Python学习笔记
和字符串相关的常用操作
字符串的定义
双引号
string_in_double_quotes = "Hello, Python!"单引号
string_in_single_quotes = 'Hello, Python!'三引号（用于多行输入）
string_in_triple_quotes = '''Hello, Python! I believe that ‘BOXUE’ will let me become stronger and stronger. However, only “hard work” makes me better than before.'''
当字符串本身需要引号时，就使用另外的引号形式将其包裹，防止歧义
数字通过类型转换来定义
number = 1145141919810 number_to_string = str(number)
python中的字符串是只读的，不能根据存储位置进行修改
字符串的拼接
action = 'Hello'
name = 'Leo'
welcome = action + ", " +name
字符串全部大写/小写
welcome.upper()
welcome.lower()
字符串去掉首尾空格
welcome.strip()
查看字符串可用的方法
dir(welcome)
help(welcome.upper)
分割字符串
可以像C一样，使用单个字符的位置读取内容
welcome[0]也可以使用一个range，截取字符串的一部分
slice = welcome[0:5] #range是一个左闭右开区间
字符串的template
以C为例：
printf("Hello %s", "Leo");
故效仿C的形式：
action = 'Hello'
name = 'Leo'
print('Hello %s' % 'Leo')
print('{0} {1}!'.format(action, name))
print("Pi : %.2f" % 3.14)
welcome = {action: "Hello", name: "Leo"}
print("{action} {name} !".format(**welcome))
Python中的数组——list
创建list
list1 = [] # This is an empty list, no any value
list2 = list() # a way to create a list
number_list = [8, 6, 5, 7, 2] # This is a list filled with numbers
mixed_list = [1, 'one', 2, "two", 3] # This is a list with mixed value consist of number and strings
final_list = [number_list, mixed_list] # This is a list consists of two different list
对list进行变换
合并list
final_list = number_list + mixed_list print(final_list)为list追加元素
number_list.append([9, 10]) print(number_list)为list进行排序（无返回值，升序）
number_list1 = [3, 5, 6, 1, 2] number_list1.sort() print(number_list1)
append为O(1)算法，故最好只用1次(整体加入)，不要让list反复遍历
为list进行复制操作
number_list2 = number_list1.copy() # 可以保留原list操作后的值
sort, append等方法皆为对原有对象的直接引用
查找list上特定的值
print(number_list1[0]) # 查找对应位置的单个值 print(number_list1[0:2]) # 查找n个值，返回的是一个新的list print(number_list1[0:-2]) # 负数代表倒数顺序在指定位置上插入元素
number_list1.insert(1, 9) # 指定某位置的插入值，已有元素顺延 print(number_list1)删除指定位置的元素
number_list1.remove(2) # 移除该值，已有元素重新定序号，若没有此元素则报错 del (number_list1[0:5:2]) # 在左闭右开区间内，每隔n个元素删除一个元素，且从范围内的第一个元素开始计数和删除,然后剩下的元素重新排序
Python中的只读集合——Tuple
简单来说，它就是一个只读的list，一旦创建之后，就只能访问它的值，而不能再对其修改，包括修改已有元素的值，以及添加删除元素，都不可以。
创建Tuple
empty = () # create an empty tuple
number_tuple = (1, 2, 3) # use the elements to create a tuple directly
mix = tuple([1, 2, "Three"]) # use the list to pass its value to create a tuple
Tuple相关操作
# Tuple
from typing import Tuple
# 1. create the tuple
empty = () # create an empty tuple
number_tuple: tuple[int, int, int] = (1, 2, 3) # use the elements to create a tuple directly
mix = tuple([1, 2, "Three"]) # use the list to pass its value to create a tuple
# 2. how to use the tuple
# print the tuple
print(number_tuple)
# add some tuples
print(number_tuple * 2)
# find some elements in the tuples
print(number_tuple[0])
print(number_tuple[0:2])
print(number_tuple[0:3:2])
print(3 in number_tuple)
# get length of the tuple
print(len(number_tuple))
# get the maximum of the tuple
print(max(number_tuple))
# get the minium of the tuple
print(min(number_tuple))
# if there is a specific element?
print(3 in number_tuple)
# delete the whole tuple
del number_tuple
Python中的Dictionary
又称哈希表，指用某个具体的值而不是位置来指代另一个值，前者称为key，后者为value
创建Dictionary
# 1.1 Create an empty dictionary
empty_dictionary = {}
# 1.2 Create a dictionary directly
user = {'email': 'aoi@s1no1style.com', 'password': '1145141919810', 'ID': 1}
# 1.3 use the keyword dict() to create(not recommended)
another_user = dict({'email': 'aoi@'})
# 1.4 use another dictionary to extend it
user.update({'address': 'Tokyo, Japan'})
元素删除
# 2. delete the key and value in the dictionary
del user['password']
# del user # of course it can also delete the whole dictionary
print(user)
# Caution: the key must be read-only, so only the value, tuple and dictionary could be the key.
# List can't be the key!
其他方法
print(user.values()) # get all the values in the dictionary
print(user.keys()) # get all the keys in the dictionary
print('email' in user) # if there is a key like this in the dictionary?
分支语句和Boolean表达式
秉承着“一件事件只有一种做法”的原则，在Python里，我们只能使用
if ... elif ... else在多个条件中选择
# if ... elif ... else
# remember, no switch ... case ...
num = 21
if num < 10:
content = "{0} is less than 10".format(num)
print(content)
elif 10 < num < 20:
content = "{0} is greater than 10 and less than 20".format(num)
print(content)
elif num in [10, 20]:
content = "{0} is equal to 10 or 20".format(num)
print(content)
else:
content = "{0} is greater than 20".format(num)
print(content)
一切空值都会被判断为false， None表示空
if __name__ == '__main__':
# only be used to run some codes independently, maybe be used to debug
所以示例文件可以变换为：
# if ... elif ... else
# remember, no switch ... case ...
def conditional(func_num):
if func_num < 10:
content = "{0} is less than 10".format(func_num)
print(content)
elif 10 < func_num < 20:
content = "{0} is greater than 10 and less than 20".format(func_num)
print(content)
elif func_num in [10, 20]:
content = "{0} is equal to 10 or 20".format(func_num)
print(content)
else:
content = "{0} is greater than 20".format(func_num)
print(content)
if __name__ == '__main__':
print('The Script "Conditional.py" is running.')
num = int(input('Enter a number: '))
conditional(num)
Python中的循环
在Python里，和其他编程语言类似，我们可以使用for和while实现循环。
# for and while
# for num in [1, 2, 3, 4]:
# print(num)
for num in range(1, 5):
if num == 2:
print(num)
break
else:
print("Error!")
user = {'id': 10, "Name": "Leo"}
for info in user:
print(info) # info means the keys of the dictionary
print(user[info])
value = 1
while value <= 10:
if value % 2 == 0:
value += 1
continue
print(value)
value += 1
Python中的Comprehension
一个for循环配合if的特殊用法，简化代码用
# About the list comprehension
numbers = [i for i in range(1, 5) if i % 2 == 0]
print(numbers)
strings = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flatten = [value for key in strings for value in key]
print(flatten)
x_samples = [1, 2, 3, 4, 5, 6, 7, 8, 9]
y_samples = [1, 2, 3, 4, 5, 6, 7, 8, 9]
need_points = [(x, y) for x in x_samples for y in y_samples if x != y]
print(need_points)
Python中的异常处理
在现实世界中，无论我们多么小心，编写的代码总是会因为一些外部的原因发生错误。例如，被粗心的开发者传递了错误的参数、要打开的文件被误删除了、操作系统发生了未知错误等等。哪怕这些问题都不存在，用户还可以在你的程序过程中通过Ctrl + C强制结束呢。所以，就像我们要一丝不苟的编写逻辑正确的代码一样，在错误处理上，我们同样马虎不得。
不过好在，相比哪些有很多种表达错误方式的语言，Python只有一种表达错误的方式，就是Exception，自然，处理错误的方式，也就只有一种了。
# Error Handling
# Remember, only the 'exception'
# Exception
# AttributeError
# IndexError
# NameError
# SyntaxError
# TypeError
# ValueError
# ZeroDivisionError
# So how to handle these errors?
try:
3/0
except ZeroDivisionError:
print("Divided by Zero!")
except NameError:
print("Invalid Name!")
else:
print("No Errors.")
finally:
print("Clean up actions.")
# Divided by Zero!
# Clean up actions.
Python引入第三方代码
Python之所以广泛流行，依靠的当然不仅仅是它流畅简单的语法，还有在其背后给他站台撑腰的众多模块代码。依靠它们，可以让我们更简单的完成日常的开发工作。之前的例子里，我们使用的，都是属于Python核心功能的代码，因此，无须引入任何内容。
引入方式分为：Module 和 Package
# about import
# In Python, it consists of modules and packages
# Of course, a package includes some modules.
# For example:
# import this
import math as m
# from math import * # Not recommended!
print(m.gcd(4, 6)) # 2
Python自定义函数
# about functions
def a_custom_function():
pass # just like the nop in asm
def add(*args):
tmp = 0
for i in args:
tmp += i
return tmp
def another_custom_function(*args, **kwargs):
tmp = 0
print(kwargs)
for i in args:
tmp += i
for i in kwargs:
tmp += kwargs[i]
return tmp
print(add(1, 2, 3, 4, 5)) # 15
print(another_custom_function(1, 2, n1=4, n2=5, n3=6))
Python中的Class
# the beginning of the class
# take an example
class Person:
"""A general person class"""
# Initial this class
def __init__(self, name, age):
self.name = name
self.age = age
def wakeup(self, at):
print("{0} wake up at {1} am".format(self.name, at))
def __del__(self):
print("This person {0} has been deleted".format(self.name))
mars = Person("Mars", 30)
eleven = mars
print(id(mars))
print(id(eleven))
del eleven
print("only 1 ref")
del mars
Class的继承和静态属性
class Person:
"""A general person class"""
# Initial this class
def __init__(self, name, age):
self.name = name
self.age = age
def wakeup(self, at):
print("{0} wake up at {1} am".format(self.name, at))
def __del__(self):
print("This person {0} has been deleted".format(self.name))
from Person import Person
class Employee(Person):
__counter = 0
def __init__(self, name, age, work_id):
Person.__init__(self, name, age)
self.work_id = work_id
Employee.__counter += 1
def __str__(self):
return "Employee"
def __eq__(self, other):
return self.name == other.name \
and self.age == other.age \
and self.work_id == other.work_id
from Employee import Employee
from Person import Person
mars = Employee('Mars', 30, 11)
eleven = Employee('Mars', 30, 11)
print(mars == eleven) # if you want this value is true, you need to define the attribute "__eq__"
# print(Employee.__counter)
print(issubclass(Employee, Person))
print(isinstance(mars, Employee))
print(isinstance(mars, Person))
print(mars.__repr__())
print(mars.__str__())
# str use the repr, which include more information about developer
# str include more information about user, it can be written.
