Introduction to Python

Python is a programming language used to write programs, give computer commands and perform tasks and operations. It is relatively simpler and more forgiving than C in syntax. For example it doesn’t require semicolons at the end of each command in C, and doesn’t need data specifiers like %d or %f etc.

print("Hello World")

Python is an interpreted language, meaning the code is read and run line by line by the Python interpreter, instead of being compiled into a separate executable file first like in C. This makes it quicker to write and test, since you can change a line and run it again right away without a compile step. The trade off is that Python is generally slower than C when it comes to raw speed, but for most everyday tasks you will never notice.

Variables and Data Types

In C you have to tell the computer what type a variable is before using it (int, float, char). Python figures this out on its own, which is called dynamic typing. You just give the variable a name and a value.

age = 19            # int
price = 49.99       # float
name = "Aryan"      # string (str)
is_student = True   # boolean (bool)

You can check what type a variable is with type():

print(type(age))    # <class 'int'>
print(type(name))   # <class 'str'>

Indentation

This is the biggest difference you will notice from C. Python doesn’t use curly braces { } to group code, it uses indentation (spaces at the start of the line). Everything indented under a line like if, for or while belongs to it.

if age >= 18:
    print("Adult")
    print("Can vote")
print("This line runs no matter what")

If the indentation is wrong, Python gives an error, so it is a good habit to always use 4 spaces.

Comments

Comments are notes in the code that Python ignores. A single line comment starts with #, and for longer ones you can use triple quotes.

# This is a single line comment

"""
This is a
multi line comment
"""

Taking Input and Showing Output

print() shows output and input() takes whatever the user types. Note that input() always gives back a string, so if you want a number you have to convert it.

name = input("Enter your name: ")
age = int(input("Enter your age: "))
print("Hello", name, "you are", age, "years old")

Compare this to C, where you would need scanf("%d", &age); and format specifiers. Python does it in one line.

Basic Operators

Most operators work the same as in C, with a few extra ones.

print(10 + 3)    # 13   addition
print(10 - 3)    # 7    subtraction
print(10 * 3)    # 30   multiplication
print(10 / 3)    # 3.33 division (always gives a float)
print(10 // 3)   # 3    floor division
print(10 % 3)    # 1    remainder
print(10 ** 3)   # 1000 power

Decision Making

Conditions use if, elif (short for else if) and else.

marks = 72

if marks >= 90:
    print("Grade A")
elif marks >= 60:
    print("Grade B")
else:
    print("Grade C")

Running a Python File

Save your code in a file ending with .py (for example hello.py) and run it from the terminal:

python hello.py

You can also type python alone in the terminal to open an interactive shell, where each line you type runs immediately. This is great for quickly testing small things.