Python Fundamentals contents

Variables and Data Types

Variables, the core types (int, float, str, bool), operators, type conversion and reading input.

Beginner4 min readbasicstypesoperatorsinput

Variables

A variable is a name attached to a value. You create it by assigning:

python

age = 16
price = 19.99
name = "Ana"
is_student = True

print(age, price, name, is_student)

There is no declaration and the same name can later refer to a value of another type. Python also lets you assign several names at once, and swap without a temporary:

python

a, b = 1, 2
a, b = b, a
assert (a, b) == (2, 1)

x = y = 0          # both names refer to 0

Naming rules

An identifier can contain letters, digits and underscores, and cannot start with a digit. Names are case-sensitive (Total and total are different) and cannot be a keyword (if, for, class, ...). The convention for variables and functions is snake_case.

The basic data types

Type	Example	Meaning
`int`	`42`, `-7`, `10**100`	whole numbers of any size
`float`	`3.14`, `1e-9`	real numbers with limited precision
`str`	`"hello"`, `'a'`	text
`bool`	`True`, `False`	truth values
`NoneType`	`None`	"no value"

type() tells you what something is:

python

print(type(5), type(5.0), type("5"), type(True), type(None))

text

<class 'int'> <class 'float'> <class 'str'> <class 'bool'> <class 'NoneType'>

Operators

Arithmetic

Operator	Meaning	Example	Result
`+` `-` `*`	add, subtract, multiply	`6 * 7`	`42`
`/`	true division (always a float)	`7 / 2`	`3.5`
`//`	floor division	`7 // 2`, `-7 // 2`	`3`, `-4`
`%`	remainder	`7 % 3`, `-7 % 3`	`1`, `2`
`**`	power	`2 ** 5`	`32`

python

assert 7 // 2 == 3 and -7 // 2 == -4       # floors toward minus infinity
assert -7 % 3 == 2                          # result has the sign of the divisor
assert divmod(17, 5) == (3, 2)              # quotient and remainder together
assert (7 // 2) * 2 + 7 % 2 == 7            # the identity that always holds

Comparison (==, !=, <, <=, >, >=) give a bool, and can be chained: 0 < x < 10.

Logical: and, or, not.

Augmented assignment is shorthand: x += 1 means x = x + 1 (same for -=, *=, //=, %=, **=).

Converting between types

Use the type name as a function:

python

assert int("42") + 1 == 43
assert float("3.5") * 2 == 7.0
assert str(2024) + "!" == "2024!"
assert int(3.99) == 3            # truncates, does not round
assert round(3.5) == 4 and round(2.5) == 2     # banker's rounding: ties go to even
assert bool(0) is False and bool("") is False and bool([]) is False
assert bool("0") is True         # a non-empty string is always truthy

Converting something that is not a valid number raises a ValueError (see Exceptions).

Reading input

input() always returns a string, so convert what you read:

python

n = int(input())                          # one integer on a line
a, b = map(int, input().split())          # two integers on one line
values = list(map(int, input().split()))  # any number of integers
name = input()                            # a line of text

For an input line 5 7, input().split() gives ['5', '7'] and map(int, ...) converts each piece. A complete program for the problem sum of two numbers:

python

a, b = map(int, input().split())
print(a + b)

To feel what these calls do without a keyboard, apply them to a literal string:

python

line = "5 7"
a, b = map(int, line.split())
assert a + b == 12

Formatting output

print accepts several values and separates them by a space; use sep and end to change that. f-strings embed expressions in text:

python

x, y = 3, 4
print(x, y)                       # 3 4
print(x, y, sep=", ")             # 3, 4
print("no newline", end="")       # stays on the same line
print()
print(f"{x} squared is {x**2}")   # 3 squared is 9
print(f"{22/7:.3f}")              # 3.143  (3 decimals)
print(f"{5:03d}")                 # 005    (zero-padded)

Exercises

Read two integers and print their sum, difference, product, quotient (//) and remainder.
Convert a temperature from Celsius to Fahrenheit: $F = C \cdot 9/5 + 32$ .
Read a three-digit number and print the sum of its digits using only // and %.
Explain why int("3.5") fails but int(float("3.5")) works.

Practice

Apply this on the platform and get an instant verdict.

suma a doua numere celsius fahrenheit