[Tut] How to Remove Empty Lists from a List of Lists in Python?

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How to Remove Empty Lists from a List of Lists in Python?

Short answer: You can remove all empty lists from a list of lists by using the list comprehension statement

Code:

[x for x in list if x]

to filter the list.



In the following, you’ll learn about the two methods using list comprehension and the filter() function to remove all empty lists from a list of lists.

But before that, feel free to play with the code yourself:

https://trinket.io/embed/python/659a4282bd

Method 1: List Comprehension

How can you remove all empty lists from a list of lists? Say, you’ve got a list of lists

Code:

[[1, 2, 3], [1, 2], [], [], [], [1, 2, 3, 4], [], []]

and you want all empty lists removed to obtain the list of lists

Code:

[[1, 2, 3], [1, 2], [1, 2, 3, 4]]

.

Solution: Use list comprehension

Code:

[x for x in list if x]

to filter the list and remove all lists that are empty.

Code:

lst = [[1, 2, 3], [1, 2], [], [], [], [1, 2, 3, 4], [], []] print([x for x in lst if x]) # [[1, 2, 3], [1, 2], [1, 2, 3, 4]]

The condition if

Code:

x

evaluates to

Code:

False

only if the list

Code:

x

is empty. In all other cases, it evaluates to

Code:

True

and the element is included in the new list.

You can visualize the execution flow here by clicking the “Next” button:

https://pythontutor.com/iframe-embed.htm...nces=false

Method 2: filter()

An alternative is to use the

Code:

filter()

function to remove all empty lists from a list of lists:

Code:

lst = [[1, 2, 3], [1, 2], [], [], [], [1, 2, 3, 4], [], []] print(list(filter(lambda x: x, lst)))

The filter() function takes two arguments:

• the filter decision function to check for each element whether it should be included in the filtered iterable (it returns a Boolean value), and
  
  
• the iterable to be filtered.
  

As filter decision function, you use the identity function that just passes the list through. Why does this work? Because only an empty list will be evaluated to

Code:

False

. All other lists will be evaluated to

Code:

True

(and, thus, pass the filtering test).

Related articles:

• How to filter a list in Python?
  
  
• Filter vs List Comprehension (Speed)
  
  
• List Comprehension
  
  
• Lists of Lists
  

Where to Go From Here?

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