5j print(type(x)) print(type(y)) print(type(z))

Question 31

Type Conversion

Answer 31

You can convert from one type to another with the int(), float(), and complex() methods: Example 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.

Question 32

Random Number

Answer 32

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: Example Import the random module, and display a random number between 1 and 9: import random print(random.randrange(1, 10))

Question 33

Specify a Variable Type

Answer 33

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'

Question 34

Strings

Answer 34

Strings in python are surrounded by either single quotation marks, or double quotation marks. 'hello' is the same as "hello".

Question 35

Assign String to a Variable

Answer 35

Assigning a string to a variable is done with the variable name followed by an equal sign and the string: Example a = "Hello" print(a)

Question 36

Multiline Strings

Answer 36

You can assign a multiline string to a variable by using three quotes: Example 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: Example a = '''Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.''' print(a)

Question 37

Strings are Arrays

Answer 37

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. Example Get the character at position 1 (remember that the first character has the position 0): a = "Hello, World!" print(a[1])

Question 38

Looping Through a String

Answer 38

Since strings are arrays, we can loop through the characters in a string, with a for loop. Example Loop through the letters in the word "banana": for x in "banana": print(x)

Question 39

String Length

Answer 39

To get the length of a string, use the len() function. Example The len() function returns the length of a string: a = "Hello, World!" print(len(a))

Question 40

Check String

Answer 40

To check if a certain phrase or character is present in a string, we can use the keyword in. Example 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: Example Print only if "free" is present: txt = "The best things in life are free!" if "free" in txt: print("Yes, 'free' is present.")

Question 41

Check if NOT

Answer 41

To check if a certain phrase or character is NOT present in a string, we can use the keyword not in. Example 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: Example 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.")

Question 42

Slicing

Answer 42

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. ExampleGet your own Python Server Get the characters from position 2 to position 5 (not included): b = "Hello, World!" print(b[2:5])

Question 43

Slice From the Start

Answer 43

By leaving out the start index, the range will start at the first character: Example Get the characters from the start to position 5 (not included): b = "Hello, World!" print(b[:5])

Question 44

Slice To the End

Answer 44

By leaving out the end index, the range will go to the end: Example Get the characters from position 2, and all the way to the end: b = "Hello, World!" print(b[2:])

Question 45

Upper Case

Answer 45

The upper() method returns the string in upper case: a = "Hello, World!" print(a.upper())

Question 46

Lower Case

Answer 46

Example The lower() method returns the string in lower case: a = "Hello, World!" print(a.lower())

Question 47

Remove Whitespace

Answer 47

Whitespace is the space before and/or after the actual text, and very often you want to remove this space. Example The strip() method removes any whitespace from the beginning or the end: a = " Hello, World! " print(a.strip()) # returns "Hello, World!"

Question 48

Replace String

Answer 48

Example The replace() method replaces a string with another string: a = "Hello, World!" print(a.replace("H", "J"))

Question 49

Split String

Answer 49

The split() method returns a list where the text between the specified separator becomes the list items. Example The split() method splits the string into substrings if it finds instances of the separator: a = "Hello, World!" print(a.split(",")) # returns ['Hello', ' World!']

Question 50

String Concatenation

Answer 50

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) Example To add a space between them, add a " ": a = "Hello" b = "World" c = a + " " + b print(c)

Question 51

String Format

Answer 51

As we learned in the Python Variables chapter, we cannot combine strings and numbers like this: Example 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: Example 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: Example 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: Example 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))

Question 52

Escape Character

Answer 52

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: Example 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 \": Example 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."

Question 53

Escape Characters

Answer 53

Other escape characters used in Python: Code Result Try it \' Single Quote \\ Backslash \n New Line \r Carriage Return \t Tab \b Backspace \f Form Feed \ooo Octal value \xhh Hex value

Question 54

String Methods

Answer 54

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 isascii() Returns True if all characters in the string are ascii characters 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

Question 55

Python Booleans

Answer 55

Booleans represent one of two values: True or False.

Question 56

Boolean Values

Answer 56

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: ExampleGet your own Python Server print(10 > 9) print(10 == 9) print(10 < 9) When you run a condition in an if statement, Python returns True or False: Example 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")

Question 57

Evaluate Values and Variables

Answer 57

The bool() function allows you to evaluate any value, and give you True or False in return, Example Evaluate a string and a number: print(bool("Hello")) print(bool(15)) Example 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.

Question 58

Some Values are False

Answer 58

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. Example 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: Example class myclass(): def __len__(self): return 0 myobj = myclass() print(bool(myobj))

Question 59

Functions can Return a Boolean

Answer 59

You can create functions that returns a Boolean Value: Example Print the answer of a function: def myFunction() : return True print(myFunction()) You can execute code based on the Boolean answer of a function: Example 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: Example Check if an object is an integer or not: x = 200 print(isinstance(x, int))

Question 60

Python Operators

Answer 60

Operators are used to perform operations on variables and values. In the example below, we use the + operator to add together two values: Example print(10 + 5)

Question 61

Python divides the operators in the following groups:

Answer 61

Arithmetic operators Assignment operators Comparison operators Logical operators Identity operators Membership operators Bitwise operators

Question 62

Python Arithmetic Operators

Answer 62

Arithmetic operators are used with numeric values to perform common mathematical operations: Operator Name Example Try it + Addition x + y - Subtraction x - y * Multiplication x * y / Division x / y % Modulus x % y ** Exponentiation x ** y // Floor division x // y

Question 63

Python Assignment Operators

Answer 63

Assignment operators are used to assign values to variables: Operator Example Same As Try it = 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

Question 64

Python Comparison Operators

Answer 64

Comparison operators are used to compare two values: Operator Name Example Try it == 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

Question 65

Python Logical Operators

Answer 65

Logical operators are used to combine conditional statements: Operator Description Example Try it 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)

Question 66

Python Identity Operators

Answer 66

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 Try it 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

Question 67

Python Membership Operators

Answer 67

Membership operators are used to test if a sequence is presented in an object: Operator Description Example Try it 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

Question 68

Python Bitwise Operators

Answer 68

Bitwise operators are used to compare (binary) numbers: Operator Name Description Example Try it & 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

Question 69

Operator Precedence

Answer 69

Operator precedence describes the order in which operations are performed. Example Parentheses has the highest precedence, meaning that expressions inside parentheses must be evaluated first: print((6 + 3) - (6 + 3)) Example 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 Try it () 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. Example Addition + and subtraction - has the same precedence, and therefor we evaluate the expression from left to right: print(5 + 4 - 7 + 3)