Python 备忘清单

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

Hello, World!

age = 18      # 年龄是 int 类型

name = "John" # name 现在是 str 类型

print(name)

>>> msg = "Hello, World!"

>>> print(msg[2:5])

llo

mylist = []

mylist.append(1)

mylist.append(2)

for item in mylist:

    print(item) # 打印输出 1,2

num = 200

if num > 0:

    print("num is greater than 0")

else:

    print("num is not greater than 0")

for item in range(6):

    if item == 3: break

    print(item)

else:

    print("Finally finished!")

>>> def my_function():

...     print("来自函数的你好")

...

>>> my_function()

来自函数的你好

with open("myfile.txt", "r", encoding='utf8') as file:

    for line in file:

        print(line)

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

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'

hello = "Hello World"

hello = 'Hello World'

multi_string = """Multiline Strings

Lorem ipsum dolor sit amet,

consectetur adipiscing elit """

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))

my_tuple = (1, 2, 3)

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

set1 = {"a", "b", "c"}   

set2 = set(("a", "b", "c"))

>>> 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

x = int(1)       # x 将是 1

y = int(2.8)     # y 将是 2

z = int("3")     # z 将是 3

x = float(1)     # x 将为 1.0

y = float(2.8)   # y 将是 2.8

z = float("3")   # z 将为 3.0

w = float("4.2") # w 将是 4.2

x = str("s1")    # x 将是's1'

y = str(2)       # y 将是 '2'

z = str(3.0)     # z 将是 '3.0'

>>> hello = "Hello, World"

>>> print(hello[1])

e

>>> print(hello[-1])

d

>>> for char in "foo":

...     print(char)

f

o

o

>>> s = 'mybacon'

>>> s[2:5]

'bac'

>>> s[0:2]

'my'

>>> s = 'mybacon'

>>> s[:2]

'my'

>>> s[2:]

'bacon'

>>> s[:2] + s[2:]

'mybacon'

>>> s[:]

'mybacon'

>>> s = 'mybacon'

>>> s[-5:-1]

'baco'

>>> s[2:6]

'baco'

>>> s = '12345' * 5

>>> s

'1234512345123451234512345'

>>> s[::5]

'11111'

>>> s[4::5]

'55555'

>>> s[::-5]

'55555'

>>> s[::-1]

'5432154321543215432154321'

>>> hello = "Hello, World!"

>>> print(len(hello))

13

>>> 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 = "John"

print("Hello, %s!" % name)

name = "John"

age = 23

print("%s is %d years old." % (name, age))

txt1 = "My name is {fname}, I'm {age}".format(fname="John", age=36)

txt2 = "My name is {0}, I'm {1}".format("John", 36)

txt3 = "My name is {}, I'm {}".format("John", 36)

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

Enter your name: Tom

>>> name

'Tom'

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

'John#Peter#Vicky'

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

True

>>> website = 'Reference'

>>> f"Hello, {website}"

"Hello, Reference"

>>> num = 10

>>> f'{num} + 10 = {num + 10}'

'10 + 10 = 20'

>>> f"""He said {"I'm John"}"""

"He said I'm John"

>>> f'5 {"{stars}"}'

'5 {stars}'

>>> f'5 {"stars"}'

'{5} stars'

>>> name = 'Eric'

>>> age = 27

>>> f"""Hello!

...     I'm {name}.

...     I'm {age}."""

"Hello!\n    I'm Eric.\n    I'm 27."

>>> 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'{10:b}'     # binary 二进制类型

'1010'

>>> f'{10:o}'     # octal 八进制类型

'12'

>>> f'{200:x}'    # hexadecimal 十六进制类型

'c8'

>>> f'{200:X}'

'C8'

>>> f'{345600000000:e}' # 科学计数法

'3.456000e+11'

>>> f'{65:c}'       # 字符类型

'A'

>>> f'{10:#b}'      # [类型] 带符号（基础）

'0b1010'

>>> f'{10:#o}'

'0o12'

>>> f'{10:#x}'

'0xa'

>>> 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]

>>> list(filter(lambda x : x % 2 == 1, range(1, 20)))

[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

>>> [x ** 2 for x in range (1, 11) if  x % 2 == 1]

[1, 9, 25, 49, 81]

>>> [x for x in [3, 4, 5, 6, 7] if x > 5]

[6, 7]

>>> list(filter(lambda x: x > 5, [3, 4, 5, 6, 7]))

[6, 7]

>>> 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]

a_list[start:end]

a_list[start:end:step]

>>> a = ['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

>>> a[2:5]

['bacon', 'tomato', 'ham']

>>> a[-5:-2]

['egg', 'bacon', 'tomato']

>>> a[1:4]

['egg', 'bacon', 'tomato']

>>> a[:4]

['spam', 'egg', 'bacon', 'tomato']

>>> a[0:4]

['spam', 'egg', 'bacon', 'tomato']

>>> a[2:]

['bacon', 'tomato', 'ham', 'lobster']

>>> a[2:len(a)]

['bacon', 'tomato', 'ham', 'lobster']

>>> a

['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

>>> a[:]

['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

>>> a[0:6:2]

['spam', 'bacon', 'ham']

>>> a[1:6:2]

['egg', 'tomato', 'lobster']

>>> a[6:0:-2]

['lobster', 'tomato', 'egg']

>>> a

['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

>>> a[::-1]

['lobster', 'ham', 'tomato', 'bacon', 'egg', 'spam']

>>> 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

>>> odd = [1, 3, 5]

>>> odd.extend([9, 11, 13])

>>> odd

[1, 3, 5, 9, 11, 13]

>>> odd = [1, 3, 5]

>>> odd + [9, 11, 13]

[1, 3, 5, 9, 11, 13]

>>> li = [3, 1, 3, 2, 5]

>>> li.sort()

>>> li

[1, 2, 3, 3, 5]

>>> li.reverse()

>>> li

[5, 3, 3, 2, 1]

>>> 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))

result = [x**2 for x in range(10) if x % 2 == 0]

print(result)

# [0, 4, 16, 36, 64]

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)

with open("myfile.txt") as file:

    for line in file:

        print(line)

file = open('myfile.txt', 'r')

for i, line in enumerate(file, start=1):

    print("Number %s: %s" % (i, line))

contents = {"aa": 12, "bb": 21}

with open("myfile1.txt", "w+") as file:

    file.write(str(contents))

with open('myfile1.txt', "r+") as file:

    contents = file.read()

print(contents)

contents = {"aa": 12, "bb": 21}

with open("myfile2.txt", "w+") as file:

    file.write(json.dumps(contents))

with open('myfile2.txt', "r+") as file:

    contents = json.load(file)

print(contents)

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 MyClass:

    class_variable = "A class variable!"

# => 一个类变量！

print(MyClass.class_variable)

x = MyClass()

# => 一个类变量！

print(x.class_variable)

class ParentClass:

    def print_test(self):

        print("Parent Method")

class ChildClass(ParentClass):

    def print_test(self):

        print("Child Method")

        # 调用父级的 print_test ()

        super().print_test()

>>> child_instance = ChildClass()

>>> child_instance.print_test()

Child Method

Parent Method

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

values = (-x for x in [1,2,3,4,5])

gen_to_list = list(values)

# => [-1, -2, -3, -4, -5]

print(gen_to_list)

try:

    # 使用 “raise” 来引发错误

    raise IndexError("这是一个索引错误")

except IndexError as e:

    pass                 # pass 只是一个空操作。 通常你会在这里做恢复。

except (TypeError, NameError):

    pass                 # 如果需要，可以一起处理多个异常。

else:                    # try/except 块的可选子句。 必须遵循除块之外的所有内容

    print("All good!")   # 仅当 try 中的代码未引发异常时运行

finally:                 # 在所有情况下执行

    print("我们可以在这里清理资源")