Python 速查表

变量与类型

# 变量
x = 10                    # int
y = 3.14                  # float
name = "Hello"            # str
flag = True               # bool
nothing = None            # NoneType

# 类型转换
int("42")      # 42
str(100)       # "100"
float("3.14")  # 3.14
bool(0)        # False

字符串

s = "Hello, World!"
len(s)              # 13
s.upper()           # "HELLO, WORLD!"
s.lower()           # "hello, world!"
s.strip()           # 去除首尾空格
s.split(",")        # ["Hello", " World!"]
",".join(["A","B"]) # "A,B"
s.replace("H","J")  # "Jello, World!"
"Hello" in s        # True

# 切片
s[0:5]    # "Hello"
s[::-1]   # "!dlroW ,olleH"

# f-string
name = "小明"; age = 25
f"{name}今年{age}岁"  # "小明今年25岁"

列表

arr = [1, 2, 3, 4, 5]
arr.append(6)        # [1,2,3,4,5,6]
arr.insert(0, 0)     # [0,1,2,3,4,5,6]
arr.pop()            # 6, arr = [0,1,2,3,4,5]
arr.remove(3)        # [0,1,2,4,5]
arr[2]               # 2
arr[-1]              # 5（最后一个）
arr[1:3]             # [1,2]

# 列表推导式
[x*2 for x in range(5)]           # [0,2,4,6,8]
[x for x in arr if x > 2]         # 过滤

# 常用操作
len(arr)             # 长度
sum(arr)             # 求和
max(arr)             # 最大值
sorted(arr)          # 排序（返回新列表）
arr.sort()           # 原地排序

字典

d = {"name": "小明", "age": 25}
d["name"]            # "小明"
d.get("city", "未知") # 安全获取
d["city"] = "北京"    # 添加/修改
d.pop("age")         # 删除并返回
"name" in d          # True
d.keys()             # 所有键
d.values()           # 所有值
d.items()            # 键值对

# 字典推导式
{v: k for k, v in d.items()}  # 键值互换

集合

s = {1, 2, 3}
s.add(4)             # {1,2,3,4}
s.remove(2)          # {1,3,4}

a = {1,2,3}; b = {2,3,4}
a | b    # 并集 {1,2,3,4}
a & b    # 交集 {2,3}
a - b    # 差集 {1}

条件与循环

# if-elif-else
if x > 0:
    print("正数")
elif x < 0:
    print("负数")
else:
    print("零")

# 三元表达式
"偶数" if x % 2 == 0 else "奇数"

# for 循环
for i in range(5): print(i)          # 0-4
for i, v in enumerate(arr): pass     # 索引+值
for k, v in d.items(): pass          # 键+值

# while
while x > 0: x -= 1

# break / continue

函数

def greet(name, greeting="Hello"):
    """文档字符串"""
    return f"{greeting}, {name}!"

# lambda
square = lambda x: x ** 2

# *args, **kwargs
def func(*args, **kwargs): pass

# 装饰器
def timer(func):
    def wrapper(*a, **kw):
        import time; t = time.time()
        r = func(*a, **kw)
        print(f"{time.time()-t:.2f}s")
        return r
    return wrapper

类

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

    def speak(self):
        return f"{self.name} 叫了一声"

class Dog(Animal):
    def speak(self):
        return f"{self.name} 汪汪"

文件操作

with open("file.txt", "r", encoding="utf-8") as f:
    content = f.read()        # 读取全部
    lines = f.readlines()     # 读取每行

with open("out.txt", "w") as f:
    f.write("Hello")

# 模式: r=读 w=写(覆盖) a=追加 rb/wb=二进制

常用内置函数

map(str, [1,2,3])      # 映射
filter(lambda x:x>0, arr)  # 过滤
zip([1,2], ["a","b"])  # 配对
enumerate(arr)         # 枚举
isinstance(42, int)    # 类型检查
hasattr(obj, "name")   # 属性检查