60個(gè)重要的Python小示例

?

17 計(jì)算表達(dá)式

將字符串str 當(dāng)成有效的表達(dá)式來(lái)求值并返回計(jì)算結(jié)果取出字符串中內(nèi)容

?

In [1]: s = "1 + 3 +5"
    ...: eval(s)
    ...:
Out[1]: 9

?

18 字符串格式化　

格式化輸出字符串，format(value, format_spec)實(shí)質(zhì)上是調(diào)用了value的__format__(format_spec)方法。

?

In [104]: print("i am {0},age{1}".format("tom",18))
i am tom,age18

?

三、 函數(shù)

19 拿來(lái)就用的排序函數(shù)

排序：

?

In [1]: a = [1,4,2,3,1]

In [2]: sorted(a,reverse=True)
Out[2]: [4, 3, 2, 1, 1]

In [3]: a = [{'name':'xiaoming','age':18,'gender':'male'},{'name':'
     ...: xiaohong','age':20,'gender':'female'}]
In [4]: sorted(a,key=lambda x: x['age'],reverse=False)
Out[4]:
[{'name': 'xiaoming', 'age': 18, 'gender': 'male'},
 {'name': 'xiaohong', 'age': 20, 'gender': 'female'}]

?

20 求和函數(shù)

求和：

?

In [181]: a = [1,4,2,3,1]

In [182]: sum(a)
Out[182]: 11

In [185]: sum(a,10) #求和的初始值為10
Out[185]: 21

?

21 nonlocal用于內(nèi)嵌函數(shù)中

關(guān)鍵詞nonlocal常用于函數(shù)嵌套中，聲明變量i為非局部變量；如果不聲明，i+=1表明i為函數(shù)wrapper內(nèi)的局部變量，因?yàn)樵趇+=1引用(reference)時(shí),i未被聲明，所以會(huì)報(bào)unreferenced variable的錯(cuò)誤。

?

def excepter(f):
    i = 0
    t1 = time.time()
    def wrapper():
        try:
            f()
        except Exception as e:
            nonlocal i
            i += 1
            print(f'{e.args[0]}: {i}')
            t2 = time.time()
            if i == n:
                print(f'spending time:{round(t2-t1,2)}')
    return wrapper

?

22 global 聲明全局變量

先回答為什么要有g(shù)lobal，一個(gè)變量被多個(gè)函數(shù)引用，想讓全局變量被所有函數(shù)共享。有的伙伴可能會(huì)想這還不簡(jiǎn)單，這樣寫：

?

i = 5
def f():
    print(i)

def g():
    print(i)
    pass

f()
g()

?

但是，如果我想要有個(gè)函數(shù)對(duì)i遞增，這樣：

?

def h():
    i += 1

h()

?

global就是為解決此問(wèn)題而被提出，在函數(shù)h內(nèi)，顯式地告訴編譯器i為全局變量，然后編譯器會(huì)在函數(shù)外面尋找i的定義，執(zhí)行完i+=1后，i還為全局變量，值加1：

?

i = 0
def h():
    global i
    i += 1

h()
print(i)

?

23 交換兩元素

?

def swap(a, b):
    return b, a


print(swap(1, 0))  # (0,1)

?

24 操作函數(shù)對(duì)象

?

In [31]: def f():
    ...:     print('i'm f')
    ...:

In [32]: def g():
    ...:     print('i'm g')
    ...:

In [33]: [f,g][1]()
i'm g

?

25 生成逆序序列

?

list(range(10,-1,-1)) # [10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

?

26 函數(shù)的五類參數(shù)使用例子

python五類參數(shù)：位置參數(shù)，關(guān)鍵字參數(shù)，默認(rèn)參數(shù)，可變位置或關(guān)鍵字參數(shù)的使用。

?

def f(a,*b,c=10,**d):
  print(f'a:{a},b:,c:{c},d:usksguy')

?

調(diào)用f:

?

In [10]: f(1,2,5,width=10,height=20)
a:1,b:(2, 5),c:10,d:{'width': 10, 'height': 20}

?

再次調(diào)用f:

?

In [11]: f(a=1,c=12)
a:1,b:(),c:12,d:{}

?

注意觀察參數(shù)a, 既可以f(1),也可以f(a=1)?其可讀性比第一種更好，建議使用f(a=1)。如果要強(qiáng)制使用f(a=1)，需要在前面添加一個(gè)星號(hào):

?

def f(*,a,**b):
  print(f'a:{a},b:')

?

此時(shí)f(1)調(diào)用，將會(huì)報(bào)錯(cuò)：TypeError: f() takes 0 positional arguments but 1 was given

只能f(a=1)才能OK.

說(shuō)明前面的*發(fā)揮作用，它變?yōu)橹荒軅魅腙P(guān)鍵字參數(shù)，那么如何查看這個(gè)參數(shù)的類型呢？借助python的inspect模塊：

?

In [22]: for name,val in signature(f).parameters.items():
    ...:     print(name,val.kind)
    ...:
a KEYWORD_ONLY
b VAR_KEYWORD

?

但是，如果f定義為：

?

def f(a,*b):
  print(f'a:{a},b:')

In [24]: for name,val in signature(f).parameters.items():
    ...:     print(name,val.kind)
    ...:
a POSITIONAL_OR_KEYWORD
b VAR_POSITIONAL

?

27使用slice對(duì)象

生成關(guān)于蛋糕的序列cake1：

?

In [1]: cake1 = list(range(5,0,-1))

In [2]: b = cake1[1:10:2]

In [3]: b
Out[3]: [4, 2]

In [4]: cake1
Out[4]: [5, 4, 3, 2, 1]

In [5]: from random import randint
   ...: cake2 = [randint(1,100) for _ in range(100)]
   ...: # 同樣以間隔為2切前10個(gè)元素，得到切片d
   ...: d = cake2[1:10:2]
In [6]: d
Out[6]: [75, 33, 63, 93, 15]

?

那么，為什么不把這種切法封裝為一個(gè)對(duì)象呢？于是就有了slice對(duì)象。

定義slice對(duì)象極為簡(jiǎn)單，如把上面的切法定義成slice對(duì)象：

?

perfect_cake_slice_way = slice(1,10,2)
#去切cake1
cake1_slice = cake1[perfect_cake_slice_way]
cake2_slice = cake2[perfect_cake_slice_way]

In [11]: cake1_slice
Out[11]: [4, 2]

In [12]: cake2_slice
Out[12]: [75, 33, 63, 93, 15]

?

對(duì)于逆向序列切片，slice對(duì)象一樣可行：

?

a = [1,3,5,7,9,0,3,5,7]
a_ = a[5:1:-1]

named_slice = slice(5,1,-1)
a_slice = a[named_slice]

In [14]: a_
Out[14]: [0, 9, 7, 5]

In [15]: a_slice
Out[15]: [0, 9, 7, 5]

?

28 lambda 函數(shù)的動(dòng)畫演示

有些讀者反映，lambda函數(shù)不太會(huì)用，問(wèn)我能不能解釋一下。

比如，下面求這個(gè)?lambda函數(shù)：

?

def max_len(*lists):
    return max(*lists, key=lambda v: len(v))

?

參數(shù)v的取值？

lambda函數(shù)有返回值嗎？如果有，返回值是多少？

調(diào)用上面函數(shù)，求出以下三個(gè)最長(zhǎng)的列表：

?

r = max_len([1, 2, 3], [4, 5, 6, 7], [8])
print(f'更長(zhǎng)的列表是{r}')

?

結(jié)論：

參數(shù)v的可能取值為*lists，也就是?tuple?的一個(gè)元素。

lambda函數(shù)返回值，等于lambda v冒號(hào)后表達(dá)式的返回值。

四、 數(shù)據(jù)結(jié)構(gòu)

29 轉(zhuǎn)為字典　　

創(chuàng)建數(shù)據(jù)字典

?

In [1]: dict()
Out[1]: {}

In [2]: dict(a='a',b='b')
Out[2]: {'a': 'a', 'b': 'b'}

In [3]: dict(zip(['a','b'],[1,2]))
Out[3]: {'a': 1, 'b': 2}

In [4]: dict([('a',1),('b',2)])
Out[4]: {'a': 1, 'b': 2}

?

30 凍結(jié)集合　　

創(chuàng)建一個(gè)不可修改的集合。

?

In [1]: frozenset([1,1,3,2,3])
Out[1]: frozenset({1, 2, 3})

?

31 轉(zhuǎn)為集合類型

返回一個(gè)set對(duì)象，集合內(nèi)不允許有重復(fù)元素：

?

In [159]: a = [1,4,2,3,1]

In [160]: set(a)
Out[160]: {1, 2, 3, 4}

?

32 轉(zhuǎn)為切片對(duì)象

class?slice(start,?stop[,?step])

返回一個(gè)表示由 range(start, stop, step) 所指定索引集的 slice對(duì)象，它讓代碼可讀性、可維護(hù)性變好。

?

In [1]: a = [1,4,2,3,1]

In [2]: my_slice_meaning = slice(0,5,2)

In [3]: a[my_slice_meaning]
Out[3]: [1, 2, 1]

?

33 轉(zhuǎn)元組

tuple()?將對(duì)象轉(zhuǎn)為一個(gè)不可變的序列類型

?

In [16]: i_am_list = [1,3,5]
In [17]: i_am_tuple = tuple(i_am_list)
In [18]: i_am_tuple
Out[18]: (1, 3, 5)

?

五、 類和對(duì)象

34 是否可調(diào)用　　

檢查對(duì)象是否可被調(diào)用

?

In [1]: callable(str)
Out[1]: True

In [2]: callable(int)
Out[2]: True

In [18]: class Student():
    ...:     def __init__(self,id,name):
    ...:         self.id = id
    ...:         self.name = name
    ...:     def __repr__(self):
    ...:         return 'id = '+self.id +', name = '+self.name
    ...

In [19]: xiaoming = Student('001','xiaoming')

In [20]: callable(xiaoming)
Out[20]: False

In [1]: class Student():
    ...:     def __init__(self,id,name):
    ...:         self.id = id
    ...:         self.name = name
    ...:     def __repr__(self):
    ...:         return 'id = '+self.id +', name = '+self.name
    ...:     def __call__(self):
    ...:         print('I can be called')
    ...:         print(f'my name is {self.name}')
    ...:

In [2]: t = Student('001','xiaoming')

In [3]: t()
I can be called
my name is xiaoming

?

35 ascii 展示對(duì)象　　

調(diào)用對(duì)象的?__repr__?方法，獲得該方法的返回值，如下例子返回值為字符串

?

>>> class Student():
    def __init__(self,id,name):
        self.id = id
        self.name = name
    def __repr__(self):
        return 'id = '+self.id +', name = '+self.name

?

調(diào)用：

?

>>> xiaoming = Student(id='1',name='xiaoming')
>>> xiaoming
id = 1, name = xiaoming
>>> ascii(xiaoming)
'id = 1, name = xiaoming'

?

36 類方法　

classmethod?裝飾器對(duì)應(yīng)的函數(shù)不需要實(shí)例化，不需要?self?參數(shù)，但第一個(gè)參數(shù)需要是表示自身類的 cls 參數(shù)，可以來(lái)調(diào)用類的屬性，類的方法，實(shí)例化對(duì)象等。

?

In [1]: class Student():
    ...:     def __init__(self,id,name):
    ...:         self.id = id
    ...:         self.name = name
    ...:     def __repr__(self):
    ...:         return 'id = '+self.id +', name = '+self.name
    ...:     @classmethod
    ...:     def f(cls):
    ...:         print(cls)

?

37 動(dòng)態(tài)刪除屬性　　

刪除對(duì)象的屬性

?

In [1]: delattr(xiaoming,'id')

In [2]: hasattr(xiaoming,'id')
Out[2]: False

?

38 一鍵查看對(duì)象所有方法　

不帶參數(shù)時(shí)返回當(dāng)前范圍內(nèi)的變量、方法和定義的類型列表；帶參數(shù)時(shí)返回參數(shù)的屬性，方法列表。

?

In [96]: dir(xiaoming)
Out[96]:
['__class__',
 '__delattr__',
 '__dict__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__gt__',
 '__hash__',
 '__init__',
 '__init_subclass__',
 '__le__',
 '__lt__',
 '__module__',
 '__ne__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 '__weakref__',
 
 'name']

?

39 動(dòng)態(tài)獲取對(duì)象屬性　

獲取對(duì)象的屬性

?

In [1]: class Student():
   ...:     def __init__(self,id,name):
   ...:         self.id = id
   ...:         self.name = name
   ...:     def __repr__(self):
   ...:         return 'id = '+self.id +', name = '+self.name

In [2]: xiaoming = Student(id='001',name='xiaoming')
In [3]: getattr(xiaoming,'name') # 獲取xiaoming這個(gè)實(shí)例的name屬性值
Out[3]: 'xiaoming'

?

40 對(duì)象是否有這個(gè)屬性

?

In [1]: class Student():
   ...:     def __init__(self,id,name):
   ...:         self.id = id
   ...:         self.name = name
   ...:     def __repr__(self):
   ...:         return 'id = '+self.id +', name = '+self.name

In [2]: xiaoming = Student(id='001',name='xiaoming')
In [3]: hasattr(xiaoming,'name')
Out[3]: True

In [4]: hasattr(xiaoming,'address')
Out[4]: False

?

41 對(duì)象門牌號(hào)　

返回對(duì)象的內(nèi)存地址

?

In [1]: id(xiaoming)
Out[1]: 98234208

?

42 isinstance

判斷object是否為類classinfo的實(shí)例，是返回true

?

In [1]: class Student():
   ...:     def __init__(self,id,name):
   ...:         self.id = id
   ...:         self.name = name
   ...:     def __repr__(self):
   ...:         return 'id = '+self.id +', name = '+self.name

In [2]: xiaoming = Student(id='001',name='xiaoming')

In [3]: isinstance(xiaoming,Student)
Out[3]: True

?

43 父子關(guān)系鑒定

?

In [1]: class undergraduate(Student):
    ...:     def studyClass(self):
    ...:         pass
    ...:     def attendActivity(self):
    ...:         pass

In [2]: issubclass(undergraduate,Student)
Out[2]: True

In [3]: issubclass(object,Student)
Out[3]: False

In [4]: issubclass(Student,object)
Out[4]: True

In [1]: issubclass(int,(int,float))
Out[1]: True

?

44 所有對(duì)象之根

object 是所有類的基類

?

In [1]: o = object()

In [2]: type(o)
Out[2]: object

?

45 創(chuàng)建屬性的兩種方式

返回 property 屬性，典型的用法：

?

class C:
    def __init__(self):
        self._x = None

    def getx(self):
        return self._x

    def setx(self, value):
        self._x = value

    def delx(self):
        del self._x
    # 使用property類創(chuàng)建 property 屬性
    x = property(getx, setx, delx, "I'm the 'x' property.")

class C:
    def __init__(self):
        self._x = None

    @property
    def x(self):
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

    @x.deleter
    def x(self):
        del self._x

?

46 查看對(duì)象類型

class?type(name,?bases,?dict)

傳入一個(gè)參數(shù)時(shí)，返回?object?的類型：

?

In [1]: class Student():
   ...:     def __init__(self,id,name):
   ...:         self.id = id
   ...:         self.name = name
   ...:     def __repr__(self):
   ...:         return 'id = '+self.id +', name = '+self.name
   ...:

In [2]: xiaoming = Student(id='001',name='xiaoming')
In [3]: type(xiaoming)
Out[3]: __main__.Student

In [4]: type(tuple())
Out[4]: tuple

?

47 元類

xiaoming,?xiaohong,?xiaozhang?都是學(xué)生，這類群體叫做?Student.

Python 定義類的常見(jiàn)方法，使用關(guān)鍵字?class

?

In [36]: class Student(object):
    ...:     pass

xiaoming = Student()
xiaohong = Student()
xiaozhang = Student()

In [38]: xiaoming.__class__
Out[38]: __main__.Student

In [39]: xiaoming.__class__.__class__
Out[39]: type

?

那么，我們不妨猜測(cè)：Student?類，類型就是?type，換句話說(shuō)，Student類就是一個(gè)對(duì)象，它的類型就是?type，所以，Python 中一切皆對(duì)象，類也是對(duì)象

Python 中，將描述?Student?類的類被稱為：元類。

按照此邏輯延伸，描述元類的類被稱為：元元類，開玩笑了~ 描述元類的類也被稱為元類。

聰明的朋友會(huì)問(wèn)了，既然?Student?類可創(chuàng)建實(shí)例，那么?type?類可創(chuàng)建實(shí)例嗎？如果能，它創(chuàng)建的實(shí)例就叫：類 了。你們真聰明！

說(shuō)對(duì)了，type?類一定能創(chuàng)建實(shí)例，比如?Student?類了。

?

In [40]: Student = type('Student',(),{})

In [41]: Student
Out[41]: __main__.Student

?

Python 的類，因?yàn)橛质菍?duì)象，所以和?xiaoming，xiaohong?對(duì)象操作相似。支持：

賦值

拷貝

添加屬性

作為函數(shù)參數(shù)

?

In [43]: StudentMirror = Student # 類直接賦值 # 類直接賦值
In [44]: Student.class_property = 'class_property' # 添加類屬性
In [46]: hasattr(Student, 'class_property')
Out[46]: True

?

“元類就是深度的魔法，99%的用戶應(yīng)該根本不必為此操心。

六、工具

48 枚舉對(duì)象　　

返回一個(gè)可以枚舉的對(duì)象，該對(duì)象的next()方法將返回一個(gè)元組。

?

In [1]: s = ["a","b","c"]
    ...: for i ,v in enumerate(s,1):
    ...:     print(i,v)
    ...:
1 a
2 b
3 c

?

49 查看變量所占字節(jié)數(shù)

?

In [1]: import sys

In [2]: a = {'a':1,'b':2.0}

In [3]: sys.getsizeof(a) # 占用240個(gè)字節(jié)
Out[3]: 240

?

50 過(guò)濾器　　

在函數(shù)中設(shè)定過(guò)濾條件，迭代元素，保留返回值為True的元素：

?

In [1]: fil = filter(lambda x: x>10,[1,11,2,45,7,6,13])

In [2]: list(fil)
Out[2]: [11, 45, 13]

?

51 返回對(duì)象的哈希值　　

返回對(duì)象的哈希值，值得注意的是自定義的實(shí)例都是可哈希的，list,?dict,?set等可變對(duì)象都是不可哈希的(unhashable)

?

In [1]: hash(xiaoming)
Out[1]: 6139638

In [2]: hash([1,2,3])
# TypeError: unhashable type: 'list'

?

52 一鍵幫助　

返回對(duì)象的幫助文檔

?

In [1]: help(xiaoming)
Help on Student in module __main__ object:

class Student(builtins.object)
 |  Methods defined here:
 |
 |  __init__(self, id, name)
 |
 |  __repr__(self)
 |
 |  Data descriptors defined here:
 |
 |  __dict__
 |      dictionary for instance variables (if defined)
 |
 |  __weakref__
 |      list of weak references to the object (if defined)

?

53 獲取用戶輸入　

獲取用戶輸入內(nèi)容

?

In [1]: input()
aa
Out[1]: 'aa'

?

54 創(chuàng)建迭代器類型

使用iter(obj, sentinel), 返回一個(gè)可迭代對(duì)象, sentinel可省略(一旦迭代到此元素，立即終止)

?

In [1]: lst = [1,3,5]

In [2]: for i in iter(lst):
    ...:     print(i)
    ...:
1
3
5

In [1]: class TestIter(object):
    ...:     def __init__(self):
    ...:         self.l=[1,3,2,3,4,5]
    ...:         self.i=iter(self.l)
    ...:     def __call__(self):  #定義了__call__方法的類的實(shí)例是可調(diào)用的
    ...:         item = next(self.i)
    ...:         print ("__call__ is called,fowhich would return",item)
    ...:         return item
    ...:     def __iter__(self): #支持迭代協(xié)議(即定義有__iter__()函數(shù))
    ...:         print ("__iter__ is called!!")
    ...:         return iter(self.l)
In [2]: t = TestIter()
In [3]: t() # 因?yàn)閷?shí)現(xiàn)了__call__，所以t實(shí)例能被調(diào)用
__call__ is called,which would return 1
Out[3]: 1

In [4]: for e in TestIter(): # 因?yàn)閷?shí)現(xiàn)了__iter__方法，所以t能被迭代
    ...:     print(e)
    ...:
__iter__ is called!!
1
3
2
3
4
5

?

55 打開文件

返回文件對(duì)象

?

In [1]: fo = open('D:/a.txt',mode='r', encoding='utf-8')

In [2]: fo.read()
Out[2]: 'ufefflife is not so long,
I use Python to play.'

?

56 創(chuàng)建range序列

range(stop)

range(start, stop[,step])

生成一個(gè)不可變序列：

?

In [1]: range(11)
Out[1]: range(0, 11)

In [2]: range(0,11,1)
Out[2]: range(0, 11)

?

57 反向迭代器

?

In [1]: rev = reversed([1,4,2,3,1])

In [2]: for i in rev:
     ...:     print(i)
     ...:
1
3
2
4
1

?

58 聚合迭代器

創(chuàng)建一個(gè)聚合了來(lái)自每個(gè)可迭代對(duì)象中的元素的迭代器：

?

In [1]: x = [3,2,1]
In [2]: y = [4,5,6]
In [3]: list(zip(y,x))
Out[3]: [(4, 3), (5, 2), (6, 1)]

In [4]: a = range(5)
In [5]: b = list('abcde')
In [6]: b
Out[6]: ['a', 'b', 'c', 'd', 'e']
In [7]: [str(y) + str(x) for x,y in zip(a,b)]
Out[7]: ['a0', 'b1', 'c2', 'd3', 'e4']

?

59 鏈?zhǔn)讲僮?/p>

?

from operator import (add, sub)


def add_or_sub(a, b, oper):
    return (add if oper == '+' else sub)(a, b)


add_or_sub(1, 2, '-')  # -1

?

60 對(duì)象序列化

對(duì)象序列化，是指將內(nèi)存中的對(duì)象轉(zhuǎn)化為可存儲(chǔ)或傳輸?shù)倪^(guò)程。很多場(chǎng)景，直接一個(gè)類對(duì)象，傳輸不方便。

但是，當(dāng)對(duì)象序列化后，就會(huì)更加方便，因?yàn)榧s定俗成的，接口間的調(diào)用或者發(fā)起的 web 請(qǐng)求，一般使用 json 串傳輸。

實(shí)際使用中，一般對(duì)類對(duì)象序列化。先創(chuàng)建一個(gè) Student 類型，并創(chuàng)建兩個(gè)實(shí)例。

?

class Student():
    def __init__(self,**args):
        self.ids = args['ids']
        self.name = args['name']
        self.address = args['address']
xiaoming = Student(ids = 1,name = 'xiaoming',address = '北京')
xiaohong = Student(ids = 2,name = 'xiaohong',address = '南京')

import json

with open('json.txt', 'w') as f:
    json.dump([xiaoming,xiaohong], f, default=lambda obj: obj.__dict__, ensure_ascii=False, indent=2, sort_keys=True)

[
    {
        "address":"北京",
        "ids":1,
        "name":"xiaoming"
    },
    {
        "address":"南京",
        "ids":2,
        "name":"xiaohong"
    }
]

?

?