Python Basics: Hello World, Print Function, Operators, Input/Output, Data Types, Control Flow, and Simple Exercises

01 - Hello World

Python relies on indentation; use four spaces per level. The following example demonstrates a simple hello‑world print and an indented if‑statement.

print('hello world!')

if 3 > 0:
    print('OK')
    print('yes')

x = 3; y = 4   # not recommended, should be on separate lines
print(x + y)

02 - print Function

The print function can output multiple arguments, concatenate strings, customize separators, repeat characters, and suppress the trailing newline.

print('hello world!')
print('hello', 'world!')  # prints with a space
print('hello' + 'world!')  # concatenates without space
print('hello', 'world', sep='***')  # uses *** as separator
print('#' * 50)  # repeats character 50 times
print('how are you?', end='')  # suppresses newline

03 - Basic Operators

Demonstrates arithmetic, comparison, and logical operators with their precedence.

print(5 / 2)  # 2.5
print(5 // 2)  # integer division
print(5 % 2)  # remainder
print(5 ** 3)  # exponentiation
print(5 > 3)  # True
print(3 > 5)  # False
print(20 > 10 > 5)  # chained comparison
print(20 > 10 and 10 > 5)  # equivalent logical expression
print(not 20 > 10)  # False

04 - input

Shows how to read user input and convert types.

number = input("请输入数字：")  # input reads a string
print(number)
print(type(number))  # shows <class 'str'>

print(number + 10)  # TypeError
print(int(number) + 10)  # converts to int
print(number + str(10))  # concatenates as strings

05 - Basic I/O Exercise

username = input('username: ')
print('welcome', username)   # default space separator
print('welcome ' + username)  # explicit space

06 - String Basics

Illustrates quoting, escaping, multi‑line strings, slicing, and common string operations.

sentence = 'tom\'s pet is a cat'  # escape single quote
sentence2 = "tom's pet is a cat"
sentence3 = "tom said:\"hello world!\""
sentence4 = 'tom said:"hello world"'
# Triple quotes for multi‑line string
words = """
hello
world
abcd
"""
print(words)

py_str = 'python'
len(py_str)
py_str[0]
'python'[0]
py_str[-1]
py_str[2:4]
py_str[2:]
py_str[:2]
py_str[:]
py_str[::2]
py_str[1::2]
py_str[::-1]

py_str + ' is good'
py_str * 3

't' in py_str
'th' in py_str
'to' in py_str
'to' not in py_str

07 - List Basics

alist = [10, 20, 30, 'bob', 'alice', [1,2,3]]
len(alist)
alist[-1]  # last element
alist[-1][-1]  # nested list element
[1,2,3][-1]
alist[-2][2]
alist[3:5]   # ['bob', 'alice']
10 in alist
'o' in alist
100 not in alist
alist[-1] = 100  # modify
alist.append(200)  # add element

08 - Tuple Basics

atuple = (10, 20, 30, 'bob', 'alice', [1,2,3])
len(atuple)
10 in atuple
atuple[2]
atuple[3:5]
# atuple[-1] = 100  # error, tuple is immutable

09 - Dictionary Basics

# key‑value mapping, unordered
adict = {'name': 'bob', 'age': 23}
len(adict)
'bob' in adict  # False, checks keys
'name' in adict  # True
adict['email'] = '[email protected]'  # add new key
adict['age'] = 25  # update existing key

10 - Basic Truthiness

if 3 > 0:
    print('yes')
    print('ok')

if 10 in [10, 20, 30]:
    print('ok')

if -0.0:
    print('yes')  # False for zero

if [1, 2]:
    print('yes')  # True for non‑empty list

if ' ':
    print('yes')  # True for non‑empty string

11 - Conditional Expression (Ternary)

a = 10
b = 20

if a < b:
    smaller = a
else:
    smaller = b
print(smaller)

s = a if a < b else b  # equivalent
print(s)

12 - Login Check Exercise

import getpass

username = input('username: ')
password = getpass.getpass('password: ')

if username == 'bob' and password == '123456':
    print('Login successful')
else:
    print('Login incorrect')

13 - Number Guessing (Basic)

import random

num = random.randint(1, 10)
answer = int(input('guess a number: '))
if answer > num:
    print('猜大了')
elif answer < num:
    print('猜小了')
else:
    print('猜对了')

print('the number:', num)

14 - Score Classification 1

score = int(input('分数：'))

if score >= 90:
    print('优秀')
elif score >= 80:
    print('好')
elif score >= 70:
    print('良')
elif score >= 60:
    print('及格')
else:
    print('你要努力了')

15 - Score Classification 2

score = int(input('分数：'))

if score >= 60 and score < 70:
    print('及格')
elif 70 <= score < 80:
    print('良')
elif 80 <= score < 90:
    print('好')
elif score >= 90:
    print('优秀')
else:
    print('你要努力了')

16 - Rock Paper Scissors

import random

all_choices = ['石头', '剪刀', '布']
computer = random.choice(all_choices)
player = input('请出拳：')

print("Your choice: %s, Computer's choice: %s" % (player, computer))
if player == '石头':
    if computer == '石头':
        print('平局')
    elif computer == '剪刀':
        print('You WIN!!!')
    else:
        print('You LOSE!!!')
elif player == '剪刀':
    if computer == '石头':
        print('You LOSE!!!')
    elif computer == '剪刀':
        print('平局')
    else:
        print('You WIN!!!')
else:
    if computer == '石头':
        print('You WIN!!!')
    elif computer == '剪刀':
        print('You LOSE!!!')
    else:
        print('平局')

17 - Improved Rock Paper Scissors

import random

all_choices = ['石头', '剪刀', '布']
win_list = [['石头', '剪刀'], ['剪刀', '布'], ['布', '石头']]
prompt = "(0) 石头\n(1) 剪刀\n(2) 布\n请选择 (0/1/2): "
computer = random.choice(all_choices)
ind = int(input(prompt))
player = all_choices[ind]

print("Your choice: %s, Computer's choice: %s" % (player, computer))
if player == computer:
    print('\033[32;1m 平局、033[0m')
elif [player, computer] in win_list:
    print('\033[31;1mYou WIN!!!\033[0m')
else:
    print('\033[31;1mYou LOSE!!!\033[0m')

18 - Guess Number Until Correct

import random

num = random.randint(1, 10)
running = True

while running:
    answer = int(input('guess the number: '))
    if answer > num:
        print('猜大了')
    elif answer < num:
        print('猜小了')
    else:
        print('猜对了')
        running = False

19 - Guess Number with 5 Attempts

import random

num = random.randint(1, 10)
counter = 0

while counter < 5:
    answer = int(input('guess the number: '))
    if answer > num:
        print('猜大了')
    elif answer < num:
        print('猜小了')
    else:
        print('猜对了')
        break
    counter += 1
else:
    print('the number is:', num)

20 - While Loop Summation to 100

sum100 = 0
counter = 1

while counter < 101:
    sum100 += counter
    counter += 1

print(sum100)

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