Essential Python 3 code for lists

Essential Python 3 code for lists

Python 3 examples for all of the things I listed in my previous article. There are variations on much of this syntax, but so long as you know one way, you'll be fine. I tried to find the Pythonic way in most cases. You can use this as a cheat sheet.


# Create a list
mylist = []

Add an element to the front or back

mylist.append( "abc" ) mylist.insert( 0, "def" )

Pop element off front or back

end = mylist.pop() start = mylist.pop( 0 )

Forward iterate over elements

for item in mylist: print( item )

Get the length of list

len( mylist )

Test if empty

if not mylist: print( "list is empty" )

Location Based Operations

# Get item at location

Insert an item at location

mylist.insert( 3, "abc" )

Remove an item from location

del mylist[2]

Replace/Assign item at location

mylist[1] = "def"

Sorting and searching

# Find an item
if item in mylist:
    index = mylist.index(item)

Using index and error handling

try: index = mylist.index( 'abc' ) except ValueError: index = None

Using next and filtering

next((x for x in mylist if x == 'ghif'), None)

Find and remove an item

if item in mylist: mylist.remove( item )

with error handling

try: mylist.remove( item ) except ValueError: pass

Find last matching item

Index of found item, or None

next( (index for index in reversed( range( len( mylist ) ) ) if mylist[index] == item), None)

Alternately, reverse list and use "Find an item", but that copies the list

revlist = mylist[::-1] if item in revlist: index = revlist.index( item )

Sort by natural order

in-place sort


Sort with custom comparator

mylist = [ ('a', 10), ('b', 7), ('c',13), ('d',1) ]

sort by a key (sub-element

mylist.sort( key = lambda item: item[1] )

custom comparator

def compare_fn( a, b ): return some_cond( a, b ) mylist.sort( key = functools.cmp_to_key( compare_fn ) )

Segment Manipulation

# Split the list at arbitrary location
tail_of_list = mylist[2:]
head_of_list = mylist[:2]

Multiple splits based on a match

mylist = ['a', 'b', 'c', 'd', 'b', 'e'] [list(y) for x, y in itertools.groupby( mylist, lambda z: z == 'b') if not x]

Clear the list


Remove segment

delete from position 1 up to, but excluding position 3

del mylist[1:3]

Concatenate lists

mylist + other_list

Insert list at location

list slicing replaces the segment of list with another one, here we replace a zero-length slice

mylist[1:1] = other_list

Get a sublist

sublist starting at position 1 up to, but excluding, position 3


More Iteration

# Backward
for item in reversed( mylist ):
    print( item )

Partial segment iteration

using itertools.islice avoids copying the list (which is what would happen if you used a slice)

for item in itertools.islice( mylist, 1, 4 ): print( item )

Skipping elements

step from element 1 to 6 (exclusive) by 2

for item in itertools.islice( mylist, 1, 6, 2 ): print( item )


# Create from a static list of items
mylist = [ 'abc', 'def', 'ghi']

Create a range of numbers

a list of numbers from 10..20 (exclusive)

numbers = list( range( 10, 20 ) )

Data Manipulation

# Mapping
[number * 10 for number in numbers]


[number for number in numbers if number % 2 == 0]

Fold / Reduce

Summing up numbers using builtin add

functools.reduce( operator.add, numbers )

Joining string representations of items

functools.reduce( lambda left,right: str( left ) + '/' + str( right ), mylist )


the zip function produces a list of tuples

zip( lista, listb )

to alternate items into one list use reduce

functools.reduce( operator.add, zip( lista, listb ) )


# Swap elements at two locations
mylist[3], mylist[5] = mylist[5], mylist[3]

Reserve capacity

Python lists do not expose capacity

Replace content in a list

mylist[:] = other_list

Compare two lists

lista == listb

Search a sorted list

bisect_left/bisect_right work with sorted lists,

find an item ndx using bisect_left, finds the left-most item

ndx = bisect_left( numbers, 4 ) if ndx != len(numbers) and numbers[ndx] == 4 print( "Found at {}".format(ndx) )


Manually stepping through an iterator

myiter = iter( mylist ) while True: try: n = next( myiter ) print(n) except StopIteration: break

Multiple iterators at the same time

itera = iter( lista ) iterb = iter( listb ) while True: try: a = next( itera ) b = next( iterb ) print( a, b ) except StopIteration: break

Do you want to learn more ways to become a great programmer? Read my book What is Programming?. I look at the People, the reason software exists, the code at the heart of that software, and you, the person behind the keyboard.

Originally published by edA‑qa mort‑ora‑y at

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