Python – Theory 1 — Vatiables, Types, Working with digits of a number

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Python Programming: Module 1 – Theory. Introduction to Python, Syntax, and Basic Operations.

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Python Programming: Module 1 – Theory

Introduction to Python, Syntax, and Basic Operations  |  For English‑speaking students



📖 What is a Program?

A computer program is a list of instructions for the computer. Instructions can be:

  • Read data from the keyboard.
  • Perform arithmetic calculations (+, -, *, /).
  • Display information on the screen.

Programs are written in programming languages — a set of rules that allows humans to write instructions a computer can understand and execute. The text of a program is called source code.

Examples of programming languages: Python, C++, Java, C#, Ruby, PHP.

Example 1: Compiled vs Interpreted Languages

Compiled languages (C, C++, Java): The entire source code is translated into machine code before execution. Requires a compilation step — errors are caught early.

Source → Compiler → Machine Code → Execute

Interpreted languages (Python, Ruby, JavaScript): Code is executed line by line by an interpreter. No separate compilation — easier to test and debug.

Source → Interpreter → Execute line by line

💡 Подробнее о Python

Python was created by Guido van Rossum and is widely used in science and IT. It is named after the show «Monty Python’s Flying Circus», not the snake.

Key characteristics: High-level, Interpreted, Object-oriented, General-purpose, Open-source.

Advantages: Platform-independent, simple syntax, huge library of classes, dynamic typing.

Disadvantages: Relatively slow execution speed.

Interesting fact: Python consistently ranks among the top 3 programming languages in popularity (TIOBE index).



📝 Python Syntax Basics

Python has a clean and readable syntax. Key rules:

  • Indentation defines code blocks (no curly braces { }).
  • Case-sensitive — myVar ≠ myvar.
  • Comments start with #.
  • Statements are usually on separate lines.
  • Colon (:) indicates the start of an indented block (e.g., after if, for).
  • No semicolons required at line ends.
Example 2: Basic Syntax Example
💡 Open an example 
# This is a comment
name = "Alice"
if name == "Alice":
    print("Hello, Alice!")  # indented block



🖨️ Output: print()

The print() function displays output to the console. It can take any number of arguments.

Example 3: Basic print() Usage
💡 Open an example 
print("Hello, World!")
print(42)
print("The answer is", 42)
# Output: The answer is 42

Quotes can be either single or double: print('Python') and print("Python") work the same.

Calling print() with no arguments creates an empty line.

Example 4: The sep Parameter

The optional sep parameter allows you to specify a set of characters that will separate arguments when outputting data.

💡 Open an example 
print('a', 'b', 'c')          # a b c
print('d', 'e', 'f')          # d e f
 
print('a', 'b', sep='*')      # a*b
print('d', 'e', 'f', sep='**') # d**e**f

Example 5: The end Parameter

By default, print() adds a newline after output. The optional end parameter allows you to set a special ending character.

💡 Open an example 
print('a', 'b', 'c')                 # a b c
print('d', 'e', 'f')                 # d e f
 
print('a', 'b', 'c', end='@')        # a b c@
print('d', 'e', 'f', end='@@')       # d e f@@

Example 6: Combining sep and end
💡 Open an example 
print("a", "b", "c", sep="*")                 # a*b*c
print("Hello", end=" ")                        # Hello World
print("World")
print("One", "Two", "Three", sep="-", end="\n") # One-Two-Three

Notes:

  • Default values: sep=' ' (space), end='\n' (newline).
  • \n is the newline character.
  • To remove all additional output characters: print('a', 'b', 'c', sep='', end='')



⌨️ Input: input()

The input() function reads a line of text from the user. Its syntax is input([prompt]). It always returns a string — even if the user types digits.

Example 7: Reading User Input
💡 Open an example 
name = input("Enter your name: ")
print("Hello,", name)
# Example run:
# Enter your name: Alice
# Hello, Alice

Important: input() always returns a string!

age = input("Enter age: ")   # "20" (string)
# age + 1                     # ERROR! Can't add string and int



📦 Variables and Data Types

A variable is a named reference to a value. Assignment uses the = operator: variable_name = value.

Python is dynamically typed — the type is determined by the assigned value.

Example 8: Variables and Dynamic Typing
💡 Open an example 
x = 5          # integer
x = "hello"    # now a string
print(x)       # hello

📌 Variable Naming Rules

  1. Can contain only Latin letters a-z, A-Z, digits, and underscores (_).
  2. Cannot start with a digit.
  3. Should reflect its purpose whenever possible.

Correct names: name, my_variable, sum, TotalAmount, qwerty12my.

Incorrect names: 5name (starts with digit), nepeMeHHaa (non‑Latin), surname$% (invalid characters).

📌 Style (PEP 8)

Use lower_case_with_underscores for variables and functions.

Example 9: Common Data Types
💡 Open an example 
name = "Alice"        # string  <class 'str'>
age = 20              # integer <class 'int'>
pi = 3.14159          # float   <class 'float'>
is_student = True     # boolean <class 'bool'>

Built‑in types include:

  • int — integer numbers (e.g., 42, -7)
  • float — floating‑point numbers (e.g., 3.14, -0.5)
  • str — strings (e.g., "hello")
  • bool — boolean (True / False)
  • list — ordered, mutable collection (e.g., [1, 2, 3])
  • tuple — ordered, immutable collection (e.g., (1, 2, 3))
  • dict — key‑value pairs (e.g., {"name": "Alice"})
  • set — unordered collection of unique items (e.g., {1, 2, 3})

Interesting fact: In Python, variables are references to objects, not memory containers. Even int and float are objects — everything is an object.

Example 10: Integers and Underscore Separator
💡 Open an example 
num1 = 7
num2 = 10
num3 = num1 + num2
print(num3)  # 17
 
# Underscore as separator for readability:
big_num = 25_000_000
print(big_num)  # 25000000

Note: int is short for integer. Python supports long arithmetic — integer variables have no limits.



🧮 Arithmetic Operations

Python supports the following arithmetic operators:

Operator Description Example Result
+ Addition 3 + 2 5
- Subtraction 3 - 2 1
* Multiplication 3 * 2 6
/ True division 3 / 2 1.5
** Exponentiation 3 ** 2 9
// Floor division 3 // 2 1
% Modulo (remainder) 3 % 2 1
Example 11: Basic Arithmetic
💡 Open an example 
a = 3
b = 2
print(a + b)   # 5
print(a - b)   # 1
print(a * b)   # 6
print(a / b)   # 1.5

Example 12: Order of Operations

Operator precedence (from highest to lowest):

  1. **
  2. *, //, %
  3. +, -

Use parentheses () to override precedence.

💡 Open an example 
print(2 + 3 * 4)    # 14
print((2 + 3) * 4)  # 20
print(10 // 3)      # 3
print(10 % 3)       # 1

Example 13: Operations with Floats
💡 Open an example 
a = 13.5
b = 2.0
print(a + b)   # 15.5
print(a - b)   # 11.5
print(a * b)   # 27.0
print(a / b)   # 6.75
print(a ** b)  # 182.25

You can also use // and % with floats:

print(13.5 // 2.0)  # 6.0
print(13.5 % 2.0)   # 1.5



🔄 Type Conversion

You can convert between types using built‑in functions:

  • int(x) — convert to integer
  • float(x) — convert to float
  • str(x) — convert to string

This is necessary when reading numbers with input() (which returns a string).

Example 14: Converting Strings to Integers
💡 Open an example 
age = '1992'
year = int(age)          # year is an integer type
grade = int(input())     # grade is an integer type

int() converts the specified value into an integer.

Example 15: Program: Sum of Two Integers
💡 Open an example 
num1 = int(input())
num2 = int(input())
print(num1 + num2)

Example 16: Converting Strings to Float
💡 Open an example 
num = float(input())    # conversion of the read string
n = float('1.2345')

Example 17: Conversion Between int and float

Implicit conversion: any integer can be used where a float is expected.

Explicit conversion: a float cannot be used where an integer is expected — use int().

💡 Open an example 
num1 = 17.89
num2 = -13.56
num3 = int(num1)   # 17
num4 = int(num2)   # -13
print(num3)
print(num4)

The int() function simply discards the fractional part.

Example 18: Common Conversions
💡 Open an example 
# String to int
num = int("123")        # 123
 
# String to float
num = float("3.14")     # 3.14
 
# Int to string
text = str(123)         # "123"
 
# Float to int (truncates)
num = int(3.99)         # 3



🔢 Working with Digits

Using the // (floor division) and % (modulo) operations, you can extract individual digits from a number. This technique is used in digital clocks, check digit algorithms, and more.

📌 Integer Division and Remainder

  • // — calculates the integer part of division (how many times the divisor «fits» into the number).
  • % — calculates the remainder after division.

Examples:

  • 10 // 2 = 5; 22 // 7 = 3; 65 // 10 = 6
  • 10 % 2 = 0; 22 % 7 = 1; 65 % 10 = 5
Example 19: Extracting Digits from a Two‑Digit Number
💡 Open an example 
num = 17
a = num % 10    # 7 (last digit)
b = num // 10   # 1 (first digit)
print(a)
print(b)

Example 20: Extracting Digits from a Three‑Digit Number
💡 Open an example 
num = 754
a = num % 10              # 4 (last digit)
b = (num % 100) // 10     # 5 (middle digit)
c = num // 100            # 7 (first digit)
print(a)
print(b)
print(c)

General formula for the k‑th digit (from right, 0‑based): (n // 10**k) % 10

Example 21: Standard Form of a Number

Any number can be disassembled into digits using powers of 10. This rule is called the standard form of a number:

123 = 1*100 + 2*10 + 3

Using the same divisor K and operations // and %, you can restore the number by the formula: quotient * K + remainder.

💡 Open an example 
# 10 // 2 = 5; 10 % 2 = 0  =>  5 * 2 + 0 = 10
# 22 // 7 = 3; 22 % 7 = 1  =>  3 * 7 + 1 = 22
# 65 // 10 = 6; 65 % 10 = 5 =>  6 * 10 + 5 = 65

Example 22: Rules to Get Digits of a Three‑Digit Integer
  • Hundreds (first digit): number // 100 (e.g., 123 // 100 = 1)
  • Tens (second digit): (number % 100) // 10 (e.g., 123 % 100 = 23, then 23 // 10 = 2)
  • Units (third digit): number % 10 (e.g., 123 % 10 = 3)
💡 Open an example 
num = 754
last = num % 10              # 4
middle = (num // 10) % 10    # 5
first = num // 100           # 7
print(first, middle, last)   # 7 5 4

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