Python 基础语法
概述
Python 是一种高级编程语言，以简洁易读的语法著称。本文档涵盖了 Python 的核心语法知识。
基本语法结构
Hello World
print("Hello, World!")注释
# 单行注释
print("Hello") # 行尾注释
"""
多行注释
可以使用三引号
"""
'''
另一种多行注释方式
'''变量与数据类型
变量赋值
# 变量命名：小写字母和下划线
name = "Alice"
age = 25
height = 1.75
is_student = True基本数据类型
| 类型 | 示例 | 说明 |
|---|---|---|
int | 42 | 整数 |
float | 3.14 | 浮点数 |
str | "hello" | 字符串 |
bool | True | 布尔值 |
None | None | 空值 |
# 类型检查与转换
x = 42
print(type(x)) # <class 'int'>
print(float(x)) # 42.0
print(str(x)) # "42"字符串操作
# 字符串定义
s1 = "双引号"
s2 = '单引号'
s3 = """多行
字符串"""
# 字符串格式化
name = "Bob"
age = 30
print(f"我叫{name}，今年{age}岁") # f-string (推荐)
print("我叫{}，今年{}岁".format(name, age))
# 字符串方法
text = "Hello World"
print(text.lower()) # "hello world"
print(text.upper()) # "HELLO WORLD"
print(text.split()) # ["Hello", "World"]
print(len(text)) # 11运算符
算术运算符
a, b = 10, 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) # 取余: 1
print(a ** b) # 幂运算: 1000比较运算符
x, y = 5, 10
print(x == y) # False
print(x != y) # True
print(x < y) # True
print(x > y) # False
print(x <= y) # True
print(x >= y) # False逻辑运算符
a, b = True, False
print(a and b) # False
print(a or b) # True
print(not a) # False控制流
条件语句
age = 18
if age < 18:
print("未成年")
elif age == 18:
print("刚成年")
else:
print("成年人")
# 三元运算符
status = "成年" if age >= 18 else "未成年"for 循环
# 遍历列表
fruits = ["apple", "banana", "orange"]
for fruit in fruits:
print(fruit)
# 使用 range
for i in range(5):
print(i) # 0, 1, 2, 3, 4
for i in range(1, 6, 2):
print(i) # 1, 3, 5
# 遍历字典
person = {"name": "Alice", "age": 25}
for key, value in person.items():
print(f"{key}: {value}")while 循环
count = 0
while count < 5:
print(count)
count += 1
# break 和 continue
i = 0
while i < 10:
if i == 3:
i += 1
continue # 跳过本次
if i == 7:
break # 退出循环
print(i)
i += 1数据结构
列表 (List)
# 创建列表
numbers = [1, 2, 3, 4, 5]
empty_list = []
# 常用操作
numbers.append(6) # 添加元素: [1,2,3,4,5,6]
numbers.insert(0, 0) # 插入: [0,1,2,3,4,5,6]
numbers.remove(3) # 删除值3
numbers.pop() # 删除最后一个
numbers[0] = 99 # 修改元素
# 切片
print(numbers[1:4]) # [1, 2, 4]
print(numbers[:3]) # [99, 1, 2]
print(numbers[-2:]) # [5, 6]
print(numbers[::2]) # 每隔一个取值
# 列表推导式
squares = [x**2 for x in range(5)] # [0, 1, 4, 9, 16]
evens = [x for x in range(10) if x % 2 == 0] # [0, 2, 4, 6, 8]字典 (Dict)
# 创建字典
person = {
"name": "Alice",
"age": 25,
"city": "Beijing"
}
# 访问和修改
print(person["name"]) # "Alice"
print(person.get("email", "未设置")) # "未设置" (默认值)
person["age"] = 26
person["email"] = "alice@example.com"
# 常用方法
print(person.keys()) # dict_keys([...])
print(person.values()) # dict_values([...])
print(person.items()) # dict_items([...])
del person["city"] # 删除键
person.pop("email") # 删除并返回值集合 (Set)
# 创建集合
numbers = {1, 2, 3, 4}
numbers.add(5)
numbers.discard(2)
# 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: {1, 2, 4, 5}元组 (Tuple)
# 元组是不可变的
point = (3, 4)
single = (42,) # 单元素元组需要逗号
# 解包
x, y = point
print(x, y) # 3 4函数
定义和调用
def greet(name):
"""向某人打招呼"""
return f"Hello, {name}!"
message = greet("Alice")
print(message) # "Hello, Alice!"参数
# 默认参数
def power(base, exponent=2):
return base ** exponent
print(power(3)) # 9
print(power(3, 3)) # 27
# 可变参数
def sum_all(*args):
return sum(args)
print(sum_all(1, 2, 3, 4)) # 10
# 关键字参数
def create_user(name, **kwargs):
user = {"name": name}
user.update(kwargs)
return user
user = create_user("Alice", age=25, city="Beijing")Lambda 函数
# 简单的匿名函数
square = lambda x: x ** 2
print(square(5)) # 25
# 常用于函数式编程
numbers = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x**2, numbers))
evens = list(filter(lambda x: x % 2 == 0, numbers))异常处理
try:
result = 10 / 0
except ZeroDivisionError:
print("不能除以零")
except Exception as e:
print(f"发生错误: {e}")
else:
print("没有错误")
finally:
print("总是执行")
# 自定义异常
class CustomError(Exception):
pass
try:
raise CustomError("自定义错误")
except CustomError as e:
print(f"捕获到: {e}")类与对象
基本类定义
class Person:
def __init__(self, name, age):
self.name = name # 实例属性
self.age = age
def greet(self):
return f"我是 {self.name}，{self.age} 岁"
@classmethod
def from_birth_year(cls, name, birth_year):
age = 2026 - birth_year
return cls(name, age)
# 使用类
person = Person("Alice", 25)
print(person.greet())
# 使用类方法
bob = Person.from_birth_year("Bob", 1990)
print(bob.age) # 36继承
class Animal:
def __init__(self, name):
self.name = name
def speak(self):
pass
class Dog(Animal):
def speak(self):
return f"{self.name} 说: 汪汪!"
class Cat(Animal):
def speak(self):
return f"{self.name} 说: 喵喵!"
dog = Dog("旺财")
cat = Cat("咪咪")
print(dog.speak()) # 旺财 说: 汪汪!
print(cat.speak()) # 咪咪 说: 喵喵!文件操作
读写文件
# 写入文件
with open("example.txt", "w", encoding="utf-8") as f:
f.write("Hello, World!\n")
f.write("你好，世界！")
# 读取文件
with open("example.txt", "r", encoding="utf-8") as f:
content = f.read() # 读取全部
print(content)
# 逐行读取
with open("example.txt", "r", encoding="utf-8") as f:
for line in f:
print(line.strip()) # 去除换行符常用内置函数
# 类型转换
int("42") # 42
float("3.14") # 3.14
str(42) # "42"
list("abc") # ['a', 'b', 'c']
# 序列操作
len([1, 2, 3]) # 3
max([1, 2, 3]) # 3
min([1, 2, 3]) # 1
sum([1, 2, 3]) # 6
sorted([3, 1, 2]) # [1, 2, 3]
# 输入输出
name = input("请输入名字: ")
print(f"你好, {name}!")导入模块
# 导入整个模块
import math
print(math.sqrt(16)) # 4.0
# 导入特定函数
from math import sqrt, pi
print(sqrt(16)) # 4.0
print(pi) # 3.14159...
# 使用别名
import numpy as np
import pandas as pd学习建议
- 多动手练习，边学边写代码
- 阅读 Python 官方文档
- 参与开源项目积累经验
注意事项
- Python 使用缩进表示代码块（通常是4个空格）
- 变量命名避免使用 Python 关键字
- 注意列表的可变性（深拷贝 vs 浅拷贝）