To find the length of a list use the built-in len() function which takes a single parameter that is iterable. For example this could be a string, list, tuple or dictionary and here is how it would look:

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>>> my_list = ['a', 'b', 'c']
>>> len(my_list)
3
>>> my_dict = {'a': 100, 'b': 200, 'c': 300}
>>> len(my_dict)
3
>>> my_str = 'How long is a piece of string?'
>>> len(my_str)
30

This may be the easiest and quickest way of being able to find the length of a list and other data types, but what if you want to exclude certain items from your list?

How do you find the length of a list without including certain types of elements, such as NoneType, or empty strings?

Count List Length Excluding Certain Elements

Suppose you have a list that contains empty elements, as None, in your list, like so:

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>>> my_list = ['a', None, 'c']

If you needed to know the length of this list excluding certain elements, such as the element listed as None, how could you do so? Using the built-in len() function would produce the following:

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>>> my_list = ['a', None, 'c']
>>> len(my_list)
3

To count the length of a list where certain elements are excluded would require a filter. By filtering a list using certain conditions and having that produce a new list you could then use the len on the new list.

In this example by using a list comprehension, which is just a way of looping through a list using one line, and applying an if condition on each element you can produce the desired list needed:

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>>> my_list = ['a', None, 'c']
>>> filtered_list = [x for x in my_list if x is not None]
>>> len(filtered_list)
2

Applying the same formula len() to the new list helps to achieve the solution when only counting the length of a list when excluding certain criteria.

You could also have wrapped the two lines into one if you wanted to get it all in one line:

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>>> my_list = ['a', None, 'c']
>>> len([x for x in my_list if x is not None]
2

Summary

To determine the length of a list in Python use the built-in len() function which not only counts the length of a string but also a list as well.

If required you can count the length of a list if you need to exclude certain elements from your original list by using list comprehensions.