Merging Lists in Python: A Guide

In Python, a list is a data type that contains an ordered sequence of objects and is written as a series of comma-separated values between square brackets. Merging lists can be done in many ways in Python. 

Let’s explore the different methods in detail.

How to Merge a List in Python

1. Append

• The append() method adds an entire object (like a list) as a single item to the end of the list.
• The length of the list will be increased by one.
• It will update the original list itself.
• Return value is None.

num1=[1,2,3]
num2=[4,5,6]
num1.append(num2)
print (num1)
#Output:[1, 2, 3, [4, 5, 6]]

2. Extend

• The extend method will extend the list by appending all the items from the iterable. In Python, an iterable is any object that can be passed to a for loop or used with functions that expect a sequence of values — such as list(), tuple() or sorted().
• The length of the list will increase depending on the length of the iterable.
• It will update the original list itself.
• Return type is none.

num1=[1,2,3]
num2=[4,5,6]
num1.extend(num2)
print (num1)
#Output:[1, 2, 3, 4, 5, 6]

3. Concatenation

• Python lists also support concatenation operations.
• We can add two or more lists using the + operator.
• It won’t update the original list.
• Return type is a new list.

Concatenating Two Lists Example

num1=[1,2,3]
num2=[4,5,6]
num3=num1+num2
print (num3)
#Output:[1, 2, 3, 4, 5, 6]

Concatenating Two or More Lists Example

num1=[1,2,3]
num2=[4,5,6]
num3=[7,8]
num4=[9,10]
num5=num1+num2+num3+num4
print (num5)
#Output:[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

4. Unpacking With Asterisk (*)

An asterisk * denotes iterable unpacking. Its operand must be iterable. The iterable is expanded into a sequence of items, which are included in the new tuple, list or set, at the site of the unpacking.

list1=[*list2,*list3]

It unpacks elements from the original lists into a new list.

num1=[1,2,3]
num2=[4,5,6]
num3=[*num1,*num2]
print (num3)
#Output:[1, 2, 3, 4, 5, 6]

5. Itertools.chain

Itertools.chain is used for creating an iterator from two or more iterables:  itertools.chain(*iterables). It will accept iterables as an argument. It returns each item from the first iterable then proceeds to the next, until there are no more iterables left and no more elements to return.

import itertools
num1=itertools.chain([1,2,3],[4,5,6])
#Returns an iterator object
print (num1)#Output:<itertools.chain object at 0x029FE4D8>
#converting iterator object to list object
print(list(num1))#Output:[1, 2, 3, 4, 5, 6]

6. List Comprehension

List comprehension is used to create a new list based on existing iterables. It’s used to write complex functionality in a single line of code. It creates a new list based on the existing values from the iterables.

Syntax for List Comprehension

new_list= [expression for element in iterable if condition]

Joining Two lists Using List Comprehension 

num1=[1,2,3]
num2=[4,5,6]
num3=[x for n in (num1,num2) for x in n]
print (num3)#Output:[1, 2, 3, 4, 5, 6]

7. For Loop

• The first for loop goes through the lists (for n in (num1,num2).
• The second for loop goes through the elements in the list (for x in n)
• Then, it will append each element to the empty list created before num3.

num1=[1,2,3]
num2=[4,5,6]
num3=[]
for n in (num1,num2):
   for x in n:
       num3.append(x)
print (num3)
#Output:[1, 2, 3, 4, 5, 6]

More on PythonAn Introduction to Python Linked List and How to Create One

How to Join All Strings in a List

str.join(iterable)

I will join all the elements in the iterable and return a string. We can mention the separator to join the elements in the iterable.

Example 1

In this first example, we’re going to join all strings in the list when the separator is given as space.

num1=["Welcome", "to", "python", "programming", "language"]
num2=" ".join(num1)
print (num2)
#Output:Welcome to python programming language

Example 2

In the second example, we’re going to join all strings in the list when the separator is given as -.

num1=["1","2","3"]
num2="-".join(num1)
print (num2)
#Output:1-2-3

How to Remove Duplicate Elements While Merging Two Lists

Python sets don’t contain duplicate elements. To remove duplicate elements from lists we can convert the list to set using set() and then convert back to list using list() constructor.

Note: Converting to a set removes duplicates but also loses original order. Use dict.fromkeys() or collections.OrderedDict if order matters.

num1=[1,2,3,4,5]
num2=[1,3,5,7,9]
num3=list(set(num1+num2))
print (num3)
#Output:[1, 2, 3, 4, 5, 7, 9]

More on Python5 Ways to Remove Characters From a String in Python

Speed Comparison of Ways to Merge Lists in Python

time.time() → float

Return the time in seconds since the epoch as a floating-point number.

1. Append Method Merge List Time 

import time
num1=list(range(1,1000000))
num2=list(range(1000000,2000000))
start=time.time()
num1.append(num2)
print (time.time()-start)
#Output:0.0019817352294921875

2. Extend Method Merge List Time

import time
num1=list(range(1,1000000))
num2=list(range(1000000,2000000))
start1=time.time()
num1.extend(num2)
print (time.time()-start1)
#Output:0.00701141357421875

3. Concatenation Operator Merge List Time

import time
num1=list(range(1,1000000))
num2=list(range(1000000,2000000))
start2=time.time()
num1+num2
print (time.time()-start2)
#Output:0.014112710952758789

4. Unpacking Method Merge List Time

import time
num1=list(range(1,1000000))
num2=list(range(1000000,2000000))
start3=time.time()
[*num1,*num2]
print (time.time()-start3)
#Output:0.020873069763183594

5. itertools.chain() Merge List Time

import itertools
import time
num1=list(range(1,1000000))
num2=list(range(1000000,2000000))
start4=time.time()
num3=itertools.chain(num1,num2)
list(num3)
print (time.time()-start4)
#Output:0.045874595642089844

6. List Comprehension Merge List Time

import time
num1=list(range(1,1000000))
num2=list(range(1000000,2000000))
start5=time.time()
num3=[x for n in (num1,num2) for x in n]
print (time.time()-start5)
#Output:0.06680965423583984

7. For Loop Merge List Time

import time
num1=list(range(1,1000000))
num2=list(range(1000000,2000000))
start5=time.time()
num3=[]
for n in (num1,num2):
   for x in n:
       num3.append(x)
print (time.time()-start5)
#Output:0.23975229263305664

Merge List Method Time Comparisons

Note: Although append() may be faster than other merge list methods, it doesn’t flatten or merge lists the way the other methods do.

More on PythonPython Tuples vs. Lists: When to Use Tuples Instead of Lists

Merging Lists in Python Tips

• The append method will add the list as one element to another list. The length of the list will be increased by one only after appending one list.
• The extend method will extend the list by appending all the items from iterable (another list). The length of the list will increase depending on the length of the iterable.
• Both the append and extend method will modify the original list.
• Concatenation, unpacking and list comprehension returns a new list object. It won’t modify the original list.
• With itertools.chain(), the return type will be itertools.chain object. We can convert to a list using the list() constructor.

Frequently Asked Questions