基础

变量

name = "Alice"   # str
age = 20         # int
gpa = 3.85       # float
active = True    # bool

数据类型

str	文本：`"hello"`
int	整数：`42`
float	小数：`3.14`
bool	`True` / `False`
list	有序可变：`[1, 2, 3]`
tuple	有序不可变：`(1, 2)`
dict	键值对：`{"a": 1}`
set	唯一元素集合：`{1, 2, 3}`

算术运算

+ - *	加、减、乘
/	除法（浮点）：7/2 → 3.5
//	整除：7//2 → 3
%	取模：7%2 → 1
**	幂运算：2**3 → 8

类型转换

int("42")     # 42
float("3.14") # 3.14
str(100)      # "100"
list("abc")   # ['a','b','c']

用户输入

name = input("Your name? ")
age = int(input("Age? "))

字符串

创建字符串

s1 = 'single quotes'
s2 = "double quotes"
s3 = """triple quotes
    for multiline"""

f-String（Python 3.6+）

name = "Alice"
f"Hello, {name}!"  # Hello, Alice!
f"{2 + 3}"         # 5
f"{3.14159:.2f}"   # 3.14
f"{1000:,}"        # 1,000

字符串切片

s = "Python"
# Index: 0 1 2 3 4 5
s[0]    # 'P'
s[-1]   # 'n'
s[2:5]  # 'tho'
s[:2]   # 'Py'
s[2:]   # 'thon'
s[::-1] # 'nohtyP' (reverse)

字符串方法

len(s)	字符串长度
s.upper()	转大写
s.lower()	转小写
s.strip()	去除首尾空白
s.split(",")	分割为列表
",".join(lst)	列表合并为字符串
s.replace(a, b)	将 a 替换为 b
s.find("x")	首个匹配的索引（未找到返回 -1）
s.startswith(x)	检查前缀 → bool
s.endswith(x)	检查后缀 → bool
s.count(x)	统计出现次数
"x" in s	包含检查 → bool

列表

创建与访问

fruits = ["apple", "banana", "cherry"]
fruits[0]   # "apple"
fruits[-1]  # "cherry"
fruits[1:3] # ["banana", "cherry"]

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]

列表方法

lst.append(x)	追加到末尾
lst.extend(lst2)	追加 lst2 的所有元素
lst.insert(i, x)	在索引 i 处插入
lst.pop()	移除并返回最后一个元素
lst.pop(i)	移除并返回索引 i 处的元素
lst.remove(x)	移除第一个 x
del lst[i]	按索引删除
lst.sort()	原地排序
sorted(lst)	返回排序副本
lst.reverse()	原地反转
len(lst)	元素数量
x in lst	成员检查
lst.index(x)	x 的第一个索引
lst.count(x)	x 的出现次数

元组与集合

元组（不可变）

point = (3, 4)
x, y = point  # unpacking
point[0]      # 3 (read-only)

集合（唯一元素）

s = {1, 2, 3}
s.add(4); s.remove(1)
a & b  # intersection
a | b  # union
a - b  # difference

字典

创建与访问

student = {"name": "Alice", "age": 20}
student["name"]      # "Alice"
student.get("gpa", 0) # 0 (default)
student["gpa"] = 3.85  # add/update

字典推导式

sq = {x: x**2 for x in range(5)}
# {0:0, 1:1, 2:4, 3:9, 4:16}

遍历

for k, v in student.items():
    print(f"{k}: {v}")

字典方法

d.keys()	所有键
d.values()	所有值
d.items()	所有（键, 值）对
d.get(k, default)	获取值，带默认值
d.update(d2)	将 d2 合并到 d
d.pop(k)	移除并返回值
del d[k]	删除键
"k" in d	键是否存在 → bool
len(d)	条目数量

控制流

if / elif / else

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C"

三元表达式

status = "pass" if score >= 60 else "fail"

循环

for 循环

for fruit in ["apple", "banana"]:
    print(fruit)

range()

range(5)        # 0, 1, 2, 3, 4
range(2, 5)     # 2, 3, 4
range(0, 10, 2) # 0, 2, 4, 6, 8

while 循环

while count < 10:
    count += 1

enumerate() 与 zip()

for i, val in enumerate(["a", "b"]):
    print(i, val) # 0 a, 1 b

for a, b in zip([1, 2], ["x", "y"]):
    print(a, b) # 1 x, 2 y

break 与 continue

for x in range(10):
    if x == 5: break      # stop loop
    if x % 2 == 0: continue # skip

函数

定义与调用

def greet(name, greeting="Hi"):
    return f"{greeting}, {name}!"

greet("Alice")        # "Hi, Alice!"
greet("Bob", "Hello") # "Hello, Bob!"

多返回值

def min_max(lst):
    return min(lst), max(lst)
lo, hi = min_max([3, 1, 4, 1, 5])

*args 与 **kwargs

def total(*args): # args is a tuple
    return sum(args)
total(1, 2, 3) # 6

def info(**kwargs): # kwargs is a dict
    print(kwargs)

Lambda 函数

square = lambda x: x**2
square(5) # 25
sorted(lst, key=lambda x: x["age"])

类

class Dog:
    def __init__(self, name, breed):
        self.name = name
        self.breed = breed

    def bark(self):
        return f"{self.name} says Woof!"

dog = Dog("Rex", "Lab")
dog.bark() # "Rex says Woof!"

继承

class Puppy(Dog):
    def __init__(self, name, breed, toy):
        super().__init__(name, breed)
        self.toy = toy

异常处理

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")
finally:
    print("Always runs")

文件读写

读取文件

with open("data.txt") as f:
    content = f.read() # full text

with open("data.txt") as f:
    for line in f: # line by line
        print(line.strip())

写入文件

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

"r" = 读 "w" = 写（覆盖） "a" = 追加

CSV

import csv

with open("data.csv") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"])

with open("out.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "age"])

JSON

import json

data = json.loads('{"name":"Alice"}') # parse
text = json.dumps(data)               # serialize

with open("data.json") as f:
    data = json.load(f)               # read file
with open("out.json", "w") as f:
    json.dump(data, f, indent=2)      # write file

HTTP 请求

import requests

# GET
r = requests.get("https://api.example.com/data")
r.status_code # 200
data = r.json() # parse JSON

# POST
r = requests.post(url, json={"key": "val"})

pandas 基础

import pandas as pd
df = pd.read_csv("data.csv")
df.head()           # first 5 rows
df.shape            # (rows, cols)
df["name"]          # single column
df[df["age"] > 20]  # filter rows

常用内置函数

print()	输出到控制台
len()	长度 / 数量
type()	对象类型
range()	数字序列
enumerate()	索引 + 值对
zip()	配对多个可迭代对象
sorted()	返回排序副本
sum() min() max()	聚合函数

模块

import math
from math import sqrt, pi
import pandas as pd # alias

PYTHON 3 快速参考