# Comments

A comment in Python starts with the hash: #, and goes until the end of the line.

# This is a comment

# Variables

• In Python, variables are a storage placeholder for texts and numbers.
• It must have a name so that you are able to find it again.
• The variable is always assigned with the equal sign, followed by the value of the variable.

#  This code assigns variable a to Number 5
a = 5
#  This will assign b to string "Hello"
b = "Hello"

# Variable Types

Python has five standard data types :

• Numbers
• String
• List
• Tuple
• Dictionary

In the course, we'll mostly use just numbers and strings.

#Number
sample_number = 10

# String are enclosed inside double or single quotes.
sample_string = "This is a string"
sample_string_2 = "456"

# Lists are used to store multiple items in a single variable. Values are indexed and are changeable
sample_list = [10,20,0,"Python",30.4]

# A tuple is a collection which is ordered and unchangeable.
sample_tuple = ('Tuple item',2,4)

# A set is a collection which is both unordered and unindexed.
sample_set = {1,2,3,4,5}

# Dictionaries are used to store data values in key:value pairs.

# A dictionary is a collection which is unordered, changeable and does not allow duplicates.

# Dictionaries are written with curly brackets, and have keys and values:
sample_dictionary =  {"key1": "value1" , "Name": "John", "Age": 26}

# Type Casting

The process of converting the value of one data type (integer, string, float, etc.) to another data type explicitly is called type casting.

Consider the following:

num_1 = 123 # Type : Number
num_2 = "456" # Type : String

If you want to add num_1 and num_2 in Python, you can't do that because Python cannot convert string to a number implicitly. So, we have to explicitly convert num_2 to an integer type and proceed to addition.

num_1 = 123
num_2 = "456"

num_2 = int(num_2) # This will convert string "456" to number 456

# Now we can add num_1 and num_2
print(num_1 + num_2) # 579 would be printed

# Printing Strings and Variables

We use the print() function for printing any output.

# This program will print Hello World on the terminal

print("Hello, World!")

# Printing Variables:

There are mainly two ways of printing variables:

• Using comma operator:

  # For example lets take a = 5, b = 10
  a = 5
  b = 10
  # This is one way of printing variable
  print("The value of variable a is", a , "and value of b is", b)
  
  

  The output of the above code would be:

  The value of variable a is 5 and value of b is 10
  

  The comma operator adds space after the string as well.

• String Interpolation:

  Python 3.6 added new string interpolation method called literal string interpolation and introduced a new literal prefix f. This new way of formatting strings is powerful and easy to use.

  name = 'World'
  program = 'Python'
  
  # Just use f before the quotation marks
  print(f"Hello {name}! This is {program}")
  

  The output of the above program would be:

  Hello World! This is Python
  

  Simple and easy, right? That's why we'll be using this method in the course.

# Python Booleans

Booleans represent one of two values: True or False. When you compare two values, the expression is evaluated and Python returns the Boolean answer:

print(10 > 9) # This will print True
print(10 == 9) # False
print(10 < 9) # False

# Boolean function:

The bool() function allows you to evaluate any value, and give you True or False in return.

bool("Hello") # True
bool(15) # True
bool(None) # False

# Conditional Statements

The if...else statement is used if you want perform different action (run different code) on different condition. For example:

num = -1

if num > 0:
    print("Positive number")
elif num == 0: #elif is short for else if
    print("Zero")
else:
    print("Negative number")

# Output: Negative number

# Error Handling

When an error or an exception occurs, Python normally stops its execution and generates an error message.

Example:

print(x)
# This statement will generate error because variable x was not defined before it was printed.

In Python, these errors and exceptions can be handled using a try statement. The operation which can raise an exception is placed inside the try block. The code that handles exception is written in the except block.

try:
  print(x)
except:
  print("An exception occurred") 

Here, as the try block raises an error (x is not defined), the except block will be executed and An exception occurred would be printed on the terminal.

# Conclusion

These are all the things that you'll need for the course. Make sure you understand these well so that there isn't any knowledge gap in the future videos.