入门
介绍
- Python 官方网站 (python.org)
- Python 文档 (docs.python.org)
- Y 分钟学会 Python (learnxinyminutes.com)
- Python 中的正则表达式 (jaywcjlove.github.io)
控制台打印
>>> print("Hello, World!")
Hello, World!
著名的“Hello World”程序在 Python 中的实现
变量
age = 18 # 年龄是 int 类型
name = "John" # 名字现在是 str 类型
print(name)
- Python 不能在没有赋值的情况下声明变量
- 变量可以存放不同类型的值
内置数据类型
序列指一批有序的元素,集合指一批无序且不重复的元素
:- | :- |
---|---|
str |
文本/字符串(Text) |
int , float , complex
|
数值(Numeric) |
dict |
映射/键值对(Mapping) |
list , tuple , range
|
序列(Sequence) |
set , frozenset
|
集合(Set) |
bool |
布尔值/逻辑值(Boolean) |
bytes , bytearray , memoryview
|
二进制数据(Binary) |
查看: 数据类型
字符串切片
>>> msg = "Hello, World!"
>>> print(msg[2:5])
llo
查看: 字符串
列表
mylist = []
mylist.append(1)
mylist.append(2)
for item in mylist:
print(item) # 打印输出 1,2
查看: 列表
判断
num = 200
if num > 0:
print("num is greater than 0")
else:
print("num is not greater than 0")
查看: 判断
循环
for item in range(6):
if item == 3: break
print(item)
else:
print("Finally finished!")
查看: 循环
函数
>>> def my_function():
... print("来自函数的你好")
...
>>> my_function()
来自函数的你好
查看: 函数
文件处理
with open("myfile.txt", "r", encoding='utf8') as file:
for line in file:
print(line)
查看: 文件处理
算术
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
加等于
counter = 0
counter += 10 # => 10
counter = 0
counter = counter + 10 # => 10
message = "Part 1."
# => Part 1.Part 2.
message += "Part 2."
f-字符串 (Python 3.6+)
>>> website = 'Quick Reference'
>>> f"Hello, {website}"
"Hello, Quick Reference"
>>> num = 10
>>> f'{num} + 10 = {num + 10}'
'10 + 10 = 20'
查看: f-字符串
Python 数据类型
字符串
hello = "Hello World"
hello = 'Hello World'
multi_string = """Multiline Strings
Lorem ipsum dolor sit amet,
consectetur adipiscing elit """
查看: 字符串
数值
x = 1 # 整数
y = 2.8 # 浮点小数
z = 1j # 复数
>>> print(type(x))
<class 'int'>
只要内存足够,可以容纳无限大(小)的数值
布尔值
my_bool = True
my_bool = False
bool(0) # => False
bool(1) # => True
bool 是 int 的子类
列表
list1 = ["apple", "banana", "cherry"]
list2 = [True, False, False]
list3 = [1, 5, 7, 9, 3]
list4 = list((1, 5, 7, 9, 3))
查看: 列表
元组
my_tuple = (1, 2, 3)
my_tuple = tuple((1, 2, 3))
类似列表,但自身不可变
集合
set1 = {"a", "b", "c"}
set2 = set(("a", "b", "c"))
类似列表,但里面的元素是无序而不重复的
字典
>>> 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,一种像 JSON 那样对象
类型转换
转换为整数
x = int(1) # 得到 1
y = int(2.8) # 得到 2
z = int("3") # 得到 3
转换为浮点数
x = float(1) # 得到 1.0
y = float(2.8) # 得到 2.8
z = float("3") # 得到 3.0
w = float("4.2") # 得到 4.2
转换为字符串
x = str("s1") # 得到 "s1"
y = str(2) # 得到 "2"
z = str(3.0) # 得到 "3.0"
Python 字符串
下标访问
>>> hello = "Hello, World"
>>> print(hello[1]) # 获取第二个字符
e
>>> print(hello[-1]) # 获取倒数第一个字符
d
>>> print(type(hello[-1])) # 得到的还是字符串
<class 'str'>
循环
>>> for char in "foo":
... print(char)
f
o
o
对字符串 for-in 可以得到每个字符(类型还是字符串)
切割字符串
┌───┬───┬───┬───┬───┬───┬───┐
| m | y | b | a | c | o | n |
└───┴───┴───┴───┴───┴───┴───┘
0 1 2 3 4 5 6 7
-7 -6 -5 -4 -3 -2 -1
>>> 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'
步长
>>> s = '12345' * 5
>>> s
'1234512345123451234512345'
>>> s[::5]
'11111'
>>> s[4::5]
'55555'
>>> s[::-5]
'55555'
>>> s[::-1]
'5432154321543215432154321'
获取长度
>>> hello = "Hello, World!"
>>> print(len(hello))
13
len()
函数返回字符串的长度
重复多次
>>> s = '===+'
>>> n = 8
>>> s * n
'===+===+===+===+===+===+===+===+'
存在性判断
>>> s = 'spam'
>>> s in 'I saw spamalot!'
True
>>> s not in 'I saw The Holy Grail!'
True
判断 "spam" 这个字符串是否在其它字符串里
字符串拼接
>>> s = 'spam'
>>> t = 'egg'
>>> s + t # 可以使用加号进行拼接
'spamegg'
>>> 'spam' 'egg' # 两个字符串之间可以省略加号
'spamegg'
格式化
name = "John"
print("Hello, %s!" % name)
name = "John"
age = 23
print("%s is %d years old." % (name, age))
format() 方法
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)
控制台输入
>>> name = input("Enter your name: ")
Enter your name: Tom
>>> name
'Tom'
从控制台获取输入数据
插入分隔符拼接
>>> "、".join(["John", "Peter", "Vicky"])
'John、Peter、Vicky'
头尾判断
>>> # 是否以 H 开头
>>> "Hello, world!".endswith("H")
True
>>> # 是否以 h 开头
>>> "Hello, world!".endswith("h")
False
>>> # 是否以 ! 结尾
>>> "Hello, world!".endswith("!")
True
转义符号
转义符号 | 对应的操作 |
---|---|
\\ |
输出反斜杠 |
\' |
输出单引号 |
\" |
输出双引号 |
\n |
换行 |
\t |
水平制表符 |
\r |
光标回到首位 |
\b |
退格 |
Python f-字符串 (Python 3.6+)
f-字符串 用法
>>> 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."
它从 Python 3.6 开始可用,另见: 格式字符串字面值
填充对齐
>>> f'{"text":10}' # 使用空格填充到指定长度
'text '
>>> f'{"test":*>10}' # 向左填充
'******test'
>>> f'{"test":*<10}' # 向右填充
'test******'
>>> f'{"test":*^10}' # 居中填充
'***test***'
>>> f'{12345:0>10}' # 使用数字填充
'0000012345'
按类型输出
>>> f'{10:b}' # 输出二进制数值
'1010'
>>> f'{10:o}' # 输出八进制数值
'12'
>>> f'{200:x}' # 输出十六进制数值
'c8'
>>> f'{200:X}'
'C8'
>>> f'{345600000000:e}' # 科学计数法
'3.456000e+11'
>>> f'{65:c}' # 将整数转化为一个字符后输出
'A'
>>> f'{10:#b}' # [类型] 带符号(基础)
'0b1010'
>>> f'{10:#o}'
'0o12'
>>> f'{10:#x}'
'0xa'
显示正负号
>>> f'{12345:+}' # 显示正数的正号
'+12345'
>>> f'{-12345:+}' # 显示负数的负号
'-12345'
>>> f'{-12345:+10}' # 显示负号,并使用空格填充,直到长度为 10
' -12345'
>>> f'{-12345:+010}' # 显示负号,并使用0填充,直到长度为 10
'-000012345'
其它
>>> 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%'
Python 列表
定义
>>> 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]
生成
>>> 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]
添加元素
>>> 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]
切片
列表切片的语法:
a_list[start:end]
a_list[start:end:step]
切片
>>> 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']
省略索引
>>> 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']
间隔索引
['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']
删除
>>> li = ['bread', 'butter', 'milk']
>>> li.pop()
'milk'
>>> li
['bread', 'butter']
>>> del li[0]
>>> li
['butter']
列表边界
>>> 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
连接
>>> 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]
排序和反转
>>> li = [3, 1, 3, 2, 5]
>>> li.sort()
>>> li
[1, 2, 3, 3, 5]
>>> li.reverse()
>>> li
[5, 3, 3, 2, 1]
计数
>>> li = [3, 1, 3, 2, 5]
>>> li.count(3)
2
重复
>>> li = ["re"] * 3
>>> li
['re', 're', 're']
Python 判断
一般形式
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.")
单行形式
>>> a = 330
>>> b = 200
>>> r = "a" if a > b else "b"
>>> print(r)
a
注意条件是放在中间的
Python 循环
一般形式
primes = [2, 3, 5, 7]
for prime in primes:
print(prime)
带索引
animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
print(i, value)
while 循环
x = 0
while x < 4:
print(x)
x += 1 # Shorthand for x = x + 1
跳出循环
x = 0
for index in range(10):
x = index * 10
if index == 5:
break
print(x)
跳过一轮循环
for index in range(3, 8):
x = index * 10
if index == 5:
continue
print(x)
范围循环
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
使用 zip()
name = ['Pete', 'John', 'Elizabeth']
age = [6, 23, 44]
for n, a in zip(name, age):
print('%s is %d years old' %(n, a))
列表生成式
result = [x**2 for x in range(10) if x % 2 == 0]
print(result)
# [0, 4, 16, 36, 64]
Python 函数
基础
def hello_world():
print('Hello, World!')
返回
def add(x, y):
print("x is %s, y is %s" %(x, y))
return x + y
add(5, 6) # => 11
位置参数
def varargs(*args):
return args
varargs(1, 2, 3) # => (1, 2, 3)
args 的类型是 tuple
关键字参数
def keyword_args(**kwargs):
return kwargs
# => {"big": "foot", "loch": "ness"}
keyword_args(big="foot", loch="ness")
kwargs 的类型是 dict
返回多个
def swap(x, y):
return y, x
x = 1
y = 2
x, y = swap(x, y) # => x = 2, y = 1
默认值
def add(x, y=10):
return x + y
add(5) # => 15
add(5, 20) # => 25
匿名函数
# => True
(lambda x: x > 2)(3)
# => 5
(lambda x, y: x ** 2 + y ** 2)(2, 1)
Python 模块
导入模块
import math
print(math.sqrt(16)) # => 4.0
从一个模块导入
from math import ceil, floor
print(ceil(3.7)) # => 4.0
print(floor(3.7)) # => 3.0
导入一个模块的全部
from math import *
给模块起别名
import math as m
# => True
math.sqrt(16) == m.sqrt(16)
浏览模块的函数和属性
import math
dir(math)
Python 文件处理
读取文件
逐行
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))
字符串
写入一个字符串
contents = {"aa": 12, "bb": 21}
with open("myfile1.txt", "w+") as file:
file.write(str(contents))
读取一个字符串
with open('myfile1.txt', "r+") as file:
contents = file.read()
print(contents)
对象
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)
删除文件
import os
os.remove("myfile.txt")
检查和删除
import os
if os.path.exists("myfile.txt"):
os.remove("myfile.txt")
else:
print("The file does not exist")
删除文件夹
import os
os.rmdir("myfolder")
Python 类和继承
定义
class MyNewClass:
pass
# 类的实例化
my = MyNewClass()
构造函数
class Animal:
def __init__(self, voice):
self.voice = voice
cat = Animal('Meow')
print(cat.voice) # => Meow
dog = Animal('Woof')
print(dog.voice) # => Woof
方法
class Dog:
# 类的方法
def bark(self):
print("Ham-Ham")
charlie = Dog()
charlie.bark() # => "Ham-Ham"
类变量
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)
super() 函数
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
repr() 方法
class Employee:
def __init__(self, name):
self.name = name
def __repr__(self):
return self.name
john = Employee('John')
print(john) # => John
用户定义的异常
class CustomError(Exception):
pass
多态性
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
重写
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
继承
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!
Python 类型标注 (Python 3.5+)
变量
string: str = "ha"
times: int = 3
print(string * times) # => hahaha
变量
result: int = 1 + 2
print(result) # => 3
虽然标注了错误的类型,但不影响正常运行
参数
def say(name: str, start: str = "Hi"):
return start + ", " + name
print(say("Python")) # => Hi, Python
位置参数
def calc_summary(*args: int):
return sum(args)
print(calc_summary(3, 1, 4)) # => 8
表示 args 的所有元素都是 int 类型的。
返回值
def say_hello(name) -> str:
return "Hello, " + name
var = "Python"
print(say_hello(var)) # => Hello, Python
多种可能的返回值
from typing import Union
def resp200(meaningful) -> Union[int, str]:
return "OK" if meaningful else 200
表示返回值可能是 int,也可能是 str 。
关键字参数
def calc_summary(**kwargs: int):
return sum(kwargs.values())
print(calc_summary(a=1, b=2)) # => 3
表示 kwargs 的所有值都是 int 类型的。
多个返回值
def resp200() -> (int, str):
return 200, "OK"
多种可能的返回值 (3.10+)
def resp200(meaningful) -> int | str:
return "OK" if meaningful else 200
自 Python 3.10 起可用。
属性
class Employee:
name: str
age: int
def __init__(self, name, age):
self.name = name
self.age = age
self.graduated: bool = False
标注自己
class Employee:
name: str
age: int
def set_name(self, name) -> "Employee":
self.name = name
return self
这里表示 set_name() 返回了一个 Employee 对象。
标注自己 (3.11+)
from typing import Self
class Employee:
name: str
age: int
def set_name(self: Self, name) -> Self:
self.name = name
return self
标注一个值为类型的参数
from typing import TypeVar, Type
T = TypeVar("T")
# "mapper" 的值是一个像 int、str、MyClass 这样的类型
# "default" 是一个 T 类型的值,比如 314、"string"、MyClass()
# 函数的返回值也是一个 T 类型的值
def converter(raw, mapper: Type[T], default: T) -> T:
try:
return mapper(raw)
except:
return default
raw: str = input("请输入一个整数:")
result: int = converter(raw, mapper=int, default=0)
标注一个值为函数的参数
from typing import TypeVar, Callable, Any
T = TypeVar("T")
def converter(raw, mapper: Callable[[Any], T], default: T) -> T:
try:
return mapper(raw)
except:
return default
# Callable[[Any], T] 表示值是一个像这样声明的函数:
# def anynomous(arg: Any) -> T:
# pass
def is_success(value) -> bool:
return value in (0, "OK", True, "success")
resp = dict(code=0, message="OK", data=[])
successed: bool = converter(resp.message, mapper=is_success, default=False)
各种各样的
注释
# 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。
'''
生成器
def double_numbers(iterable):
for i in iterable:
yield i + i
生成器可帮助您编写惰性代码
要列出的生成器
values = (-x for x in [1,2,3,4,5])
gen_to_list = list(values)
# => [-1, -2, -3, -4, -5]
print(gen_to_list)
处理异常
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")
Python备忘单是Python3编程语言的单页参考表。它为开发人员提供了Python的快速参考,帮助他们快速查找和使用Python语言的基本语法和方法。