Python 3 备忘清单

• Python (python.org)
• Learn X in Y minutes (learnxinyminutes.com)
• Regex in python (jaywcjlove.github.io)

>>> print("Hello, World!")
Hello, World!

Python 中著名的“Hello World”程序

age = 18      # 年龄是 int 类型
name = "John" # name 现在是 str 类型
print(name)

Python 不能在没有赋值的情况下声明变量

>>> msg = "Hello, World!"
>>> print(msg[2:5])
llo

查看: Strings

mylist = []
mylist.append(1)
mylist.append(2)
for item in mylist:
    print(item) # 打印输出 1,2

查看: Lists

num = 200
if num > 0:
    print("num is greater than 0")
else:
    print("num is not greater than 0")

查看: 流程控制

for item in range(6):
    if item == 3: break
    print(item)
else:
    print("Finally finished!")

查看: Loops

>>> def my_function():
...     print("来自函数的你好")
...
>>> my_function()
来自函数的你好

查看: Functions

result = 10 + 30 # => 40
result = 40 - 10 # => 30
result = 50 * 5  # => 250
result = 16 / 4  # => 4.0 (Float Division)
result = 16 // 4 # => 4 (Integer Division)
result = 25 % 2  # => 1
result = 5 ** 3  # => 125

/ 表示 x 和 y 的商，// 表示 x 和 y 的底商，另见 StackOverflow

counter = 0
counter += 10           # => 10
counter = 0
counter = counter + 10  # => 10
message = "Part 1."
# => Part 1.Part 2.
message += "Part 2."   

>>> website = 'Quick Reference'
>>> f"Hello, {website}"
"Hello, Quick Reference"
>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'

查看: Python F-Strings

hello = "Hello World"
hello = 'Hello World'
multi_string = """Multiline Strings
Lorem ipsum dolor sit amet,
consectetur adipiscing elit """

查看: Strings

x = 1    # int
y = 2.8  # float
z = 1j   # complex
>>> print(type(x))
<class 'int'>

my_bool = True 
my_bool = False
bool(0)     # => False
bool(1)     # => True

list1 = ["apple", "banana", "cherry"]
list2 = [True, False, False]
list3 = [1, 5, 7, 9, 3]
list4 = list((1, 5, 7, 9, 3))

查看: Lists

my_tuple = (1, 2, 3)
my_tuple = tuple((1, 2, 3))

类似于 List 但不可变

set1 = {"a", "b", "c"}   
set2 = set(("a", "b", "c"))

一组独特的项目/对象

>>> empty_dict = {}
>>> a = {"one": 1, "two": 2, "three": 3}
>>> a["one"]
1
>>> a.keys()
dict_keys(['one', 'two', 'three'])
>>> a.values()
dict_values([1, 2, 3])
>>> a.update({"four": 4})
>>> a.keys()
dict_keys(['one', 'two', 'three', 'four'])
>>> a['four']
4

Key：值对，JSON 类对象

>>> hello = "Hello, World"
>>> print(hello[1])
e
>>> print(hello[-1])
d

获取位置 1 或最后的字符

>>> for char in "foo":
...     print(char)
f
o
o

遍历单词 foo 中的字母

>>> hello = "Hello, World!"
>>> print(len(hello))
13

len() 函数返回字符串的长度

>>> s = '===+'
>>> n = 8
>>> s * n
'===+===+===+===+===+===+===+===+'

>>> s = 'spam'
>>> s in 'I saw spamalot!'
True
>>> s not in 'I saw The Holy Grail!'
True

>>> s = 'spam'
>>> t = 'egg'
>>> s + t
'spamegg'
>>> 'spam' 'egg'
'spamegg'

>>> name = input("Enter your name: ")
Enter your name: Tom
>>> name
'Tom'

从控制台获取输入数据

>>> "#".join(["John", "Peter", "Vicky"])
'John#Peter#Vicky'

>>> "Hello, world!".endswith("!")
True

>>> f'{"text":10}'   # [宽度]
'text      '
>>> f'{"test":*>10}' # 向左填充
'******test'
>>> f'{"test":*<10}' # 向右填充
'test******'
>>> f'{"test":*^10}' # 填充中心
'***test***'
>>> f'{12345:0>10}'  # 填写数字
'0000012345'

>>> f'{12345:+}'      # [sign] (+/-)
'+12345'
>>> f'{-12345:+}'
'-12345'
>>> f'{-12345:+10}'
'    -12345'
>>> f'{-12345:+010}'
'-000012345'

>>> f'{-12345:0=10}'  # 负数
'-000012345'
>>> f'{12345:010}'    # [0] 快捷方式（不对齐）
'0000012345'
>>> f'{-12345:010}'
'-000012345'
>>> import math       # [.precision]
>>> math.pi
3.141592653589793
>>> f'{math.pi:.2f}'
'3.14'
>>> f'{1000000:,.2f}' # [分组选项]
'1,000,000.00'
>>> f'{1000000:_.2f}'
'1_000_000.00'
>>> f'{0.25:0%}'      # 百分比
'25.000000%'
>>> f'{0.25:.0%}'
'25%'

>>> li1 = []
>>> li1
[]
>>> li2 = [4, 5, 6]
>>> li2
[4, 5, 6]
>>> li3 = list((1, 2, 3))
>>> li3
[1, 2, 3]
>>> li4 = list(range(1, 11))
>>> li4
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

>>> li = []
>>> li.append(1)
>>> li
[1]
>>> li.append(2)
>>> li
[1, 2]
>>> li.append(4)
>>> li
[1, 2, 4]
>>> li.append(3)
>>> li
[1, 2, 4, 3]

>>> li = ['bread', 'butter', 'milk']
>>> li.pop()
'milk'
>>> li
['bread', 'butter']
>>> del li[0]
>>> li
['butter']

>>> li = ['a', 'b', 'c', 'd']
>>> li[0]
'a'
>>> li[-1]
'd'
>>> li[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

>>> li = [3, 1, 3, 2, 5]
>>> li.count(3)
2

>>> li = ["re"] * 3
>>> li
['re', 're', 're']

num = 5
if num > 10:
    print("num is totally bigger than 10.")
elif num < 10:
    print("num is smaller than 10.")
else:
    print("num is indeed 10.")

>>> a = 330
>>> b = 200
>>> r = "a" if a > b else "b"
>>> print(r)
a

value = True
if not value:
    print("Value is False")
elif value is None:
    print("Value is None")
else:
    print("Value is True")

primes = [2, 3, 5, 7]
for prime in primes:
    print(prime)

animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
    print(i, value)

x = 0
while x < 4:
    print(x)
    x += 1  # Shorthand for x = x + 1

x = 0
for index in range(10):
    x = index * 10
    if index == 5:
        break
    print(x)

for index in range(3, 8): 
    x = index * 10
    if index == 5:
        continue
    print(x)

for i in range(4):
    print(i) # Prints: 0 1 2 3
for i in range(4, 8):
    print(i) # Prints: 4 5 6 7
for i in range(4, 10, 2):
    print(i) # Prints: 4 6 8

name = ['Pete', 'John', 'Elizabeth']
age = [6, 23, 44]
for n, a in zip(name, age):
    print('%s is %d years old' %(n, a))

def hello_world():  
    print('Hello, World!')

def add(x, y):
    print("x is %s, y is %s" %(x, y))
    return x + y
add(5, 6)    # => 11

def varargs(*args):
    return args
varargs(1, 2, 3)  # => (1, 2, 3)

def keyword_args(**kwargs):
    return kwargs
# => {"big": "foot", "loch": "ness"}
keyword_args(big="foot", loch="ness")

def swap(x, y):
    return y, x
x = 1
y = 2
x, y = swap(x, y)  # => x = 2, y = 1

def add(x, y=10):
    return x + y
add(5)      # => 15
add(5, 20)  # => 25

# => True
(lambda x: x > 2)(3)
# => 5
(lambda x, y: x ** 2 + y ** 2)(2, 1)

import math
print(math.sqrt(16))  # => 4.0

from math import ceil, floor
print(ceil(3.7))   # => 4.0
print(floor(3.7))  # => 3.0

from math import *

import math as m
# => True
math.sqrt(16) == m.sqrt(16)

import math
dir(math)

import os
os.remove("myfile.txt")

import os
if os.path.exists("myfile.txt"):
    os.remove("myfile.txt")
else:
    print("The file does not exist")

import os
os.rmdir("myfolder")

class MyNewClass:
    pass
# Class Instantiation
my = MyNewClass()

class Animal:
    def __init__(self, voice):
        self.voice = voice
 
cat = Animal('Meow')
print(cat.voice)    # => Meow
 
dog = Animal('Woof') 
print(dog.voice)    # => Woof

class Dog:
    # 类的方法
    def bark(self):
        print("Ham-Ham")
 
charlie = Dog()
charlie.bark()   # => "Ham-Ham"

class Employee:
    def __init__(self, name):
        self.name = name
 
    def __repr__(self):
        return self.name
 
john = Employee('John')
print(john)  # => John

class CustomError(Exception):
    pass

class ParentClass:
    def print_self(self):
        print('A')
 
class ChildClass(ParentClass):
    def print_self(self):
        print('B')
 
obj_A = ParentClass()
obj_B = ChildClass()
 
obj_A.print_self() # => A
obj_B.print_self() # => B

class ParentClass:
    def print_self(self):
        print("Parent")
 
class ChildClass(ParentClass):
    def print_self(self):
        print("Child")
 
child_instance = ChildClass()
child_instance.print_self() # => Child

class Animal: 
    def __init__(self, name, legs):
        self.name = name
        self.legs = legs
        
class Dog(Animal):
    def sound(self):
        print("Woof!")
 
Yoki = Dog("Yoki", 4)
print(Yoki.name) # => YOKI
print(Yoki.legs) # => 4
Yoki.sound()     # => Woof!

# 这是单行注释

""" 可以写多行字符串
    使用三个"，并且经常使用
    作为文档。
"""

''' 可以写多行字符串
    使用三个'，并且经常使用
    作为文档。
'''

def double_numbers(iterable):
    for i in iterable:
        yield i + i

生成器可帮助您编写惰性代码

values = (-x for x in [1,2,3,4,5])
gen_to_list = list(values)
# => [-1, -2, -3, -4, -5]
print(gen_to_list)