Variables, types and f-strings

A name is a label, not a box

In C, int a = 5; reserves 4 bytes and writes 5 into them. In Python, a = 5 creates an integer object with the value 5 and sticks the label a on it. The difference shows up here.

a = 5
b = a
a = 7
print(b)      # 5, not 7

b = a copied the label, not the value. Both labels pointed at the same 5. Then a was moved to point at a 7. The 5 never changed, because integers in Python cannot be changed. They are immutable.

The four types you will use every day

The int row is worth stopping on. A 32-bit int in C holds up to 2,147,483,647. Add one and it wraps around to -2,147,483,648. Python has no such limit.

print(2 ** 100)
# 1267650600228229401496703205376

Python grows the integer as needed, using more memory. That is why Python never overflows and C does.

Floats do not do what you expect

print(0.1 + 0.2)          # 0.30000000000000004
print(0.1 + 0.2 == 0.3)   # False

This is not a Python bug. A float stores a number in binary, and 0.1 in binary is a repeating fraction, exactly like 1/3 is 0.3333... in decimal. It gets cut off, so a tiny error remains. Every language with floats has this, C included.

Conversion, and the input() trap

n = input("Enter a number: ")
print(type(n))        # <class 'str'>  - always a string

input() gives you a string every single time, even if the user typed 42.

n = int(input("Enter a number: "))
print(n * 2)

Wrap it in int() or float(). If you forget, n 2 with n = "42" does not give 84 - it gives "4242", because on a string repeats it. That silent wrong answer is worse than a crash.

Operators that are not obvious

print(7 / 2)      # 3.5    true division, always a float
print(7 // 2)     # 3      floor division, throws away the remainder
print(7 % 2)      # 1      remainder
print(7 ** 2)     # 49     power
print(-7 // 2)    # -4     floors downward, not toward zero

That last line catches people. C's integer division truncates toward zero, so -7 / 2 is -3 in C. Python floors, so it is -4. If you are converting a C algorithm to Python and the numbers go wrong at negative values, this is usually why.

f-strings

The old way of building output was clumsy. This is the modern way.

name = "Ananya"
cgpa = 8.7346
sem = 3

print(f"{name} is in semester {sem} with CGPA {cgpa:.2f}")
# Ananya is in semester 3 with CGPA 8.73

Put f before the quote, then put any expression inside {}. The :.2f after the colon is a format spec: show it as a float with 2 decimal places.

More specs worth memorising:

value = 1234.5678
print(f"{value:.1f}")     # 1234.6
print(f"{value:10.2f}")   #    1234.57   padded to width 10
print(f"{value:,.2f}")    # 1,234.57     thousands separators
print(f"{42:05d}")        # 00042        zero padded
print(f"{0.256:.1%}")     # 25.6%        as a percentage

You can put real expressions inside the braces, not just names.

marks = [72, 65, 88]
print(f"Average: {sum(marks) / len(marks):.2f}")
# Average: 75.00

Naming

Python's convention is lower_case_with_underscores for variables and functions, ALL_CAPS for constants. Follow it. avgMarks works but marks you out as someone who learned Java first, and in a code review that is the first thing anyone says.

Names cannot start with a digit, cannot contain spaces, and must not shadow built-ins. Naming a variable list or sum or input is legal and will break the rest of your program in a way that is very hard to find.