Python Basics

Why Python?

Ans:

1. Huge community support, easy to understand and implement.

2. Libraries make your life easy; there is a library for literally everything.

3. Jobs, freelancing, different roles, automation, and endless opportunitiesUnderstand variables and data types.

Variables are containers for storing data values.

x = 5
y = "John"
print(x)
print(y)

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

x = 4       # x is of type int
x = "Sally" # x is now of type str
print(x)

Casting

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

Get the Type

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

x = 5
y = "John"
print(type(x))
print(type(y))

Multi Words Variable Names

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"

Many Values to Multiple Variables

Python allows you to assign values to multiple variables in one line:

x, y, z = "Orange", "Banana", "Cherry"
print(x)
print(y)
print(z)

One Value to Multiple Variables

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

x = y = z = "Orange"
print(x)
print(y)
print(z)

Unpack a Collection

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.

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

Global Variables

Variables that are created outside of a function (as in all of the examples above) are known as global variables.

Global variables can be used by everyone, both inside of functions and outside.

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)

The global Keyword

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.

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

def myfunc():
  global x
  x = "fantastic"

myfunc()

print("Python is " + x)

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

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)

Python Data Types

Built-in Data Types

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

Getting the Data Type

You can get the data type of any object by using the type() function:

Print the data type of the variable x:

x = 5
print(type(x))

Setting the Data Type

In Python, the data type is set when you assign a value to a variable:

Setting the Specific Data Type

If you want to specify the data type, you can use the following constructor functions:

Python Numbers/

There are three numeric types in Python:

• int

• float

• complex

Variables of numeric types are created when you assign a value to them:

x = 1    # int
y = 2.8  # float
z = 1j   # complex

To verify the type of any object in Python, use the type() function:

print(type(x))
print(type(y))
print(type(z))

Int

Int, or integer, is a whole number, positive or negative, without decimals, of unlimited length.

Integers:

x = 1
y = 35656222554887711
z = -3255522

print(type(x))
print(type(y))
print(type(z))

Float

Float, or "floating point number" is a number, positive or negative, containing one or more decimals.

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.

Floats:

x = 35e3
y = 12E4
z = -87.7e100

print(type(x))
print(type(y))
print(type(z))

Complex

Complex numbers are written with a "j" as the imaginary part:

Complex:

x = 3+5j
y = 5j
z = -5j

print(type(x))
print(type(y))
print(type(z))

Type Conversion

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

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))

Random Number

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:

Import the random module, and display a random number between 1 and 9:

import random

print(random.randrange(1, 10))

Strings are Arrays

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

Get the character at position 1 (remember that the first character has the position 0):

a = "Hello, World!"
print(a[1])

Looping Through a String

Since strings are arrays, we can loop through the characters in a string, with a for loop.

Loop through the letters in the word "banana":

for x in "banana":
  print(x)

String Length

To get the length of a string, use the len() function.

The len() function returns the length of a string:

a = "Hello, World!"
print(len(a))

Check String

To check if a certain phrase or character is present in a string, we can use the keyword in.

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:

Print only if "free" is present:

txt = "The best things in life are free!"
if "free" in txt:
  print("Yes, 'free' is present.")

Check if NOT

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

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:

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.")

Python - Slicing Strings

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.

Get the characters from position 2 to position 5 (not included):

b = "Hello, World!"
print(b[2:5])

Slice From the Start

By leaving out the start index, the range will start at the first character:

Get the characters from the start to position 5 (not included):

b = "Hello, World!"
print(b[:5])

Slice To the End

By leaving out the end index, the range will go to the end:

Get the characters from position 2, and all the way to the end:

b = "Hello, World!"
print(b[2:])

Negative Indexing

Use negative indexes to start the slice from the end of the string:

Get the characters:

From: "o" in "World!" (position -5)

To, but not included: "d" in "World!" (position -2):

b = "Hello, World!"
print(b[-5:-2])

Upper Case

The upper() method returns the string in upper case:

a = "Hello, World!"
print(a.upper())

Lower Case

The lower() method returns the string in lower case:

a = "Hello, World!"
print(a.lower())

Replace String

The replace() method replaces a string with another string:

a = "Hello, World!"
print(a.replace("H", "J"))

Python - String Concatenation

To concatenate, or combine, two strings you can use the + operator.

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)

Python - String Methods

Python has a set of built-in methods that you can use on strings.

Python Booleans

Booleans represent one of two values: True or False.

Boolean Values

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:

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")

Evaluate Values and Variables

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))

Most Values are True

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.

Example

The following will return True:

bool("abc")
bool(123)
bool(["apple", "cherry", "banana"])

Some Values are False

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:

class myclass():
  def __len__(self):
    return 0

myobj = myclass()
print(bool(myobj))

Functions can Return a Boolean

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))

Python Operators

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)

Python divides the operators in the following groups:

• Arithmetic operators

• Assignment operators

• Comparison operators

• Logical operators

• Identity operators

• Membership operators

• Bitwise operators

Python Arithmetic Operators

Arithmetic operators are used with numeric values to perform common mathematical operations:

Python Assignment Operators

Assignment operators are used to assign values to variables:

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Python Comparison Operators

Comparison operators are used to compare two values:

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Python Logical Operators

Logical operators are used to combine conditional statements:

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Python Identity Operators

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:

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Python Membership Operators

Membership operators are used to test if a sequence is presented in an object:

---

Python Bitwise Operators

Bitwise operators are used to compare (binary) numbers:

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Operator Precedence

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)