Python 基础语法小抄

变量和赋值

name = "张三"
age = 25
height = 1.75
is_student = True

数据类型

str_var = "文本"        # 字符串
int_var = 42           # 整数
float_var = 3.14       # 浮点数
bool_var = True        # 布尔值 (True/False)
none_var = None        # 空值

fruits = ["苹果", "香蕉", "橙子"]
numbers = [1, 2, 3, 4, 5]
mixed = [1, "文本", True, 3.14]  # 可以混合类型

coords = (10, 20)
colors = ("红", "绿", "蓝")

person = {
    "name": "李四",
    "age": 30,
    "city": "上海"
}

unique_nums = {1, 2, 3, 4, 5}

运算符

a = 10 + 5    # 加法 = 15
b = 10 - 5    # 减法 = 5
c = 10 * 5    # 乘法 = 50
d = 10 / 5    # 除法 = 2.0
e = 10 // 3   # 整除 = 3
f = 10 % 3    # 取余 = 1
g = 2 ** 3    # 幂运算 = 8

5 == 5     # 等于 = True
5 != 3     # 不等于 = True
5 > 3      # 大于 = True
5 < 3      # 小于 = False
5 >= 5     # 大于等于 = True
5 <= 3     # 小于等于 = False

True and False   # 与 = False
True or False    # 或 = True
not True         # 非 = False

字符串

# 多种引号方式
str1 = "双引号"
str2 = '单引号'
str3 = """三引号
可以换行"""

# 拼接
full_name = "张" + "三"          # "张三"

# f-string（推荐）
name = "Julien"
age = 25
msg = f"我叫{name}，今年{age}岁"  # "我叫Julien，今年25岁"

# 格式化
msg2 = "我叫{}，今年{}岁".format(name, age)

text = "Hello World"
len(text)              # 11 (长度)
text.lower()           # "hello world"
text.upper()           # "HELLO WORLD"
text.strip()           # 去除首尾空格
text.replace("H", "h") # "hello World"
text.split(" ")        # ["Hello", "World"]
"World" in text        # True (包含检查)
text.startswith("H")   # True
text[0]                # "H" (索引)
text[0:5]              # "Hello" (切片)

列表（List）

fruits = ["苹果", "香蕉", "橙子"]

# 访问
fruits[0]          # "苹果"
fruits[-1]         # "橙子" (倒数第一个)
fruits[0:2]        # ["苹果", "香蕉"] (切片)

# 添加
fruits.append("葡萄")        # 末尾添加
fruits.insert(1, "西瓜")     # 指定位置插入

# 删除
fruits.remove("香蕉")        # 删除指定元素
fruits.pop()                # 删除末尾
fruits.pop(0)               # 删除指定索引

# 其他
len(fruits)                 # 长度
"苹果" in fruits            # True (检查是否存在)
fruits.sort()               # 排序
fruits.reverse()            # 反转

字典（Dictionary）

user = {
    "name": "王五",
    "age": 28,
    "city": "北京"
}

# 访问
user["name"]              # "王五"
user.get("age")           # 28
user.get("email", "无")   # "无" (不存在时返回默认值)

# 修改和添加
user["age"] = 29          # 修改
user["email"] = "wang@example.com"  # 添加

# 删除
del user["city"]
user.pop("age")

# 遍历
for key in user:                    # 遍历键
    print(key, user[key])

for key, value in user.items():     # 遍历键值对
    print(key, value)

user.keys()                         # 所有键
user.values()                       # 所有值

条件语句

age = 20

if age >= 18:
    print("成年人")
elif age >= 13:
    print("青少年")
else:
    print("儿童")

status = "成年" if age >= 18 else "未成年"

循环

# 遍历列表
fruits = ["苹果", "香蕉", "橙子"]
for fruit in fruits:
    print(fruit)

# 遍历范围
for i in range(5):        # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):     # 1, 2, 3, 4, 5
    print(i)

for i in range(0, 10, 2): # 0, 2, 4, 6, 8 (步长为2)
    print(i)

# 遍历字典
person = {"name": "张三", "age": 25}
for key, value in person.items():
    print(f"{key}: {value}")

i = 0
while i < 5:
    print(i)
    i += 1

for i in range(10):
    if i == 3:
        continue  # 跳过当前循环
    if i == 7:
        break     # 终止循环
    print(i)

函数

def greet(name):
    return f"你好, {name}"

message = greet("Julien")  # "你好, Julien"

def greet(name="朋友", greeting="你好"):
    return f"{greeting}, {name}"

greet()                      # "你好, 朋友"
greet("李明")                # "你好, 李明"
greet("李明", "早上好")      # "早上好, 李明"

def calculate(a, b):
    return a + b, a - b, a * b

sum_val, diff, prod = calculate(10, 5)

add = lambda a, b: a + b
result = add(3, 5)  # 8

# 常用于 map, filter 等
numbers = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x**2, numbers))  # [1, 4, 9, 16, 25]

列表推导式

# 基本形式
squares = [x**2 for x in range(1, 6)]  # [1, 4, 9, 16, 25]

# 带条件
evens = [x for x in range(10) if x % 2 == 0]  # [0, 2, 4, 6, 8]

# 字符串处理
words = ["apple", "banana", "cherry"]
upper_words = [w.upper() for w in words]  # ["APPLE", "BANANA", "CHERRY"]

常用内置函数

# 类型转换
int("123")      # 123
float("3.14")   # 3.14
str(123)        # "123"
list("abc")     # ["a", "b", "c"]

# 数学函数
abs(-5)         # 5 (绝对值)
max(1, 5, 3)    # 5
min(1, 5, 3)    # 1
sum([1,2,3,4])  # 10
round(3.7)      # 4 (四舍五入)

# 其他
len([1,2,3])    # 3 (长度)
type(123)       # <class 'int'> (类型)
range(5)        # 0~4 的范围
input("输入:")  # 获取用户输入
print("输出")   # 打印输出

文件操作

# 读取全部内容
with open("file.txt", "r", encoding="utf-8") as f:
    content = f.read()
    print(content)

# 按行读取
with open("file.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())

# 覆盖写入
with open("file.txt", "w", encoding="utf-8") as f:
    f.write("Hello World\n")
    f.write("第二行")

# 追加写入
with open("file.txt", "a", encoding="utf-8") as f:
    f.write("追加内容\n")

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("无论如何都会执行")

导入模块

# 导入整个模块
import math
print(math.pi)  # 3.141592653589793

# 导入特定函数
from math import sqrt, pow
print(sqrt(16))  # 4.0

# 导入并重命名
import pandas as pd
import numpy as np

常用技巧

a, b, c = 1, 2, 3
x = y = z = 0

a, b = b, a

numbers = [1, 2, 3]
a, b, c = numbers

fruits = ["苹果", "香蕉", "橙子"]
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

names = ["张三", "李四", "王五"]
ages = [25, 30, 28]
for name, age in zip(names, ages):
    print(f"{name}: {age}岁")