How to remove an element from a list by index in Python?

You probably want pop:

a = ['a', 'b', 'c', 'd']
a.pop(1)

# now a is ['a', 'c', 'd']

By default, pop without any arguments removes the last item:

a = ['a', 'b', 'c', 'd']
a.pop()

# now a is ['a', 'b', 'c']

Like others mentioned pop and del are the efficient ways to remove an item of given index. Yet just for the sake of completion ( since the same thing can be done via many ways in python ):

Using slices ( This does not do inplace removal of item from original list ) :

( Also this will be the least efficient method when working with python list but this could be useful ( but not efficient, I reiterate ) when working with user defined objects that do not support pop, yet do define a __getitem__ ):

>>> a = [  1, 2, 3, 4, 5, 6 ]
>>> index = 3 # Only Positive index

>>> a = a[:index] + a[index+1 :]
# a is now [ 1, 2, 3, 5, 6 ]

Note: Please note that this method does not modify the list inplace like pop and del. It instead makes two copies of lists ( one from the start until the index but without it ( a[:index] ) and one after the index till the last element ( a[index+1:] ) ) and creates a new list object by adding both. This is then reassigned to the list variable ( a ). The old list object is hence dereferenced and hence garbage collected ( provided the original list object is not referenced by any variable other than a )

This makes this method very inefficient and it can also produce undesirable side effects ( especially when other variables point to the original list object which remains un-modified )

Thanks to @MarkDickinson for pointing this out ...

This Stack Overflow answer explains the concept of slicing.

Also note that this works only with positive indices.

While using with objects, the __getitem__ method must have been defined and more importantly the __add__ method must have been defined to return an object containing items from both the operands.

In essence this works with any object whose class definition is like :

class foo(object):
    def __init__(self, items):
        self.items = items

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

    def __add__(self, right):
        return foo( self.items + right.items )

This works with list which defines __getitem__ and __add__ methods.

Comparison of the three ways in terms of efficiency:

Assume the following is predefined :

a = range(10)
index = 3

The del object[index] method:

By far the most efficient method. Works will all objects that define a __del__ method.

The disassembly is as follows :

Code:

def del_method():
    global a
    global index
    del a[index]

Disassembly:

 10           0 LOAD_GLOBAL              0 (a)
              3 LOAD_GLOBAL              1 (index)
              6 DELETE_SUBSCR       # This is the line that deletes the item
              7 LOAD_CONST               0 (None)
             10 RETURN_VALUE        
None

pop method:

Less efficient than the del method. Used when you need to get the deleted item.

Code:

def pop_method():
    global a
    global index
    a.pop(index)

Disassembly:

 17           0 LOAD_GLOBAL              0 (a)
              3 LOAD_ATTR                1 (pop)
              6 LOAD_GLOBAL              2 (index)
              9 CALL_FUNCTION            1
             12 POP_TOP             
             13 LOAD_CONST               0 (None)
             16 RETURN_VALUE        

The slice and add method.

The least efficient.

Code:

def slice_method():
    global a
    global index
    a = a[:index] + a[index+1:]

Disassembly:

 24           0 LOAD_GLOBAL              0 (a)
              3 LOAD_GLOBAL              1 (index)
              6 SLICE+2             
              7 LOAD_GLOBAL              0 (a)
             10 LOAD_GLOBAL              1 (index)
             13 LOAD_CONST               1 (1)
             16 BINARY_ADD          
             17 SLICE+1             
             18 BINARY_ADD          
             19 STORE_GLOBAL             0 (a)
             22 LOAD_CONST               0 (None)
             25 RETURN_VALUE        
None

Note : In all three disassembles ignore the last 2 lines which basically are return None Also the first 2 lines are loading the global values a and index.

pop is also useful to remove and keep an item from a list. Where del actually trashes the item.

>>> x = [1, 2, 3, 4]

>>> p = x.pop(1)
>>> p
    2

Generally, I am using the following method:

>>> myList = [10,20,30,40,50]
>>> rmovIndxNo = 3
>>> del myList[rmovIndxNo]
>>> myList
[10, 20, 30, 50]

As previously mentioned, best practice is del(); or pop() if you need to know the value.

An alternate solution is to re-stack only those elements you want:

    a = ['a', 'b', 'c', 'd'] 

    def remove_element(list_,index_):
        clipboard = []
        for i in range(len(list_)):
            if i is not index_:
                clipboard.append(list_[i])
        return clipboard

    print(remove_element(a,2))

    >> ['a', 'b', 'd']

eta: hmm... will not work on negative index values, will ponder and update

I suppose

if index_<0:index_=len(list_)+index_

would patch it... but suddenly this idea seems very brittle. Interesting thought experiment though. Seems there should be a 'proper' way to do this with append() / list comprehension.

pondering

One can either use del or pop, but I prefer del, since you can specify index and slices, giving the user more control over the data.

You can use either del or pop to remove element from list based on index. Pop will print member it is removing from list, while list delete that member without printing it.

>>> a=[1,2,3,4,5]
>>> del a[1]
>>> a
[1, 3, 4, 5]
>>> a.pop(1)
 3
>>> a
[1, 4, 5]
>>> 

It doesn't sound like you're working with a list of lists, so I'll keep this short. You want to use pop since it will remove elements not elements that are lists, you should use del for that. To call the last element in python it's "-1"

>>> test = ['item1', 'item2']
>>> test.pop(-1)
'item2'
>>> test
['item1']