Python 基础语法全面指南

Python 是一种高级、解释型、通用的编程语言，以其简洁的语法和强大的功能而闻名。本文将全面介绍 Python 的基础语法，适合初学者快速掌握 Python 编程的核心概念。

1. Python 基础

1.1 第一个 Python 程序

# 这是一个简单的Python程序
print("Hello, World!")  # 输出: Hello, World!

1.2 注释

# 这是单行注释

"""
这是多行注释
可以跨越多行
"""

'''
这也是多行注释
使用单引号
'''

2. 变量与数据类型

2.1 变量

# 变量赋值
x = 5           # 整数
y = 3.14        # 浮点数
name = "Alice"  # 字符串
is_active = True # 布尔值

print(x, y, name, is_active)  # 输出: 5 3.14 Alice True

2.2 基本数据类型

# 整数
a = 10
b = -5

# 浮点数
pi = 3.14159
temperature = -10.5

# 字符串
s1 = '单引号字符串'
s2 = "双引号字符串"
s3 = """多行
字符串"""

# 布尔值
t = True
f = False

# 空值
n = None

print(type(a), type(pi), type(s1), type(t), type(n))
# 输出: <class 'int'> <class 'float'> <class 'str'> <class 'bool'> <class 'NoneType'>

2.3 类型转换

# 显式类型转换
num_str = "123"
num_int = int(num_str)  # 字符串转整数
num_float = float(num_str)  # 字符串转浮点数
str_num = str(123)  # 数字转字符串

print(num_int, num_float, str_num)  # 输出: 123 123.0 '123'

3. 运算符

3.1 算术运算符

a = 10
b = 3

print(a + b)   # 加法 13
print(a - b)   # 减法 7
print(a * b)   # 乘法 30
print(a / b)   # 除法 3.333...
print(a // b)  # 整除 3
print(a % b)   # 取模 1
print(a ** b)  # 幂运算 1000

3.2 比较运算符

x = 5
y = 8

print(x == y)  # False
print(x != y)  # True
print(x > y)   # False
print(x < y)   # True
print(x >= y)  # False
print(x <= y)  # True

3.3 逻辑运算符

a = True
b = False

print(a and b)  # False
print(a or b)   # True
print(not a)    # False

3.4 赋值运算符

x = 5
x += 3  # 等同于 x = x + 3
print(x)  # 8

x -= 2  # 等同于 x = x - 2
print(x)  # 6

x *= 4  # 等同于 x = x * 4
print(x)  # 24

x /= 3  # 等同于 x = x / 3
print(x)  # 8.0

4. 控制流

4.1 条件语句

# if 语句
age = 18

if age >= 18:
    print("你是成年人")
else:
    print("你是未成年人")

# elif 语句
score = 85

if score >= 90:
    print("优秀")
elif score >= 80:
    print("良好")
elif score >= 60:
    print("及格")
else:
    print("不及格")

4.2 循环语句

while 循环

# while 循环
count = 0
while count < 5:
    print(count)
    count += 1
# 输出: 0 1 2 3 4

# break 和 continue
num = 0
while num < 10:
    num += 1
    if num % 2 == 0:
        continue  # 跳过偶数
    if num > 7:
        break    # 当num大于7时退出循环
    print(num)
# 输出: 1 3 5 7

for 循环

# for 循环
for i in range(5):  # range(5) 生成0-4的序列
    print(i)
# 输出: 0 1 2 3 4

# 遍历列表
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
# 输出: apple banana cherry

# 带索引的遍历
for index, fruit in enumerate(fruits):
    print(index, fruit)
# 输出: 0 apple 1 banana 2 cherry

5. 数据结构

5.1 列表 (List)

# 创建列表
numbers = [1, 2, 3, 4, 5]
fruits = ["apple", "banana", "cherry"]
mixed = [1, "hello", 3.14, True]

# 访问元素
print(numbers[0])    # 1 (第一个元素)
print(fruits[-1])    # "cherry" (最后一个元素)

# 切片操作
print(numbers[1:3])  # [2, 3] (索引1到2的元素)
print(numbers[:3])   # [1, 2, 3] (前三个元素)
print(numbers[2:])   # [3, 4, 5] (从索引2开始的所有元素)

# 修改列表
fruits[1] = "blueberry"  # 修改元素
fruits.append("orange")  # 添加元素
fruits.insert(1, "mango") # 在指定位置插入元素
fruits.remove("apple")    # 删除元素
popped = fruits.pop()     # 移除并返回最后一个元素

# 列表操作
combined = numbers + fruits  # 列表拼接
repeated = numbers * 2       # 列表重复

# 列表方法
length = len(numbers)        # 获取长度
numbers.sort()               # 排序
numbers.reverse()            # 反转
index = fruits.index("mango") # 查找索引

5.2 元组 (Tuple)

# 创建元组
coordinates = (10, 20)
colors = ("red", "green", "blue")
single_element = (5,)  # 注意逗号，表示是元组而不是括号表达式

# 访问元素
print(coordinates[0])  # 10
print(colors[-1])      # "blue"

# 元组是不可变的
# colors[1] = "yellow"  # 会报错

# 元组解包
x, y = coordinates
print(x, y)  # 10 20

5.3 字典 (Dictionary)

# 创建字典
person = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

# 访问元素
print(person["name"])  # "Alice"
print(person.get("age"))  # 25

# 修改字典
person["age"] = 26           # 更新值
person["job"] = "Engineer"   # 添加新键值对

# 字典方法
keys = person.keys()         # 获取所有键
values = person.values()     # 获取所有值
items = person.items()       # 获取所有键值对

# 遍历字典
for key, value in person.items():
    print(f"{key}: {value}")

5.4 集合 (Set)

# 创建集合
fruits = {"apple", "banana", "cherry"}
numbers = set([1, 2, 3, 4, 5])

# 集合操作
fruits.add("orange")       # 添加元素
fruits.remove("banana")    # 移除元素
fruits.discard("grape")    # 安全移除元素(如果存在)

# 集合运算
a = {1, 2, 3}
b = {3, 4, 5}

print(a | b)  # 并集 {1, 2, 3, 4, 5}
print(a & b)  # 交集 {3}
print(a - b)  # 差集 {1, 2}
print(a ^ b)  # 对称差集 {1, 2, 4, 5}

6. 函数

6.1 定义函数

# 简单函数
def greet():
    print("Hello!")

greet()  # 输出: Hello!

# 带参数的函数
def greet_name(name):
    print(f"Hello, {name}!")

greet_name("Alice")  # 输出: Hello, Alice!

# 带返回值的函数
def add(a, b):
    return a + b

result = add(3, 5)
print(result)  # 8

# 默认参数
def power(base, exponent=2):
    return base ** exponent

print(power(3))     # 9 (使用默认指数2)
print(power(3, 3))  # 27

# 可变参数
def sum_all(*numbers):
    total = 0
    for num in numbers:
        total += num
    return total

print(sum_all(1, 2, 3))       # 6
print(sum_all(1, 2, 3, 4, 5)) # 15

# 关键字参数
def person_info(**info):
    for key, value in info.items():
        print(f"{key}: {value}")

person_info(name="Alice", age=25, city="New York")
# 输出:
# name: Alice
# age: 25
# city: New York

6.2 Lambda 函数

# 简单的lambda函数
square = lambda x: x ** 2
print(square(5))  # 25

# 在排序中使用
pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
pairs.sort(key=lambda pair: pair[1])
print(pairs)  # 按第二个元素排序: [(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]

7. 文件操作

7.1 读写文件

# 写入文件
with open("example.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is a sample file.\n")

# 读取文件
with open("example.txt", "r") as file:
    content = file.read()
    print(content)
    # 输出:
    # Hello, World!
    # This is a sample file.

# 逐行读取
with open("example.txt", "r") as file:
    for line in file:
        print(line.strip())  # strip() 移除行尾的换行符

# 追加模式
with open("example.txt", "a") as file:
    file.write("Adding a new line.\n")

8. 异常处理

# 基本异常处理
try:
    result = 10 / 0
except ZeroDivisionError:
    print("不能除以零!")
else:
    print("结果是:", result)
finally:
    print("这段代码总是会执行")

# 处理多个异常
try:
    num = int(input("请输入一个数字: "))
    result = 100 / num
except ValueError:
    print("请输入有效的数字!")
except ZeroDivisionError:
    print("不能输入零!")
else:
    print(f"结果是: {result}")

# 抛出异常
def check_age(age):
    if age < 0:
        raise ValueError("年龄不能为负数")
    if age < 18:
        print("未成年人")
    else:
        print("成年人")

try:
    check_age(-5)
except ValueError as e:
    print(e)  # 输出: 年龄不能为负数

9. 面向对象编程

9.1 类和对象

# 定义一个类
class Dog:
    # 类属性
    species = "Canis familiaris"

    # 初始化方法 (构造函数)
    def __init__(self, name, age):
        self.name = name  # 实例属性
        self.age = age

    # 实例方法
    def description(self):
        return f"{self.name} is {self.age} years old"

    def speak(self, sound):
        return f"{self.name} says {sound}"

# 创建实例
dog1 = Dog("Buddy", 5)
dog2 = Dog("Molly", 3)

# 访问属性和方法
print(dog1.name)  # Buddy
print(dog2.age)   # 3
print(dog1.description())  # Buddy is 5 years old
print(dog2.speak("Woof Woof"))  # Molly says Woof Woof

9.2 继承

# 父类
class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        raise NotImplementedError("子类必须实现这个方法")

# 子类
class Cat(Animal):
    def speak(self):
        return f"{self.name} says Meow!"

class Cow(Animal):
    def speak(self):
        return f"{self.name} says Moo!"

# 使用子类
cat = Cat("Whiskers")
cow = Cow("Bessie")

print(cat.speak())  # Whiskers says Meow!
print(cow.speak())  # Bessie says Moo!

10. 模块和包

10.1 使用模块

# 假设有一个名为 mymodule.py 的文件，内容如下:
"""
def greet(name):
    print(f"Hello, {name}!")

def add(a, b):
    return a + b
"""

# 导入整个模块
import mymodule
mymodule.greet("Alice")  # 输出: Hello, Alice!
print(mymodule.add(2, 3))  # 5

# 导入特定函数
from mymodule import greet, add
greet("Bob")  # 输出: Hello, Bob!
print(add(5, 7))  # 12

# 导入并重命名
from mymodule import greet as hello
hello("Charlie")  # 输出: Hello, Charlie!

# 导入所有内容 (不推荐)
from mymodule import *
greet("Dave")  # 输出: Hello, Dave!

10.2 创建包

my_package/
    __init__.py
    module1.py
    module2.py
    subpackage/
        __init__.py
        module3.py

# 使用包
from my_package import module1
from my_package.subpackage import module3

11. 常用内置函数

# abs() - 绝对值
print(abs(-5))  # 5

# len() - 长度
print(len("hello"))  # 5
print(len([1, 2, 3]))  # 3

# max() / min() - 最大值/最小值
print(max(1, 5, 3))  # 5
print(min([10, 2, 8]))  # 2

# sum() - 求和
print(sum([1, 2, 3, 4]))  # 10

# sorted() - 排序
numbers = [3, 1, 4, 1, 5, 9, 2]
print(sorted(numbers))  # [1, 1, 2, 3, 4, 5, 9]

# enumerate() - 枚举
fruits = ['apple', 'banana', 'cherry']
for index, fruit in enumerate(fruits):
    print(index, fruit)
# 输出:
# 0 apple
# 1 banana
# 2 cherry

# zip() - 并行迭代
names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]
for name, age in zip(names, ages):
    print(f"{name} is {age} years old")
# 输出:
# Alice is 25 years old
# Bob is 30 years old
# Charlie is 35 years old

12. 列表推导式和生成器表达式

12.1 列表推导式

# 简单的列表推导式
squares = [x**2 for x in range(10)]
print(squares)  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# 带条件的列表推导式
even_squares = [x**2 for x in range(10) if x % 2 == 0]
print(even_squares)  # [0, 4, 16, 36, 64]

# 嵌套的列表推导式
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flattened = [num for row in matrix for num in row]
print(flattened)  # [1, 2, 3, 4, 5, 6, 7, 8, 9]

12.2 生成器表达式

# 生成器表达式 (惰性求值)
squares_gen = (x**2 for x in range(10))
print(squares_gen)  # <generator object <genexpr> at 0x...>
print(list(squares_gen))  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# 生成器表达式在函数中使用
sum_of_squares = sum(x**2 for x in range(10))
print(sum_of_squares)  # 285

13. 装饰器

# 简单的装饰器
def my_decorator(func):
    def wrapper():
        print("函数执行前...")
        func()
        print("函数执行后...")
    return wrapper

@my_decorator
def say_hello():
    print("Hello!")

say_hello()
# 输出:
# 函数执行前...
# Hello!
# 函数执行后...

# 带参数的装饰器
def repeat(num_times):
    def decorator(func):
        def wrapper(*args, **kwargs):
            for _ in range(num_times):
                result = func(*args, **kwargs)
            return result
        return wrapper
    return decorator

@repeat(num_times=3)
def greet(name):
    print(f"Hello, {name}!")

greet("Alice")
# 输出:
# Hello, Alice!
# Hello, Alice!
# Hello, Alice!

14. 上下文管理器

# 使用 with 语句
with open('example.txt', 'r') as file:
    content = file.read()
    print(content)
# 文件会在with块结束后自动关闭

# 自定义上下文管理器
class ManagedFile:
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode
  
    def __enter__(self):
        self.file = open(self.filename, self.mode)
        return self.file
  
    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.file:
            self.file.close()

with ManagedFile('example.txt', 'r') as f:
    content = f.read()
    print(content)

15. 虚拟环境和包管理

虽然这不是语法部分，但对Python开发至关重要：

# 创建虚拟环境
python -m venv myenv

# 激活虚拟环境 (Windows)
myenv\Scripts\activate

# 激活虚拟环境 (macOS/Linux)
source myenv/bin/activate

# 安装包
pip install requests numpy pandas

# 列出已安装的包
pip list

# 生成requirements.txt
pip freeze > requirements.txt

# 从requirements.txt安装
pip install -r requirements.txt

结语

本文涵盖了 Python 的基础语法和核心概念，包括变量、数据类型、运算符、控制流、函数、面向对象编程等。掌握这些基础知识后，你就可以开始编写功能完善的 Python 程序了。Python 的强大之处还在于其丰富的标准库和第三方库，建议在实际项目中不断练习和探索。