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基础语法
Python 是一种简洁、易读、功能强大的编程语言，广泛应用于 Web 开发、数据分析、人工智能等领域。
第一个 Python 程序
python
# hello.py
print("Hello, World!")
# 运行: python hello.py变量与数据类型
变量
python
# 变量声明（无需指定类型）
name = "张三"
age = 25
height = 1.75
is_student = True
# 多变量赋值
a, b, c = 1, 2, 3
x = y = z = 0
# 交换变量
a, b = b, a
# 查看类型
print(type(name)) # <class 'str'>
print(type(age)) # <class 'int'>
print(type(height)) # <class 'float'>
print(type(is_student)) # <class 'bool'>数字类型
python
# 整数
a = 100
b = -50
c = 0xFF # 十六进制
d = 0o77 # 八进制
e = 0b1010 # 二进制
# 浮点数
pi = 3.14159
scientific = 1.5e-10 # 科学计数法
# 复数
z = 3 + 4j
print(z.real) # 3.0
print(z.imag) # 4.0
# 数字运算
print(10 + 3) # 13 加
print(10 - 3) # 7 减
print(10 * 3) # 30 乘
print(10 / 3) # 3.333... 除（浮点）
print(10 // 3) # 3 整除
print(10 % 3) # 1 取余
print(10 ** 3) # 1000 幂运算
# 数学函数
abs(-5) # 5 绝对值
round(3.14159, 2) # 3.14 四舍五入
max(1, 2, 3) # 3 最大值
min(1, 2, 3) # 1 最小值
pow(2, 10) # 1024 幂运算
import math
math.floor(3.7) # 3 向下取整
math.ceil(3.2) # 4 向上取整
math.sqrt(16) # 4.0 平方根
math.sin(math.pi / 2) # 1.0 正弦字符串
python
# 字符串定义
s1 = 'Hello'
s2 = "World"
s3 = '''多行
字符串'''
s4 = """多行
字符串"""
# 转义字符
print("Hello\nWorld") # 换行
print("Hello\tWorld") # 制表符
print("Hello\\World") # 反斜杠
# 原始字符串
raw_str = r"C:\Users\name"
print(raw_str) # C:\Users\name
# 字符串操作
s = "Hello, World!"
print(len(s)) # 13 长度
print(s[0]) # H 索引
print(s[-1]) # ! 负索引
print(s[0:5]) # Hello 切片
print(s[7:]) # World!
print(s[:5]) # Hello
print(s[::2]) # Hlo ol 步长
print(s[::-1]) # !dlroW ,olleH 反转
# 字符串方法
s = " Hello, World! "
print(s.strip()) # "Hello, World!" 去空格
print(s.lower()) # " hello, world! "
print(s.upper()) # " HELLO, WORLD! "
print(s.replace("World", "Python"))
print(s.split(",")) # [' Hello', ' World! ']
print(",".join(["a", "b", "c"])) # "a,b,c"
print(s.startswith(" H")) # True
print(s.endswith("! ")) # True
print(s.find("World")) # 9
print(s.count("l")) # 3
# 格式化字符串
name = "张三"
age = 25
# f-string (Python 3.6+)
print(f"姓名: {name}, 年龄: {age}")
print(f"明年: {age + 1}岁")
# format 方法
print("姓名: {}, 年龄: {}".format(name, age))
print("姓名: {0}, 年龄: {1}".format(name, age))
print("姓名: {name}, 年龄: {age}".format(name=name, age=age))
# % 格式化
print("姓名: %s, 年龄: %d" % (name, age))
print("浮点数: %.2f" % 3.14159)
# 字符串判断
s = "Hello123"
print(s.isalpha()) # False 是否全是字母
print(s.isdigit()) # False 是否全是数字
print(s.isalnum()) # True 是否全是字母或数字
print(s.isupper()) # False 是否全大写
print(s.islower()) # False 是否全小写布尔值
python
# 布尔值
is_true = True
is_false = False
# 比较运算
print(5 > 3) # True
print(5 < 3) # False
print(5 == 5) # True
print(5 != 3) # True
print(5 >= 5) # True
print(5 <= 3) # False
# 逻辑运算
print(True and False) # False 与
print(True or False) # True 或
print(not True) # False 非
# 短路求值
a = 0
result = a != 0 and 10 / a > 1 # 不会报错
# 假值
# False, None, 0, 0.0, '', [], (), {}, set()类型转换
python
# 转整数
int(3.14) # 3
int("123") # 123
int("0xFF", 16) # 255
# 转浮点数
float(3) # 3.0
float("3.14") # 3.14
# 转字符串
str(123) # "123"
str(3.14) # "3.14"
str(True) # "True"
# 转布尔值
bool(1) # True
bool(0) # False
bool("") # False
bool("hello") # True
# 转列表
list("hello") # ['h', 'e', 'l', 'l', 'o']
list((1, 2, 3)) # [1, 2, 3]
# 转元组
tuple([1, 2, 3]) # (1, 2, 3)
# 转集合
set([1, 2, 2, 3]) # {1, 2, 3}运算符
算术运算符
python
a, b = 10, 3
print(a + b) # 13 加
print(a - b) # 7 减
print(a * b) # 30 乘
print(a / b) # 3.333... 除
print(a // b) # 3 整除
print(a % b) # 1 取余
print(a ** b) # 1000 幂运算
# 自增（Python 没有 ++）
a += 1 # a = a + 1
a -= 1 # a = a - 1
a *= 2 # a = a * 2
a /= 2 # a = a / 2比较运算符
python
a, b = 5, 10
print(a == b) # False
print(a != b) # True
print(a > b) # False
print(a < b) # True
print(a >= b) # False
print(a <= b) # True
# 链式比较
x = 5
print(1 < x < 10) # True
print(1 < x and x < 10) # 等价逻辑运算符
python
a, b = True, False
print(a and b) # False 与
print(a or b) # True 或
print(not a) # False 非
# 短路求值
def check():
print("检查")
return True
False and check() # 不会调用 check()
True or check() # 不会调用 check()成员运算符
python
lst = [1, 2, 3, 4, 5]
print(3 in lst) # True
print(6 in lst) # False
print(6 not in lst) # True
s = "Hello World"
print("World" in s) # True
d = {"name": "张三", "age": 25}
print("name" in d) # True（检查键）身份运算符
python
a = [1, 2, 3]
b = [1, 2, 3]
c = a
print(a is c) # True 同一对象
print(a is b) # False 不同对象
print(a is not b) # True
print(a == b) # True 值相等
# 小整数缓存
x = 256
y = 256
print(x is y) # True
x = 257
y = 257
print(x is y) # False（可能）位运算符
python
a = 5 # 0101
b = 3 # 0011
print(a & b) # 1 (0001) 按位与
print(a | b) # 7 (0111) 按位或
print(a ^ b) # 6 (0110) 按位异或
print(~a) # -6 按位取反
print(a << 1) # 10 (1010) 左移
print(a >> 1) # 2 (0010) 右移控制流程
条件语句
python
# if 语句
age = 18
if age >= 18:
print("成年")
elif age >= 12:
print("青少年")
else:
print("儿童")
# 单行 if
status = "成年" if age >= 18 else "未成年"
# 多条件
score = 85
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
elif score >= 60:
grade = "C"
else:
grade = "D"
# 检查多个条件
x = 5
if x > 0 and x < 10:
print("0 < x < 10")
if 0 < x < 10: # 更 Pythonic
print("0 < x < 10")
# 检查是否为空
lst = []
if not lst:
print("列表为空")
# 检查 None
value = None
if value is None:
print("值为 None")循环语句
for 循环
python
# 遍历列表
fruits = ["苹果", "香蕉", "橙子"]
for fruit in fruits:
print(fruit)
# 遍历字符串
for char in "Hello":
print(char)
# 使用 range
for i in range(5):
print(i) # 0, 1, 2, 3, 4
for i in range(1, 6):
print(i) # 1, 2, 3, 4, 5
for i in range(0, 10, 2):
print(i) # 0, 2, 4, 6, 8
# 带索引遍历
for index, fruit in enumerate(fruits):
print(f"{index}: {fruit}")
# 同时遍历多个列表
names = ["张三", "李四", "王五"]
ages = [25, 30, 28]
for name, age in zip(names, ages):
print(f"{name}: {age}岁")
# 遍历字典
person = {"name": "张三", "age": 25, "city": "北京"}
for key in person:
print(key)
for key, value in person.items():
print(f"{key}: {value}")
for value in person.values():
print(value)while 循环
python
# 基本循环
count = 0
while count < 5:
print(count)
count += 1
# 带条件
while True:
response = input("继续吗？(y/n): ")
if response.lower() == 'n':
break
# while-else
count = 0
while count < 5:
print(count)
count += 1
else:
print("循环结束")break、continue 和 pass
python
# break - 跳出循环
for i in range(10):
if i == 5:
break
print(i) # 0, 1, 2, 3, 4
# continue - 跳过当前迭代
for i in range(10):
if i % 2 == 0:
continue
print(i) # 1, 3, 5, 7, 9
# pass - 占位符
for i in range(5):
pass # 什么都不做
def todo():
pass # 待实现循环中的 else
python
# for-else
for i in range(5):
if i == 10:
break
else:
print("循环正常结束") # 会执行
# 查找示例
numbers = [1, 2, 3, 4, 5]
target = 3
for num in numbers:
if num == target:
print(f"找到: {target}")
break
else:
print("未找到")列表推导式
基本语法
python
# 基本形式
numbers = [1, 2, 3, 4, 5]
squares = [x ** 2 for x in numbers]
print(squares) # [1, 4, 9, 16, 25]
# 带条件
evens = [x for x in numbers if x % 2 == 0]
print(evens) # [2, 4]
# 带条件表达式
result = ["偶数" if x % 2 == 0 else "奇数" for x in numbers]
print(result) # ['奇数', '偶数', '奇数', '偶数', '奇数']
# 嵌套循环
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [num for row in matrix for num in row]
print(flat) # [1, 2, 3, 4, 5, 6, 7, 8, 9]
# 多变量
pairs = [(x, y) for x in range(3) for y in range(3)]
print(pairs) # [(0,0), (0,1), (0,2), (1,0), ...]字典推导式
python
# 基本形式
numbers = [1, 2, 3, 4, 5]
squares_dict = {x: x ** 2 for x in numbers}
print(squares_dict) # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
# 交换键值
original = {"a": 1, "b": 2, "c": 3}
swapped = {v: k for k, v in original.items()}
print(swapped) # {1: 'a', 2: 'b', 3: 'c'}
# 带条件
scores = {"张三": 85, "李四": 92, "王五": 78}
passed = {name: score for name, score in scores.items() if score >= 80}
print(passed) # {'张三': 85, '李四': 92}集合推导式
python
# 基本形式
numbers = [1, 2, 2, 3, 3, 3, 4]
unique_squares = {x ** 2 for x in numbers}
print(unique_squares) # {1, 4, 9, 16}生成器表达式
python
# 使用圆括号
numbers = [1, 2, 3, 4, 5]
squares_gen = (x ** 2 for x in numbers)
print(squares_gen) # <generator object>
# 惰性求值
for square in squares_gen:
print(square)
# 转换为列表
squares_list = list(x ** 2 for x in numbers)
# 内存效率
# 列表推导式：立即生成所有元素
# 生成器表达式：按需生成函数
函数定义
python
# 基本函数
def greet():
print("Hello!")
greet()
# 带参数
def greet(name):
print(f"Hello, {name}!")
greet("张三")
# 带返回值
def add(a, b):
return a + b
result = add(5, 3)
print(result) # 8
# 多返回值
def get_info():
return "张三", 25, "北京"
name, age, city = get_info()
# 返回字典
def get_person():
return {"name": "张三", "age": 25}
person = get_person()参数类型
python
# 位置参数
def greet(name, age):
print(f"{name}, {age}岁")
greet("张三", 25)
# 关键字参数
greet(name="张三", age=25)
greet(age=25, name="张三")
# 默认参数
def greet(name, age=18):
print(f"{name}, {age}岁")
greet("张三") # 张三, 18岁
greet("李四", 25) # 李四, 25岁
# 可变位置参数
def sum_all(*args):
return sum(args)
print(sum_all(1, 2, 3, 4, 5)) # 15
# 可变关键字参数
def print_info(**kwargs):
for key, value in kwargs.items():
print(f"{key}: {value}")
print_info(name="张三", age=25, city="北京")
# 混合参数
def func(a, b, *args, **kwargs):
print(f"a={a}, b={b}")
print(f"args={args}")
print(f"kwargs={kwargs}")
func(1, 2, 3, 4, name="张三", age=25)
# a=1, b=2
# args=(3, 4)
# kwargs={'name': '张三', 'age': 25}
# 仅位置参数 (Python 3.8+)
def greet(name, /, greeting="Hello"):
print(f"{greeting}, {name}!")
greet("张三") # 正确
greet("张三", "Hi") # 正确
# greet(name="张三") # 错误
# 仅关键字参数
def greet(*, name, age):
print(f"{name}, {age}岁")
greet(name="张三", age=25) # 正确
# greet("张三", 25) # 错误文档字符串
python
def calculate_bmi(weight, height):
"""
计算身体质量指数 (BMI)
Args:
weight: 体重（千克）
height: 身高（米）
Returns:
float: BMI 值
Raises:
ValueError: 如果身高或体重为负数
Example:
>>> calculate_bmi(70, 1.75)
22.857...
"""
if weight <= 0 or height <= 0:
raise ValueError("体重和身高必须为正数")
return weight / (height ** 2)
# 访问文档字符串
print(calculate_bmi.__doc__)
help(calculate_bmi)Lambda 函数
python
# 基本语法
square = lambda x: x ** 2
print(square(5)) # 25
# 多参数
add = lambda a, b: a + b
print(add(5, 3)) # 8
# 条件表达式
is_even = lambda x: "偶数" if x % 2 == 0 else "奇数"
print(is_even(4)) # 偶数
# 在排序中使用
students = [("张三", 85), ("李四", 92), ("王五", 78)]
students.sort(key=lambda x: x[1], reverse=True)
print(students) # [('李四', 92), ('张三', 85), ('王五', 78)]
# 在 filter 中使用
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = list(filter(lambda x: x % 2 == 0, numbers))
print(evens) # [2, 4, 6, 8, 10]
# 在 map 中使用
squares = list(map(lambda x: x ** 2, numbers))
print(squares) # [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]高阶函数
python
# map - 映射
numbers = [1, 2, 3, 4, 5]
squares = list(map(lambda x: x ** 2, numbers))
print(squares) # [1, 4, 9, 16, 25]
# 多序列
a = [1, 2, 3]
b = [4, 5, 6]
sums = list(map(lambda x, y: x + y, a, b))
print(sums) # [5, 7, 9]
# filter - 过滤
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = list(filter(lambda x: x % 2 == 0, numbers))
print(evens) # [2, 4, 6, 8, 10]
# reduce - 归约
from functools import reduce
numbers = [1, 2, 3, 4, 5]
total = reduce(lambda x, y: x + y, numbers)
print(total) # 15
# sorted - 排序
students = [{"name": "张三", "age": 25}, {"name": "李四", "age": 22}]
sorted_students = sorted(students, key=lambda x: x["age"])
print(sorted_students)闭包
python
# 闭包示例
def outer(x):
def inner(y):
return x + y
return inner
add5 = outer(5)
print(add5(3)) # 8
# 计数器
def counter():
count = 0
def increment():
nonlocal count
count += 1
return count
return increment
c = counter()
print(c()) # 1
print(c()) # 2
print(c()) # 3装饰器基础
python
# 简单装饰器
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()
# 函数执行前
# Hello!
# 函数执行后
# 带参数的装饰器
def my_decorator(func):
def wrapper(*args, **kwargs):
print("函数执行前")
result = func(*args, **kwargs)
print("函数执行后")
return result
return wrapper
@my_decorator
def add(a, b):
return a + b
result = add(5, 3)实践示例
学生成绩管理
python
def main():
students = []
while True:
print("\n===== 学生成绩管理系统 =====")
print("1. 添加学生")
print("2. 显示所有学生")
print("3. 查找学生")
print("4. 统计信息")
print("5. 退出")
choice = input("请选择: ")
if choice == "1":
name = input("姓名: ")
scores = []
for subject in ["语文", "数学", "英语"]:
score = float(input(f"{subject}成绩: "))
scores.append(score)
student = {
"name": name,
"scores": scores,
"total": sum(scores),
"average": sum(scores) / len(scores)
}
students.append(student)
print("添加成功!")
elif choice == "2":
if not students:
print("暂无学生数据")
continue
print("\n姓名\t语文\t数学\t英语\t总分\t平均")
print("-" * 50)
for s in sorted(students, key=lambda x: x["total"], reverse=True):
print(f"{s['name']}\t{s['scores'][0]}\t{s['scores'][1]}\t"
f"{s['scores'][2]}\t{s['total']}\t{s['average']:.1f}")
elif choice == "3":
name = input("请输入姓名: ")
found = [s for s in students if s["name"] == name]
if found:
s = found[0]
print(f"\n姓名: {s['name']}")
print(f"语文: {s['scores'][0]}")
print(f"数学: {s['scores'][1]}")
print(f"英语: {s['scores'][2]}")
print(f"总分: {s['total']}")
print(f"平均: {s['average']:.1f}")
else:
print("未找到该学生")
elif choice == "4":
if not students:
print("暂无学生数据")
continue
all_scores = [s["total"] for s in students]
print(f"\n学生人数: {len(students)}")
print(f"最高总分: {max(all_scores)}")
print(f"最低总分: {min(all_scores)}")
print(f"平均总分: {sum(all_scores) / len(all_scores):.1f}")
elif choice == "5":
print("再见!")
break
if __name__ == "__main__":
main()