Python 初学笔记

教程：Python As Fast as Possible - Learn Python in ~75 Minutes

• GitHub - kyclark/tiny_python_projects: Code for Tiny Python Projects (Manning, 2020, ISBN 1617297518). Learning Python through test-driven development of games and puzzles.

交互式

输入

bash
python3

看到prompt提示符>>> 就进入交互模式了

•  <Control>-P (previous)

• <Control>-N (next). 

• <Control>-D exits a session

• 工具：

  • Project Jupyter | Home

• 教程：

  • practical python
  • PEP 20 – The Zen of Python | peps.python.org

• 书籍

  • [[流畅的Python]]

数据类型

• Int
• Float
• String
• Bool

Arithmetic Operator

• will always reture a float number
  • int(x/y) => int
• 幂运算： x ** y => 2**3 = 8
• 开跟号：x //y => 10 // 3 => 3 因为返回的是int，会自动取整
• 求余 x % y

String Method

• .upper()
• .lower()
• .capitalize()
  • 首字大写
• .count()
  • 'Hello'.count('ll) = 2
• mutiply strings
  • 'hello' * 3 = 'hellohellohello
  • 'hello' + 'yes' = 'helloyes
• .replace()
• .split() 分割
  • 和JS中[[split]]的区别：
    • 当split里面没有任何参数的时候，JS中会将字符串切割成单个字符
    • 而Python会按照空格分割（By default any whitespace is a separator¹）

Conditional Operators

return -> True / False

• ==

• !=

• >=

• <=

• >

• <

• 可以比较字符串，通过比较ASCI码

  • .orc('a') 求得某个字符串的ASCI值
  • 如果有几个字符串的话，逐一比较

Chained Conditionals

• and
• or
• not
• order
  1. nor
  2. and
  3. or

If/Else/Elif

py
if x == 'Tim':
	print('You are great')
elif x == 'joe':
	print('Bye joe!')
else: 
	print('No')

js

js
if(x === 'tim') {
	console.log('You are great')
} else if (x === 'joe') {
	console.log('Bye joe!')
} else {
	console.log('No')
}

List/Tuples

• List
  • 类似JS中的 [[数组|array]]
  • .list()
  • x = [4, True, 'hi']
  • x[index]
  • .len()
    • 返回长度
    • string also works
  • .append()
    • 在末尾加值
  • .extend([])
    • 扩展数组，在末尾加值
  • .pop(index)
    • remove the last element and return it
  • list is mutable
  • deepcopy
    • `y = x[:]
• tuples
  • TS中的 tuple
  • immutable
  • y = (1,2,3,5)
• nesting
  • x=[[],(),[]]

for loops

py
for i in range(10)
	print(i)
x = [1,2,3,5,67]
for i in range(len(x))
	print(x[i])
for i,element in enumerate(x):
	print(i,element)
// 0 1
// 1 2
// 2 3
// 3 5
// 4 67

• .range()
  • arguments:
    • stop
    • start, stop
    • start, stop, end
  • not includes start and stop
• enumerate
  • enumerate()是Python中的一个内置函数,用于将一个可迭代对象(如列表、元组或字符串)组合为一个索引序列,同时列出数据和数据下标,一般用在for循环当中

While

bash
while True:
	print('run')
	i += 1

Slice

切割数组

py
x = [0,1,2,3,4,5,6,7,8]
slice = x[0,4,2]
// slice = [0,2]

• slice = x[start:stop:step]
  • x[:stop]
  • x[start:]
  • x[start:stop]
  • step默认为1
  • reverse a list: x[::-1]

Sets

unorder unique list

py
x = set()
s = { 2,3,4,5 } //set
s2 = { 6 }
s = { } // dict
print( 4 in s) => boolean
print(s.union(s2)) // => {2,3,4,5,6}

• .add()
• .remove()
• .union：合并set
• .difference：比较差异
• .intersection： 交集

Dicts

py
x = {'key': 4}
x['key2'] = 5
print( 'key' in x) // => True
//删除
del x['key']

• .value()
  • 获取所有values
• .update()²
  • update() 方法用于将一个字典的键值对更新到另一个字典中。
  • Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None.
  • update() accepts either another dictionary object or an iterable of key/value pairs (as tuples or other iterables of length two). If keyword arguments are specified, the dictionary is then updated with those key/value pairs: d.update(red=1, blue=2).

Comprehensions

py
x = [ x for x in range(5)]
x =[[0 for x in range(100)] for x in range(5)]
x = { i for i in range(100) if i % 5 == 0 }

Function

py
def func(x,y,z=None):
	print('Run',x,y,z)
	return x * y , x /y 
r1, r2 = func(10,5)
func()

Unpack Operator /* Args and ** Kwargs

• *x
• 类似于JS中的...
• 用于list

py
def fun(x,y):
	print(x,y)
pairs = [(1,2),(3,4)]
for pair in pairs 
	fun(*pair)

• **x
• key word argumens
• 用于dict

py
def fun(x,y):
	print(x,y)
for(**{'x':2, 'y': 5})
// 顺序不一也可以

Scope & Globals

• 函数内有局部作用域
• 用globel 可以将局部变量变成全局变量

py
def func():
	global x
	return x

Exceptions

• 意外事件，相当于JS中的throw new Error('')
• 会打断程序的正常执行流程

py
raise Exception('Bad')
raise FileExistsError('')

常见的异常类型有:

• StopIteration - 迭代器没有更多的值
• ImportError - 导入模块失败
• IndexError - 序列中找不到给定的索引
• KeyError - 映射中找不到给定的键
• ValueError - 传入无效的参数
• TypeError - 对类型无效的操作
• FileNotFoundError - 未找到文件
• KeyboardInterrupt - 用户中断执行

Handling Exceptions

py
try: 
	x = 7 /0
expect Exception as e
	print(e)
finally:
	print('finally')

Lambda

lambda是Python中的匿名函数。它可以有多个参数,但只能有一个表达式。

py
x = lambda x: x + 5

Map and Filter

• Map: 跟JS的[[map()]] 差不多
• 返回map object

py
x = [0,1,2,3,4,5,6,7,8]
mp = map(lambda i : i +2 , x)
// js : const mp = x.map((i)=> i+1)
print(list(mp))
mpf = filter(lambda i : i === 2 , x)

F Strings

• 类似于JS中的 [[template literal]]

py
tim = 89
x = f'hello {6+8}'
// js 
// x = `hello ${6+8}`

Buildin Functions

• [[vars()]]