Python day 9 (3) custom class, pythonday

Python day 9 (3) custom class, pythonday

1: __str _ (return user-friendly output)

 1 >>> class Student(object): 2 ...     def __init__(self, name): 3 ...         self.name = name 4 ... 5 >>> print(Student('Michael')) 6 <__main__.Student object at 0x109afb190> 7 >>> class Student(object): 8 ...     def __init__(self, name): 9 ...         self.name = name10 ...     def __str__(self):11 ...         return 'Student object (name: %s)' % self.name12 ...13 >>> print(Student('Michael'))14 Student object (name: Michael)

 

__str__()Returns the string that the user sees,__repr__()Returns the string that the developer sees.
When Student ('Michael ') is called directly instead of print

<__main__.Student object at 0x109afb190>
In this case, you need to define another_ Repr _ (), and_ Repr _ = _ str __

2: __iter _ (return an iteration object)
If a class is usedfor ... inLoop, similar to list or tuple, you must implement__iter__()Method. This method returns an iteration object. Then, the Python for loop will continuously call__next__()Method to get the next value of the loopStopIterationExit the loop when an error occurs.

1 class Fib (object): 2 def _ init _ (self): 3 self. a, self. B = 0, 1 # initialize two counters a, B 4 5 def _ iter _ (self): 6 return self # The instance itself is an iteration object, therefore, def _ next _ (self): 9 self is returned. a, self. B = self. b, self. a + self. B # Calculate the next value 10 if self. a> 100000: # condition 11 raise StopIteration () 12 return self. a # return the next value 13 14 >>> for n in Fib (): 15... print (n) 16... 17 118 119 220 321 522... 23 4636824 75025

 

3: __getitem _ (for instance search/slice, etc)
The following example shows an _ getitem _ Implementation of index calculation.

 1 class Fib(object): 2     def __getitem__(self, n)： 3         a, b = 1, 1 4         for x in range(n): 5             a, b = b, a + b 6         return a 7  8 >>> f = Fib() 9 >>> f[0]10 111 >>> f[1]12 113 >>> f[2]14 215 >>> f[3]16 317 >>> f[10]18 89

 

IV: __getattr __
Under normal circumstances, if the method or attribute of the class does not exist, an error is returned. To avoid this error, you can write__getattr__()Method to dynamically return an attribute.

1 class Student(object):2 3     def __init__(self):4         self.name = 'Michael'5 6     def __getattr__(self, attr):7         if attr=='score':8             return 99

 

When a non-existent property is called, for examplescore, The Python interpreter will try to call__getattr__(self, 'score')In this way, we have the opportunity to returnscoreValue:

1 >>> s = Student()2 >>> s.name3 'Michael'4 >>> s.score5 99

 

Of course, you can also return functions.

1 class Student(object):2 3     def __getattr__(self, attr):4         if attr=='age':5             return lambda: 25

 

The call method must be changed:

>>> s.age()25

This API is called only when no attribute is found.__getattr__, Existing attributes, suchname, Not in__getattr__.

Any call, suchs.abcWill returnNoneThis is because we have defined__getattr__The default return value isNone. To make the class only respond to specific attributes, we need to throwAttributeErrorError:

1 class Student(object):2 3     def __getattr__(self, attr):4         if attr=='age':5             return lambda: 256         raise AttributeError('\'Student\' object has no attribute \'%s\'' % attr)

In fact, all attributes and method calls of a class can be dynamically processed without any special means. Purpose: Write the SDK.

 

5: __ call __

An object instance can have its own attributes and Methods. When we call an instance method, we useinstance.method(). However, define__call__()Method, you can directly call the instance.

1 class Student(object):2     def __init__(self, name):3         self.name = name4 5     def __call__(self):6         print('My name is %s.' % self.name)

 

The call method is as follows:

1 >>> s = Student ('Michael ') 2 >>> s () # Do not input 3 My name is Michael in the self parameter.

 

The Callable function can determine whether an object can be called.

 1 >>> callable(Student()) 2 True 3 >>> callable(max) 4 True 5 >>> callable([1, 2, 3]) 6 False 7 >>> callable(None) 8 False 9 >>> callable('str')10 False

 

Pytho also has a lot of customization class, link: http://docs.python.org/3/reference/datamodel.html#special-method-names