Variables and Data Types in Python

Variables are names, not boxes

School often teaches that a variable is a box holding a value. In Python that picture is misleading.

A Python variable is a name that refers to an object. Assignment binds a name to an object; it doesn't copy a value into a container.

x = 10        # the name x now refers to the integer object 10
y = x         # y refers to the SAME object
x = 20        # x now refers to a different object; y is unchanged
print(y)      # 10

This matters later with lists, where two names referring to the same list means changing one changes "both".

Naming rules

Must:

• Begin with a letter or underscore
• Contain only letters, digits, underscores
• Not be a Python keyword (if, for, class, None…)

Case-sensitive: total, Total and TOTAL are three different names.

Convention: snake_case for variables — student_name, total_marks.

The numeric types

Integers have no upper limit. 2 ** 1000 computes exactly. This differs from C or Java, and it's a genuine convenience.

Floats do not. They are binary approximations:

0.1 + 0.2 == 0.3        # False
0.1 + 0.2               # 0.30000000000000004

This isn't a Python bug — it's how binary floating point works everywhere. Never compare floats with ==; compare within a tolerance:

abs((0.1 + 0.2) - 0.3) < 1e-9        # True

Strings

Text, in single or double quotes — no difference between them.

name = 'Anjali'
college = "Government Engineering College"
address = """Line one
Line two"""              # triple quotes span lines

Strings are immutable. You cannot change one in place:

s = "hello"
s[0] = "H"               # TypeError
s = "H" + s[1:]          # make a new string instead

Useful operations:

len(s)                   # 5
s.upper()                # 'HELLO'   (returns new, s unchanged)
s[0]                     # 'h'       first character
s[-1]                    # 'o'       last character
s[1:4]                   # 'ell'     slice, end excluded
"abc" + "def"            # 'abcdef'  concatenation
"ab" * 3                 # 'ababab'  repetition

Dynamic typing

The type belongs to the object, not the name. A name can refer to different types at different times:

x = 10          # int
x = "hello"     # now a str — legal

Convenient, but it means type errors surface at runtime rather than being caught before the program runs.

Check a type with type(x).

Conversion

int("42")        # 42
float("3.14")    # 3.14
str(42)          # '42'
int(3.9)         # 3      truncates toward zero, does NOT round
int("3.9")       # ValueError — cannot parse a decimal string as int

int(3.9) giving 3 is a common exam point: it truncates, it does not round.

Why input() needs conversion

input() always returns a string, whatever the user types.

age = input("Age: ")     # user types 20 → age is the STRING "20"
age + 1                  # TypeError
age = int(input("Age: "))  # correct

This is among the most frequent errors in first-semester lab work.