Python suggests users use len() instead of __len__() for consistency, just like other guys said. However, There're some other benefits:
For some built-in types like list, str, bytearray and so on, the Cython implementation of len() takes a shortcut. It directly returns the ob_size in a C structure, which is faster than calling __len__().
If you are interested in such details, you could read the book called "Fluent Python" by Luciano Ramalho. There're many interesting details in it, and may help you understand Python more deeply.
Python uses duck typing: it doesn't care about what an object is, as long as it has the appropriate interface for the situation at hand. When you call the built-in function len() on an object, you are actually calling its internal __len__ method. A custom object can implement this interface and len() will return the answer, even if the object is not conceptually a sequence.
For a complete list of interfaces, have a look here: http://docs.python.org/reference/datamodel.html#basic-customization
The way you take a length of anything for which that makes sense (a list, dictionary, tuple, string, ...) is to call len on it.
l = [1,2,3,4]
s = 'abcde'
len(l) #returns 4
len(s) #returns 5
The reason for the "strange" syntax is that internally python translates len(object) into object.__len__(). This applies to any object. So, if you are defining some class and it makes sense for it to have a length, just define a __len__() method on it and then one can call len on those instances.
The preferred way to get the length of any python object is to pass it as an argument to the len function. Internally, python will then try to call the special __len__ method of the object that was passed.
you can use len(arr)
as suggested in previous answers to get the length of the array. In case you want the dimensions of a 2D array you could use arr.shape returns height and width
len(list_name) function takes list as a parameter and it calls list's __len__() function.
Just use len(arr):
>>> import array
>>> arr = array.array('i')
>>> arr.append('2')
>>> arr.__len__()
1
>>> len(arr)
1
Source: Stackoverflow.com