Python notes for beginners

# comments start with a hash
print("Hello, Study Hub!")     # shows text
print(2 + 3)                   # shows 5
print("Hi", "there")           # shows  Hi there

student = "Ayush"        # str
marks = 87               # int
percent = 87.5           # float
passed = True            # bool

print(type(marks))       # <class 'int'>

# input() ALWAYS returns text - convert it before doing maths
age = int(input("Your age: "))
print("Next year:", age + 1)

# f-strings are the neatest way to build text
print(f"{student} scored {marks} ({percent}%)")

price = 499
qty = 2
gst = 0.18

subtotal = price * qty
tax = subtotal * gst
total = subtotal + tax

print(f"Subtotal: {subtotal}")
print(f"Total   : {round(total, 2)}")

# comparisons give True / False
print(total > 1000)               # False
print(0 <= 5 < 100)               # True - Python chains comparisons

# logical operators
member = True
print(total > 800 or member)      # True

marks = 78

# checked in order - the first match wins
if marks >= 90:
    grade = "A+"
elif marks >= 75:
    grade = "A"
elif marks >= 33:
    grade = "B"
else:
    grade = "F"

print(grade)                        # A

# one-line version
label = "Pass" if marks >= 33 else "Fail"
print(label)

# for - when you know the count
for i in range(1, 6):        # 1, 2, 3, 4, 5
    print(i)

# for - walk through a collection
for subject in ["Maths", "Science"]:
    print("I study", subject)

# while - until something changes
count = 1
while count <= 3:
    print("Round", count)
    count += 1               # without this it never ends

# break stops, continue skips one round
for n in range(1, 11):
    if n == 8:
        break
    if n % 2 == 0:
        continue
    print(n, "is odd")

def greet(name):
    return f"Hello, {name}!"

print(greet("Ayush"))

# several parameters
def average(a, b):
    return (a + b) / 2

print(average(80, 90))            # 85.0

# default values make an argument optional
def power(base, exponent=2):
    return base ** exponent

print(power(5))                   # 25
print(power(2, 10))               # 1024

# a function with no return gives back None
def notice():
    print("Fees due on 10th")

notice()

marks = [78, 91, 66]
marks.append(88)              # add to the end
print(marks[0], marks[-1])    # 78 88   (0 = first, -1 = last)
print(len(marks), sum(marks)) # 4 323
print(marks[0:2])             # [78, 91]  - stop not included

student = {"name": "Ayush", "marks": {"Maths": 78, "Science": 91}}
print(student["name"])
print(student["marks"]["Science"])   # 91

student["city"] = "Shimla"           # add a key
print(student.get("phone", "none"))  # safe lookup, never crashes

for subject, score in student["marks"].items():
    print(f"{subject}: {score}")

# a list of dictionaries is the most useful shape you will ever learn
class_marks = [
    {"name": "Ayush", "marks": [78, 91, 66]},
    {"name": "Riya",  "marks": [92, 88, 95]},
]

for s in class_marks:
    avg = sum(s["marks"]) / len(s["marks"])
    print(f'{s["name"]}: {avg:.1f}')

name = "  Ayush Danthta  "

print(name.strip())            # remove spaces from both ends
print(name.strip().lower())    # ayush danthta
print(name.strip().upper())    # AYUSH DANTHTA
print(name.strip().title())    # Ayush Danthta
print(len("Study"))            # 5

email = "study.hub09@gmail.com"
print("@" in email)            # True
print(email.replace("gmail", "yahoo"))
print(email.split("@"))        # ['study.hub09', 'gmail.com']

word = "PROGRAMMING"
print(word[0])                 # P
print(word[0:7])               # PROGRAM
print(word[::-1])              # GNIMMARGORP

# f-strings format numbers neatly
marks, total = 264, 300
print(f"{marks}/{total} = {marks / total * 100:.1f}%")

Traceback (most recent call last):
  File "marks.py", line 4, in <module>
    print(total / count)
ZeroDivisionError: division by zero

# read it bottom to top:
# 1. ZeroDivisionError  -> what went wrong
# 2. print(total / count) -> the line that caused it
# 3. line 4             -> where to look in your file

# stop a crash before it happens
while True:
    raw = input("Your age: ").strip()
    try:
        age = int(raw)
        break
    except ValueError:
        print(f'"{raw}" is not a number. Try again.')

print("Next year you will be", age + 1)

students = [
    {"name": "Ayush", "marks": [78, 91, 66]},
    {"name": "Riya",  "marks": [92, 88, 95]},
    {"name": "Karan", "marks": [45, 52, 60]},
]

def average(nums):
    return sum(nums) / len(nums) if nums else 0

def grade_of(avg):
    if avg >= 85: return "A"
    if avg >= 70: return "B"
    if avg >= 50: return "C"
    return "D"

print("=" * 34)
for s in students:
    avg = average(s["marks"])
    print(f'{s["name"]:<6} {avg:6.1f}   grade {grade_of(avg)}')
print("=" * 34)