How to use the __len__() function to define the length of an object in Python

How to use the __len__() function in Python to define the length of an object

In Python, we can usually get the length of an object through the built-in len() function . However, for custom objects, the len() function may not necessarily return the length of the object correctly. To solve this problem, Python provides the special method __len__(). We can define the length of the object by implementing this method in a custom class.

__len__() method is a special method used to return the length of the object. It accepts a parameter self, which represents an instance object of the class. By defining this method in the class, we can let the len() function correctly obtain the length of the object.

The following is a sample code:

class MyClass:

def __init__(self, data):
    self.data = data

def __len__(self):
    return len(self.data)

obj = MyClass([1, 2, 3, 4, 5])
print(len (obj))

In the above example code, we define a MyClass class, which contains an attribute data. We define the length of the object by implementing the __len__() method in the class. In the __len__() method, we use the len() function to get the length of self.data and use it as the return value.

Then, we created an instance object obj of the MyClass class and passed a list containing 5 elements as a parameter to the constructor of this object. Finally, we use the len() function to get the length of the obj object and print the result.

Run the above code, the output result is 5. This shows that we successfully used the __len__() method to define the length of the obj object.

In addition to the len() function, some other built-in functions and methods can also use the __len__() method to get the length of the object. For example, we can use the sum() function to calculate the sum of the elements in an object, and we will also use the __len__() method.

The following is a sample code:

class MyList:

def __init__(self, data):
    self.data = data

def __len__(self):
    return len(self.data)

def __getitem__(self, index):
    return self.data[index]

def __setitem__(self, index, value):
    self.data[index] = value

def __delitem__(self, index):
    del self.data[index]

def __iter__(self):
    return iter(self.data)

my_list = MyList([1, 2, 3, 4, 5])
print(len (my_list))
print(sum(my_list))

In the above example code, we define a MyList class, which contains an attribute data. In addition to implementing the __len__() method, we also implement methods such as __getitem__(), __setitem__(), __delitem__() and __iter__(), so that objects can be indexed, assigned, deleted and iterated like a list .

In the __len__() method, we also use the len() function to obtain the length of self.data.

Then, we created an instance object my_list of the MyList class and passed a list containing 5 elements as a parameter to the constructor of this object. Finally, we use the len() function and sum() function to obtain the length and sum of the my_list object, and print the results.

Run the above code, the output results are 5 and 15. This shows that we have successfully used the __len__() method to define the length of the my_list object, and can also correctly obtain the length of the object when using other built-in functions and methods.

To summarize, by implementing the __len__() method, we can correctly define the length of the object in the custom class. In this way, not only can the len() function correctly obtain the length of the object, but also some other built-in functions and methods can also correctly obtain the length of the object when used. At the same time, you can also customize the length definition method of the object according to your own needs, making the length of the object flexible and diverse.

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