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Python Basics

Variables & Data Types

Dynamic typing, the core types (int, float, str, bool, None), type() and conversions, multiple assignment, and how Python variables are really references.

3 min read5 runnable examples4-question quiz

A variable is created the moment you assign to it. There is no declaration and no type keyword — the value carries the type.

Python
age = 19                # int
cgpa = 8.75             # float
name = "Asha"           # str
is_enrolled = True      # bool
nothing = None          # NoneType: "no value"

print(type(age), type(cgpa), type(name), type(is_enrolled), type(nothing))
age = "nineteen"        # the same name can later hold a different type
print(type(age))
Output
<class 'int'> <class 'float'> <class 'str'> <class 'bool'> <class 'NoneType'>
<class 'str'>
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Core types

Type Example Notes
int 42, -7, 10**30 unlimited size — no overflow
float 3.14, 2e-3 64-bit double precision
complex 3 + 4j built-in complex numbers
str 'hi', "hi", """multi-line""" immutable text
bool True, False a subclass of int (True == 1)
NoneType None the absence of a value

Python integers never overflow — try a number that would break C's long long:

Python
print(2 ** 100)
print(len(str(2 ** 1000)), "digits in 2^1000")
factorial = 1
for i in range(1, 26):
    factorial *= i
print("25! =", factorial)
Output
1267650600228229401496703205376
302 digits in 2^1000
25! = 15511210043330985984000000
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Converting between types

Python
print(int("45") + 5)          # str -> int
print(float("2.5") * 2)       # str -> float
print(int(9.99))              # float -> int truncates towards zero
print(round(9.99), round(2.5), round(3.5))   # rounds to the nearest even on .5
print(str(100) + " marks")    # int -> str
print(bool(0), bool(""), bool("0"), bool([]), bool(3))
Output
50
5.0
9
10 2 4
100 marks
False False True False True
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Naming rules and conventions

  • Letters, digits and underscores; cannot start with a digit; case-sensitive.
  • Cannot be a keyword: if, for, class, def, True, None, …
  • Convention (PEP 8): snake_case for variables and functions, PascalCase for classes, UPPER_CASE for constants.

Multiple assignment and swapping

Python
a, b, c = 1, 2, 3
x = y = 0
a, b = b, a                 # swap without a temporary variable
print(a, b, c, x, y)
first, *rest = [10, 20, 30, 40]     # unpacking with a star
print(first, rest)
Output
2 1 3 0 0
10 [20, 30, 40]
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Variables are references

In C, a variable is a box that holds a value. In Python, a variable is a name that refers to an object. Assignment binds a name to an object; it does not copy the object.

Python
a = [1, 2, 3]
b = a              # b refers to the SAME list
b.append(4)
print(a)           # a sees the change
print(a is b, id(a) == id(b))

c = list(a)        # make a real copy
c.append(5)
print(a, c, a is c)
Output
[1, 2, 3, 4]
True True
[1, 2, 3, 4] [1, 2, 3, 4, 5] False
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This matters for mutable objects (lists, dicts, sets). Immutable ones (int, float, str, tuple) can never change in place, so sharing them is always safe.

Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    What is type(3 / 2)?

  2. 2

    What does int("3.7") do?

  3. 3

    Which value is truthy?

  4. 4

    After a = [1]; b = a; b.append(2), what is a?

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