Python 备忘单

• Python (python.org)
• 在 Y 分钟内学习 X (learnxinyminute.com)
• python中的正则表达式 （quickref.me）

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

Python 中著名的“Hello World”程序

x = 4        # x is of type int
x = "Sally"  # x is now of type str
print(x)

Python 没有用于声明变量的命令。

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

mylist = []
mylist.append(1)
mylist.append(2)
for x in mylist:
    print(x) # prints out 1,2

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

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

>>> def my_function():
...     print("Hello from a function")
...
>>> my_function()
Hello from a function

result = 10 + 30  # =>  40
result = 40 - 10  # =>  30
result = 50 * 5   # =>  250
result = 16 / 4   # =>  4
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."   

s = "Hello World"
s = 'Hello World'

a = """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'>

a = True 
b = 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))

a = (1, 2, 3)
a = 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

键 : 值(key : value)对，类JSON对象

整型

x = int(1)   # x will be 1
y = int(2.8) # y will be 2
z = int("3") # z will be 3

浮点型

x = float(1)     # x will be 1.0
y = float(2.8)   # y will be 2.8
z = float("3")   # z will be 3.0
w = float("4.2") # w will be 4.2

字符串

x = str("s1") # x will be 's1'
y = str(2)    # y will be '2'
z = str(3.0)  # z will be '3.0'

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

获取索引为 1 处的字符（大多数的编程语言，默认的索引都是从 0 开始。因此，索引为 1 ，代表列表中第 2 位的内容）

>>> for x in "abc":
...     print(x)
a
b
c

循环遍历单词“abc”中的字母

>>> a = "Hello, World!"
>>> print(len(a))
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

>>> 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']

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

逐行

with open("myfile.txt") as file:
    for line in file:
        print(line)

带行号

input = open('myfile.txt', 'r')
for i,line in enumerate(input, start=1):
    print("Number %s: %s" % (i, line))

写一个字符串

contents = {"aa": 12, "bb": 21}
with open("myfile1.txt", "w+") as file:
    file.write(str(contents))

读取字符串

with open('myfile1.txt', "r+") as file:
    contents = file.read()
print(contents)

写一个对象

contents = {"aa": 12, "bb": 21}
with open("myfile2.txt", "w+") as file:
    file.write(json.dumps(contents))

读取对象

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:
    '''This is a docstring.'''
    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:

    # Method of the class
    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!

# This is a single line comments.

""" Multiline strings can be written
    using three "s, and are often used
    as documentation.
"""

''' Multiline strings can be written
    using three 's, and are often used
    as documentation.
'''

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

生成器可以帮助您编写懒惰的代码。

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

# => [-1, -2, -3, -4, -5]
print(gen_to_list)