Python Basics: Print, Variables, Data Types, Control Flow, Functions, and More

Print Output (print())

Demonstrates printing strings, variable values, formatted f-strings, multiple variables with spaces, and controlling line endings.

# 输出简单的欢迎信息
print("欢迎来到Python编程世界！")

name = "Alice"
# 输出变量的值，使用逗号分隔不同的参数
print("你好,", name)

age = 25
# 使用f-string嵌入变量到字符串中
print(f"我今年{age}岁了。")

first_name = "张"
last_name = "三"
# 输出多个变量，自动以空格分隔
print(first_name, last_name)

# 使用end参数控制输出后不换行，默认是换行符'\n'
print("这是第一行", end=" ")
print("这是在同一行继续输出的内容")

Variable Definition and Assignment

Shows how to define and assign integers, floats, multiple variables at once, reassign values, and perform calculations.

# 定义一个整数类型的变量
my_age = 28
print(my_age)  # 输出变量的值

# 定义一个浮点数类型的变量
pi_value = 3.14159
print(pi_value)  # 输出变量的值

# 同时给多个变量赋值
x, y, z = 1, 2, 3
print(x, y, z)  # 输出所有变量的值

# 变量可以被重新赋值
counter = 0
counter = counter + 1
print(counter)  # 输出更新后的值

# 变量可以参与数学运算
price = 9.99
quantity = 3
total_cost = price * quantity
print(total_cost)  # 输出总成本

Data Type Conversion

Illustrates converting strings to integers, floats to strings, lists to tuples, tuples to sets, and creating dictionaries.

# 将字符串形式的数字转换为整数
num_str = "100"
num_int = int(num_str)
print(num_int)  # 输出转换后的整数

# 将浮点数转换为字符串
num_float = 3.14
num_str = str(num_float)
print(num_str)  # 输出转换后的字符串

# 将列表转换为元组
list_items = [1, 2, 3]
tuple_items = tuple(list_items)
print(tuple_items)  # 输出转换后的元组

# 将元组转换为集合，去除重复元素
tuple_data = (1, 2, 2, 3, 4, 4)
set_data = set(tuple_data)
print(set_data)  # 输出转换后的集合

# 使用键值对创建字典
dict_data = dict(name="李四", age=30)
print(dict_data)  # 输出字典

Conditional Statements

Provides examples of simple if, if‑else, if‑elif‑else, nested conditions, and compound logical checks.

score = 60
if score >= 60:
    print("及格")  # 如果分数大于等于60，输出及格

temperature = 75
if temperature > 80:
    print("天气炎热")
else:
    print("天气凉爽")

user_input = 3
if user_input == 1:
    print("选择了选项一")
elif user_input == 2:
    print("选择了选项二")
else:
    print("选择了其他选项")

is_student = True
grade = 90
if is_student:
    if grade >= 90:
        print("优秀学生")
    else:
        print("普通学生")
else:
    print("非学生")

height = 175
weight = 70
if height > 170 and weight < 80:
    print("符合标准")

Loop Statements

Shows for loops over lists, range usage, while loops, list comprehensions, and break statements.

fruits = ["苹果", "香蕉", "橙子"]
for fruit in fruits:
    print(fruit)  # 遍历列表中的每个元素，并打印出来

for i in range(1, 5):  # 从1开始到4结束（不包括5）
    print(i)  # 打印当前索引

count = 0
while count < 5:
    print(count)  # 当count小于5时，打印count的值
    count += 1  # 每次循环后增加count的值

squares = [x**2 for x in range(10)]
for square in squares:
    print(square)  # 使用列表解析生成平方数列表，并遍历打印

for number in range(1, 10):
    if number == 5:
        break  # 当number等于5时，跳出循环
    print(number)

Function Definition

Defines simple functions, functions with parameters, return values, default arguments, and variable‑length arguments.

def greet():
    """这是一个简单的问候函数"""
    print("你好，欢迎来到Python的世界！")  # 打印欢迎信息
greet()  # 调用greet函数

def say_hello(name):
    """根据传入的名字打印个性化问候"""
    print(f"你好, {name}!")  # 使用f-string格式化字符串
say_hello("小明")  # 调用say_hello函数并传递参数

def add_numbers(a, b):
    """返回两个数相加的结果"""
    return a + b  # 返回计算结果
result = add_numbers(5, 3)  # 调用add_numbers函数并接收返回值
print(result)  # 输出结果

def describe_pet(pet_name, animal_type='狗'):
    """描述宠物的信息"""
    print(f"\n我有一只{animal_type}。")
    print(f"我的{animal_type}叫{pet_name}。")
describe_pet('旺财')  # 使用默认参数调用describe_pet函数
describe_pet('汤姆', '猫')  # 传递所有参数调用describe_pet函数

def make_pizza(*toppings):
    """打印顾客点的所有配料"""
    print("\n制作披萨需要以下配料:")
    for topping in toppings:
        print(f"- {topping}")
make_pizza('蘑菇', '青椒', '培根')  # 调用make_pizza函数并传递多个参数

Function Calls

Examples of calling functions without parameters, with arguments, with return values, keyword arguments, and variable‑length arguments.

def welcome_message():
    """显示欢迎消息"""
    print("欢迎使用我们的服务！")
welcome_message()  # 直接调用函数

def display_message(message):
    """显示传递的消息"""
    print(message)
display_message("这是通过函数传递的消息。")  # 调用函数并传递参数

def get_formatted_name(first_name, last_name):
    """返回整洁的姓名"""
    full_name = f"{first_name} {last_name}"
    return full_name.title()
musician = get_formatted_name('jimi', 'hendrix')  # 调用函数获取返回值
print(musician)  # 打印返回值

def build_profile(first, last, **user_info):
    """创建一个字典，其中包含我们知道的有关用户的一切"""
    user_info['first_name'] = first
    user_info['last_name'] = last
    return user_info
user_profile = build_profile('albert', 'einstein', location='princeton', field='physics')
print(user_profile)  # 调用函数并传递关键字参数

def make_sandwich(*items):
    """列出三明治中的所有配料"""
    print("\n正在为您制作含有以下配料的三明治:")
    for item in items:
        print(f"- {item}")
make_sandwich('火腿', '奶酪', '生菜')  # 调用函数并传递多个参数

Input Functions

Shows how to read user input, convert it, store multiple inputs, process numeric input, and combine with conditional logic.

name = input("请输入您的名字: ")  # 提示用户输入
print(f"您好, {name}!")  # 输出用户输入的名字

age = int(input("请输入您的年龄: "))  # 将输入转换为整数
next_year_age = age + 1
print(f"明年您将会是 {next_year_age} 岁。")

hobbies = []  # 创建空列表来保存爱好
hobby = input("请输入您的爱好 (输入'结束'来停止): ")
while hobby != '结束':
    hobbies.append(hobby)  # 添加到列表
    hobby = input("请输入您的下一个爱好 (输入'结束'来停止): ")
print("您的爱好有:", hobbies)

height = float(input("请输入您的身高（米）: "))
weight = float(input("请输入您的体重（千克）: "))
bmi = weight / (height * height)  # 计算BMI指数
print(f"您的BMI指数是 {bmi:.2f}")  # 格式化输出保留两位小数

answer = input("您喜欢编程吗？(yes/no): ")
if answer.lower() == 'yes':
    print("太棒了，继续加油！")
else:
    print("没关系，每个人都有自己的兴趣。")

Comments

Demonstrates single‑line comments, multi‑line comments, docstrings, and commenting out code.

# 这是一个单行注释
print("Hello, World!")  # 在打印语句后添加注释

''' 
这是一段多行注释，
用来解释下面这段代码的功能。
''' 
print("这是一段测试代码。")

def square_number(n):
    """
    返回给定数字的平方。
    参数 n: 要求平方的数字
    返回: 数字的平方
    """
    return n * n
print(square_number(4))  # 调用square_number函数

# print("这条消息不会被打印出来。")
print("这条消息会被打印出来。")

# 计算阶乘
def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n-1)  # 递归调用自身
print(factorial(5))  # 输出5的阶乘

Indentation

Shows correct and incorrect indentation for if statements, loops, functions, and nested structures.

age = 20
if age >= 18:
    print("成年人")  # 正确缩进，属于if块
print("检查完成")  # 不属于if块

for i in range(5):
    print(i)  # 每次循环都会执行，并且正确缩进
print("循环结束")  # 循环结束后执行，不缩进

def my_function():
    print("这是函数内部的代码")  # 函数体内的代码必须缩进
my_function()  # 调用函数

def another_function():
print("如果没有正确缩进，会抛出IndentationError")  # 这里缺少缩进，会导致错误

if True:
    print("外层条件成立")
    if False:
        print("内层条件不成立")  # 内层条件的代码块
    else:
        print("内层条件成立")  # 内层else部分的代码块

Program Exit

Illustrates using exit() and sys.exit() with status codes, both in scripts and inside functions.

print("程序开始")
exit()  # 程序在此处停止，不会执行后续代码
print("这行代码不会被执行")  # 此行代码不会被执行

import sys
print("尝试正常退出...")
sys.exit(0)  # 以状态码0退出，表示正常结束

import sys
def divide(a, b):
    if b == 0:
        print("除数不能为零")
        sys.exit(1)  # 错误退出，状态码为1
    return a / b
result = divide(10, 0)

import sys
try:
    raise ValueError("触发一个值错误")
except ValueError:
    print("捕获到异常，准备退出程序")
    sys.exit(2)  # 根据异常类型选择不同的退出状态码

user_input = input("请输入'y'来继续：")
if user_input != 'y':
    print("用户决定退出")
    exit()  # 用户决定不继续，则退出程序
print("继续执行程序...")  # 只有当用户输入'y'时才会执行

Arithmetic Operators

Shows addition, subtraction, multiplication, division, floor division, and modulo.

a = 5
b = 3
print(f"{a} + {b} = {a + b}")  # 输出: 5 + 3 = 8

a = 10
b = 4
print(f"{a} - {b} = {a - b}")  # 输出: 10 - 4 = 6

a = 7
b = 6
print(f"{a} * {b} = {a * b}")  # 输出: 7 * 6 = 42

a = 9
b = 2
print(f"{a} / {b} = {a / b}")  # 输出: 9 / 2 = 4.5

a = 11
b = 3
print(f"{a} // {b} = {a // b}, {a} % {b} = {a % b}")  # 输出: 11 // 3 = 3, 11 % 3 = 2

Logical Operators

Demonstrates and, or, not, combined logical expressions, and usage inside loops.

age = 20
has_license = True
if age >= 18 and has_license:
    print("可以合法驾驶")  # 当年龄大于等于18且有驾照时输出此消息

is_student = False
has_discount_card = True
if is_student or has_discount_card:
    print("可以享受折扣")  # 学生或有折扣卡都可以享受折扣

raining = False
if not raining:
    print("天气不错，适合外出")  # 如果不下雨，则适合外出

temperature = 22
humidity = 70
if temperature > 20 and humidity < 80:
    print("气候宜人")  # 温度高于20且湿度低于80时气候宜人

for i in range(1, 11):
    if i % 2 == 0 and i % 3 == 0:
        print(f"{i} 同时能被2和3整除")  # 打印同时能被2和3整除的数字

Identity Operators

Shows usage of is, is not, and id() to compare object identities.

x = ["apple", "banana"]
y = x
print(x is y)  # 输出: True，因为x和y引用同一个列表对象

x = ["apple", "banana"]
z = ["apple", "banana"]
print(x is not z)  # 输出: True，尽管内容相同，但它们是两个不同的对象

x = [1, 2, 3]
y = x
print(id(x), id(y))  # 输出相同的内存地址
print(x is y)  # 输出: True

a = None
b = None
if a is b:
    print("a和b都是None，或者引用同一对象")  # 输出: a和b都是None，或者引用同一对象

x = [1, 2, 3]
y = list(x)  # 创建一个新的列表，与x内容相同
print(x == y)  # 输出: True，因为它们的内容相等
print(x is y)  # 输出: False，因为它们不是同一个对象

Membership Operators

Demonstrates in and not in with lists, strings, and dictionaries.

fruits = ["apple", "banana", "cherry"]
if "banana" in fruits:
    print("香蕉在水果列表中")  # 输出: 香蕉在水果列表中

if "orange" not in fruits:
    print("橙子不在水果列表中")  # 输出: 橙子不在水果列表中

for fruit in ["apple", "banana", "cherry"]:
    if fruit in ["banana", "cherry"]:
        print(f"{fruit} 是我喜欢吃的水果之一")  # 输出喜欢的水果

sentence = "Hello, world!"
if "world" in sentence:
    print("找到了单词 'world'")  # 输出: 找到了单词 'world'

student_scores = {"Alice": 90, "Bob": 85}
if "Alice" in student_scores:
    print(f"Alice的成绩是 {student_scores['Alice']}")  # 输出: Alice的成绩是 90

Length Operations (len())

Shows using len() on strings, lists, tuples, dictionaries, and sets.

text = "Hello, World!"
print(f"字符串 '{text}' 的长度是 {len(text)}")  # 输出: 字符串 'Hello, World!' 的长度是 13

numbers = [1, 2, 3, 4, 5]
print(f"列表 {numbers} 中有 {len(numbers)} 个元素")  # 输出: 列表 [1, 2, 3, 4, 5] 中有 5 个元素

fruits = ("apple", "banana", "cherry")
print(f"元组 {fruits} 的大小是 {len(fruits)}")  # 输出: 元组 ('apple', 'banana', 'cherry') 的大小是 3

person = {"name": "Alice", "age": 25, "city": "New York"}
print(f"字典中有 {len(person)} 对键值对")  # 输出: 字典中有 3 对键值对

unique_numbers = {1, 2, 2, 3, 4, 4, 5}
print(f"集合 {unique_numbers} 包含 {len(unique_numbers)} 个唯一元素")  # 输出: 集合 {1, 2, 3, 4, 5} 包含 5 个唯一元素

Range Generator (range())

Illustrates basic range usage, even numbers, reverse ranges, list creation, and iterating with len().

for i in range(5):
    print(i)  # 输出: 0 1 2 3 4

for i in range(2, 11, 2):
    print(i)  # 输出: 2 4 6 8 10

for i in range(9, -1, -1):
    print(i)  # 输出: 9 8 7 6 5 4 3 2 1 0

numbers_list = list(range(1, 6))
print(f"创建的列表是 {numbers_list}")  # 输出: 创建的列表是 [1, 2, 3, 4, 5]

items = ["apple", "banana", "cherry"]
for i in range(len(items)):
    print(f"第{i+1}项是{items[i]}")  # 输出: 第1项是apple 第2项是banana 第3项是cherry

Slice Operations

Shows list slicing, negative indices, step slicing, full reversal, and slice assignment.

my_list = [0, 1, 2, 3, 4, 5]
slice_of_list = my_list[1:4]
print(f"切片后的列表是 {slice_of_list}")  # 输出: 切片后的列表是 [1, 2, 3]

my_string = "Python"
reversed_slice = my_string[-3:]
print(f"切片结果是 {reversed_slice}")  # 输出: 切片结果是 hon

my_tuple = (0, 1, 2, 3, 4, 5)
even_elements = my_tuple[::2]
print(f"每隔一个元素提取的结果是 {even_elements}")  # 输出: 每隔一个元素提取的结果是 (0, 2, 4)

sequence = "abcdef"
reversed_sequence = sequence[::-1]
print(f"反转后的序列是 {reversed_sequence}")  # 输出: 反转后的序列是 fedcba

letters = ['a', 'b', 'c', 'd', 'e']
letters[1:4] = ['B', 'C', 'D']
print(f"更新后的列表是 {letters}")  # 输出: 更新后的列表是 ['a', 'B', 'C', 'D', 'e']

List Comprehensions

Creates lists using comprehensions with expressions, filters, and transformations.

squares = [x**2 for x in range(1, 6)]
print(f"平方数列表是 {squares}")  # 输出: 平方数列表是 [1, 4, 9, 16, 25]

even_numbers = [num for num in range(1, 11) if num % 2 == 0]
print(f"偶数列表是 {even_numbers}")  # 输出: 偶数列表是 [2, 4, 6, 8, 10]

words = ["hello", "world", "python"]
upper_words = [word.upper() for word in words]
print(f"大写单词列表是 {upper_words}")  # 输出: 大写单词列表是 ['HELLO', 'WORLD', 'PYTHON']

pairs = [(x, y) for x in [1, 2, 3] for y in ['a', 'b']]
print(f"笛卡尔积列表是 {pairs}")  # 输出: 笛卡尔积列表是 [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b'), (3, 'a'), (3, 'b')]

mixed_data = [0, "apple", 1, "banana", 2, None, 3, ""]
filtered_data = [item for item in mixed_data if isinstance(item, int) and item > 0]
print(f"过滤并转换后的数据是 {filtered_data}")  # 输出: 过滤并转换后的数据是 [1, 2, 3]

Tuple Definition

Shows simple tuple creation, single‑element tuple syntax, unpacking, immutability, and using tuples as dictionary keys.

simple_tuple = (1, 2, 3)
print(f"简单元组是 {simple_tuple}")  # 输出: 简单元组是 (1, 2, 3)

single_element_tuple = (42,)
print(f"单元素元组是 {single_element_tuple}")  # 输出: 单元素元组是 (42,)

coordinates = (10, 20)
x, y = coordinates
print(f"x坐标是 {x}, y坐标是 {y}")  # 输出: x坐标是 10, y坐标是 20

immutable_tuple = (1, 2, 3)
try:
    immutable_tuple[0] = 4  # 尝试修改元组中的元素会导致错误
except TypeError as e:
    print(f"错误信息: {e}")  # 输出: 错误信息: 'tuple' object does not support item assignment

student_scores = {(1, "Alice"): 90, (2, "Bob"): 85}
print(f"Alice的成绩是 {student_scores[(1, 'Alice')]}")  # 输出: Alice的成绩是 90

Dictionary Definition

Creates dictionaries using literals, dict(), empty dicts, fromkeys, and shows basic operations.

person = {"name": "Alice", "age": 25}
print(f"字典内容: {person}")  # 输出: 字典内容: {'name': 'Alice', 'age': 25}

book = dict(title="Python编程", author="张三")
print(f"书的信息: {book}")  # 输出: 书的信息: {'title': 'Python编程', 'author': '张三'}

shopping_list = {"fruits": ["apple", "banana"], "vegetables": ["carrot", "lettuce"]}
print(f"购物清单: {shopping_list}")

empty_dict = {}
empty_dict["country"] = "China"
print(f"国家信息: {empty_dict}")  # 输出: 国家信息: {'country': 'China'}

default_values = {}.fromkeys(['height', 'width'], 0)
print(f"默认尺寸: {default_values}")  # 输出: 默认尺寸: {'height': 0, 'width': 0}

Dictionary Operations

Demonstrates get(), pop(), update(), clear(), and key existence checks.

user_info = {"name": "李四", "email": "[email protected]"}
email = user_info.get("email", "无邮箱信息")
print(f"用户邮箱: {email}")  # 输出: 用户邮箱: [email protected]

scores = {"math": 90, "english": 85}
math_score = scores.pop("math")
print(f"数学成绩已移除: {math_score}")  # 输出: 数学成绩已移除: 90

first_half = {"Q1": 100, "Q2": 200}
second_half = {"Q3": 300, "Q4": 400}
first_half.update(second_half)
print(f"全年业绩: {first_half}")  # 输出: 全年业绩: {'Q1': 100, 'Q2': 200, 'Q3': 300, 'Q4': 400}

inventory = {"apples": 30, "bananas": 45}
inventory.clear()
print(f"库存清空后: {inventory}")  # 输出: 库存清空后: {}

settings = {"theme": "dark", "language": "en"}
has_theme = "theme" in settings
print(f"是否有主题设置: {has_theme}")  # 输出: 是否有主题设置: True

File Operations

Shows opening files for reading, writing, appending, line‑by‑line reading, and using multiple context managers.

with open('example.txt', 'r') as file:
    content = file.read()
    print(f"文件内容: {content}")

with open('output.txt', 'w') as file:
    file.write("这是一个测试文件。")
    print("写入完成")

with open('output.txt', 'a') as file:
    file.write("\n这是追加的内容。")
    print("追加完成")

with open('example.txt', 'r') as file:
    for line in file:
        print(line.strip())  # 去掉每行末尾的换行符

with open('source.txt', 'r') as src, open('destination.txt', 'w') as dest:
    content = src.read()
    dest.write(content)
    print("复制完成")

Exception Handling

Provides try‑except‑finally, handling specific exceptions, else clause, and exception handling inside functions.

try:
    with open('nonexistent_file.txt', 'r') as f:
        content = f.read()
except FileNotFoundError as e:
    print(f"错误: {e}")
finally:
    print("无论是否发生异常，都会执行此代码")

try:
    number = int("abc")
except ValueError as e:
    print(f"错误: {e}")

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"除零错误: {e}")
except Exception as e:
    print(f"其他错误: {e}")

try:
    number = int("123")
except ValueError:
    print("输入不是一个有效的整数")
else:
    print(f"成功转换为整数: {number}")

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        print("除数不能为零")
        return None
result = divide(10, 0)
if result is not None:
    print(f"结果是: {result}")

Module Import

Shows importing whole modules, specific objects, aliasing, wildcard import (not recommended), and dynamic imports.

import math
print(f"圆周率: {math.pi}")  # 输出: 圆周率: 3.141592653589793

from datetime import datetime
current_time = datetime.now()
print(f"当前时间: {current_time}")  # 输出: 当前时间: 2025-02-06 14:16:00.123456

import numpy as np
array = np.array([1, 2, 3])
print(f"数组: {array}")  # 输出: 数组: [1 2 3]

from os.path import *
print(f"当前工作目录: {getcwd()}")  # 输出: 当前工作目录: /path/to/current/working/directory

import importlib
json_module = importlib.import_module('json')
data = json_module.dumps({"key": "value"})
print(f"JSON字符串: {data}")  # 输出: JSON字符串: {"key": "value"}