Python 速查表

• Python (python.org)
• Python 文档 (docs.python.org)
• X分钟学习Python (learnxinyminutes.com)
• Python中的正则表达式 (r3f.cn)

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

Python 中著名的 "Hello World" 程序

age = 18      # age is of type int
name = "John" # name is now of type str
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

参见: 字符串

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

参见: 列表

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

参见: 流程控制

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

参见: 循环

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

参见: 函数

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 的向下取整除法，另请参阅 StackOverflow

counter = 0
counter += 10           # => 10
counter = 0
counter = counter + 10  # => 10

message = "Part 1."

# => Part 1.Part 2.
message += "Part 2."

>>> website = 'r3f.cn'
>>> f"Hello, {website}"
"Hello, r3f.cn"

>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'

参见: Python F-字符串

hello = "Hello World"
hello = 'Hello World'

multi_string = """Multiline Strings
Lorem ipsum dolor sit amet,
consectetur adipiscing elit """

参见: 字符串

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))

参见: 列表

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

tupla = (1, 2, 3, 'python')

print(tupla[0])       # Output: 1
print(tupla.count(1)) # 计算出现次数
print(tupla.index(2)) # 查找索引

tupla1 = (1, 2, 3)
tupla2 = ('a', 'b')

len(tuple) → 返回元素数量。
in → 检查元素是否存在于元组中。
Concatenation (+) → 合并两个元组。
Repetition (*) → 重复一个元组。
Slicing (tuple[start:end]) → 提取子元组。

print(len(tupla1))       # Output: 3
print(2 in tupla1)       # Output: True
print(tupla1 + tupla2)   # Output: (1, 2, 3, 'a', 'b')
print(tupla1[1:])        # Output: (2, 3)

# unpacking
a, b, c, d = tupla   # 每个值赋给一个变量

与列表类似，但不可变

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

唯一项目/对象的集合

>>> 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

键值对，类似 JSON 的对象

#整数

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

#浮点数

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'

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

获取位置 1 或最后一个字符

>>> 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

>>> website = 'r3f.cn'
>>> f"Hello, {website}"
"Hello, r3f.cn"

>>> 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."

自 Python 3.6 起可用，另请参阅： 格式化字符串字面值

>>> f'{"text":10}'     # [宽度]
'text      '
>>> f'{"test":*>10}'   # 左填充
'******test'
>>> f'{"test":*<10}'   # 右填充
'test******'
>>> f'{"test":*^10}'   # 居中填充
'***test***'
>>> f'{12345:0>10}'    # 用数字填充
'0000012345'

>>> f'{10:b}'        # 二进制类型
'1010'
>>> f'{10:o}'        # 八进制类型
'12'
>>> f'{200:x}'       # 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] 带表示法 (基数)
'0b1010'
>>> f'{10:#o}'
'0o12'
>>> f'{10:#x}'
'0xa'

>>> f'{-12345:0=10}'  # 负数
'-000012345'
>>> f'{12345:010}'    # [0] 快捷方式 (无对齐)
'0000012345'
>>> f'{-12345:010}'
'-000012345'
>>> import math       # [.精度]
>>> 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%'

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

>>> 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")

x = 1
match x:
  case 0:
    print("zero")
  case 1:
    print("one")
  case _:
    print("multiple")

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

打印: 2 3 5 7

animals = ["dog", "cat", "mouse"]
# enumerate() 为可迭代对象添加计数器
for i, value in enumerate(animals):
    print(i, value)

打印: 0 dog 1 cat 2 mouse

x = 0
while x < 4:
    print(x)
    x += 1  # x = x + 1 的简写

打印: 0 1 2 3

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

打印: 0 10 20 30 40

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

打印: 30 40 60 70

for i in range(4):
    print(i) # 打印: 0 1 2 3

for i in range(4, 8):
    print(i) # 打印: 4 5 6 7

for i in range(4, 10, 2):
    print(i) # 打印: 4 6 8

words = ['Mon', 'Tue', 'Wed']
nums = [1, 2, 3]
# 使用 zip 打包成元组列表
for w, n in zip(words, nums):
    print('%d:%s, ' %(n, w))

打印: 1:Mon, 2:Tue, 3:Wed,

nums = [60, 70, 30, 110, 90]
for n in nums:
    if n > 100:
        print("%d is bigger than 100" %n)
        break
else:
    print("Not found!")

另请参阅: Python 技巧

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)

"args" 的类型是元组。

def keyword_args(**kwargs):
    return kwargs

# => {"big": "foot", "loch": "ness"}
keyword_args(big="foot", loch="ness")

"kwargs" 的类型是字典。

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)

#逐行读取

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

#带行号

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

#写入字符串

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)

#写入对象

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:
    pass

# 类实例化
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!

string: str = "ha"
times: int = 3


# 错误的提示，但能正确运行
result: str = 1 + 2
print(result)  # => 3


def say(name: str, start: str = "Hi"):
    return start + ", " + name

print(say("Python"))  # => Hi, Python

from typing import Dict, Tuple, List

bill: Dict[str, float] = {
    "apple": 3.14,
    "watermelon": 15.92,
    "pineapple": 6.53,
}
completed: Tuple[str] = ("DONE",)
succeeded: Tuple[int, str] = (1, "SUCCESS")
statuses: Tuple[str, ...] = (
    "DONE", "SUCCESS", "FAILED", "ERROR",
)
codes: List[int] = (0, 1, -1, -2)

bill: dict[str, float] = {
    "apple": 3.14,
    "watermelon": 15.92,
    "pineapple": 6.53,
}
completed: tuple[str] = ("DONE",)
succeeded: tuple[int, str] = (1, "SUCCESS")
statuses: tuple[str, ...] = (
    "DONE", "SUCCESS", "FAILED", "ERROR",
)
codes: list[int] = (0, 1, -1, -2)

def calc_summary(*args: int):
    return sum(args)

print(calc_summary(3, 1, 4))  # => 8

表示所有参数的类型都是 int。

def say_hello(name) -> str:
    return "Hello, " + name

var = "Python"
print(say_hello(var))  # => Hello, Python

from typing import Union

def resp200(meaningful) -> Union[int, str]:
    return "OK" if meaningful else 200

表示返回值类型可能是 int 或 str。

def calc_summary(**kwargs: int):
    return sum(kwargs.values())

print(calc_summary(a=1, b=2))  # => 3

表示所有参数值的类型都是 int。

def resp200() -> (int, str):
    return 200, "OK"

returns = resp200()
print(returns)  # => (200, 'OK')
print(type(returns))  # tuple

def resp200(meaningful) -> int | str:
    return "OK" if meaningful else 200

自 Python 3.10 起

class Employee:
    name: str
    age: int

    def __init__(self, name, age):
        self.name = name
        self.age = age
        self.graduated: bool = False

class Employee:
    name: str

    def set_name(self, name) -> "Employee":
        self.name = name
        return self

    def copy(self) -> 'Employee':
        return type(self)(self.name)

from typing import Self

class Employee:
    name: str
    age: int

    def set_name(self: Self, name) -> Self:
        self.name = name
        return self

# 这是单行注释。

""" 可以使用三个 " 来编写多行字符串，
    并且通常用作文档。
"""

''' 可以使用三个 ' 来编写多行字符串，
    并且通常用作文档。
'''

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

生成器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)