Did you know that you can apply len python function on your classes? Let's say you have a Lessons class and when a use use len() function on your class it should return number of lessons inside of your class. For our sample let's say we have the below class:

class Lessons(object):
     def __init__(self, lessons=0):
         self.lessons = lessons

     def insert(self, data):
         # let's assume we have save data in database
         self.lessons += 1

     def __len__(self):
         return self.lessons

This is a simple class that sets a fake lesson in your class and insert a new lesson.

std_lessons = Lessons(1)
 print len(std_lessons) # output: 1

 std_lessons.insert({})
 print len(std_lessons) # output: 2

As you can see above we have used len on our class itself. In case you don't implemet __len__ on your class and use len() on it, there will be an AttributeError:

len(std_no_len)
 Traceback (most recent call last):
   File "<input>", line 1, in <module>
 AttributeError: Lessons instance has no attribute '__len__'

__len__ is built-in class and can be implemented in different scenarios. There are tons of built-in functions that you can master and use to improve code quality and prove that you are professional.

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https://spyhce.com/blog/understanding-new-and-init

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