How to PROPERLY Join Lists in Python [10 Methods]

Bashir Alam


What Does "Joining Lists" Mean in Python?

Joining lists in Python refers to the process of combining two or more lists into a single list. This can be achieved in various ways, each with its own use-cases and performance implications.


Different Methods to Join Lists in Python

  • String .join() Method: Combines a list of strings into a single string with optional separators.
  • + Operator: Concatenates two or more lists.
  • extend() Method: Appends the elements of one list to the end of another.
  • * Operator: Repeats and joins the same list multiple times.
  • itertools.chain(): Efficiently joins multiple iterable objects including lists.
  • List Comprehension: Allows for complex conditional joins.
  • map() Function: Useful for joining lists containing different data types.
  • Nested Loop Method: Manually iterating through nested lists to join them.
  • NumPy concatenate() Function: Effective for numerical lists (arrays).
  • Pandas concat() Function: For more complex list-like structures like dataframes.


Individual Methods to Join Lists in Python

1. String .join() Method

The join() method is used to concatenate a list of strings into a single string, separated by a delimiter.

delimiter = ', '
list_of_strings = ['apple', 'banana', 'cherry']
result = delimiter.join(list_of_strings)
# Output: 'apple, banana, cherry'

2. + Operator

The + operator allows you to concatenate two or more lists together.

list1 = [1, 2, 3]
list2 = [4, 5, 6]
result = list1 + list2
# Output: [1, 2, 3, 4, 5, 6]

3. extend() Method

The extend() method appends the elements of one list to the end of another.

list1 = [1, 2, 3]
list2 = [4, 5, 6]
# Output: list1 becomes [1, 2, 3, 4, 5, 6]

4. * Operator

The * operator can be used to repeat a list a given number of times and join them together.

list1 = [1, 2, 3]
result = list1 * 3
# Output: [1, 2, 3, 1, 2, 3, 1, 2, 3]

5. itertools.chain()

The itertools.chain() function is efficient for joining multiple iterable objects, including lists.

import itertools

list1 = [1, 2, 3]
list2 = [4, 5, 6]
result = list(itertools.chain(list1, list2))
# Output: [1, 2, 3, 4, 5, 6]

6. List Comprehension

List comprehension allows you to create new lists based on existing ones with conditions.

list1 = [1, 2, 3]
list2 = [4, 5, 6]
result = [x for pair in zip(list1, list2) for x in pair]
# Output: [1, 4, 2, 5, 3, 6]

7. map() Function

The map() function can be used when the lists contain different types of data and you want to join them.

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']
result = list(map(str, list1)) + list2
# Output: ['1', '2', '3', 'a', 'b', 'c']

8. Nested Loop Method

For more complex cases, you can use nested loops to join lists manually.

list1 = [[1, 2], [3, 4]]
list2 = []
for sublist in list1:
    for item in sublist:
# Output: list2 becomes [1, 2, 3, 4]

9. NumPy concatenate() Function

If you are working with numerical data, NumPy's concatenate() is very efficient.

import numpy as np

array1 = np.array([1, 2, 3])
array2 = np.array([4, 5, 6])
result = np.concatenate((array1, array2))
# Output: array([1, 2, 3, 4, 5, 6])

10. Pandas concat() Function

For more structured data like dataframes, you can use Pandas' concat() function.

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2], 'B': [3, 4]})
df2 = pd.DataFrame({'A': [5, 6], 'B': [7, 8]})
result = pd.concat([df1, df2])
# Output: DataFrame with values from both df1 and df2


Joining Nested Lists

Joining nested lists involves converting a list of lists into a single, flat list.


Flattening and Joining

Flattening is the process of converting a nested list into a flat list. You can flatten and join nested lists using different techniques.

Using List Comprehension

List comprehension is an effective way to flatten a list.

nested_list = [[1, 2], [3, 4], [5, 6]]
flat_list = [item for sublist in nested_list for item in sublist]
# Output: [1, 2, 3, 4, 5, 6]

Using extend() Method

Another common approach is to use the extend() method.

nested_list = [[1, 2], [3, 4], [5, 6]]
flat_list = []
for sublist in nested_list:
# Output: [1, 2, 3, 4, 5, 6]

Using Nested Loops

For complex scenarios or multi-level nested lists, you can use nested loops.

nested_list = [[1, 2], [3, 4], [5, 6]]
flat_list = []
for sublist in nested_list:
    for item in sublist:
# Output: [1, 2, 3, 4, 5, 6]


Using itertools.chain()

The itertools.chain() method is efficient for joining multiple iterables and also works well for flattening a list of lists.

import itertools

nested_list = [[1, 2], [3, 4], [5, 6]]
flat_list = list(itertools.chain.from_iterable(nested_list))
# Output: [1, 2, 3, 4, 5, 6]

The itertools.chain.from_iterable() function takes an iterable containing iterables as input, and returns an iterator that produces items from these iterables, effectively flattening the structure.


Joining Lists of Different Data Types

Joining lists containing different data types often requires converting non-string elements to strings. You can accomplish this in several ways.

Typecasting to String Before Joining

The simplest way is to use a list comprehension to convert each element to a string before joining.

my_list = [1, 'apple', True, 3.14]
str_list = [str(item) for item in my_list]
joined_list = ' '.join(str_list)
# Output: "1 apple True 3.14"

Using map() Function

The map() function is another efficient way to convert each element to a string before joining them. This is particularly useful for large lists, as map() is generally faster.

my_list = [1, 'apple', True, 3.14]
joined_list = ' '.join(map(str, my_list))
# Output: "1 apple True 3.14"

By using map(str, my_list), we convert each element of the list to a string. Then we can easily join them.


Joining Lists with a Separator

Sometimes you may want to join a list of strings using a specific separator, like a comma, space, or newline. Python provides a simple and efficient way to do this using the .join() method or alternative approaches for non-string lists.

Custom Separators with .join()

The .join() method allows you to specify any string as a separator. This string will be inserted between each element of the list when joining.

# Using a comma and a space as separators
my_list = ['apple', 'banana', 'cherry']
result = ', '.join(my_list)
# Output: "apple, banana, cherry"

# Using a newline as a separator
result = '\n'.join(my_list)
# Output: "apple\nbanana\ncherry"

Using str.join() with Non-String Lists

If you try to use .join() on a list that contains non-string types, Python will throw a TypeError. To avoid this, you'll have to convert the list elements to strings first. You can either use a list comprehension or map() for this, as mentioned in the previous section.

List comprehension:

my_list = [1, 2, 3]
str_list = [str(x) for x in my_list]
result = '->'.join(str_list)
# Output: "1->2->3"

Using map():

my_list = [1, 2, 3]
result = '->'.join(map(str, my_list))
# Output: "1->2->3"


Joining Lists with Conditions

At times, you may need to join lists based on certain conditions, such as only including elements that meet specific criteria. Python offers multiple ways to accomplish this, including list comprehensions and the filter() function.

Using List Comprehensions

List comprehensions provide a concise way to filter elements in a list based on conditions. Here's how you can join a list of numbers where only the even numbers are included:

my_list = [1, 2, 3, 4, 5]
even_list = [str(x) for x in my_list if x % 2 == 0]
result = ','.join(even_list)
# Output: "2,4"

Or you can use list comprehensions to join strings that meet a certain length:

my_list = ['apple', 'banana', 'cherry']
filtered_list = [x for x in my_list if len(x) > 5]
result = ','.join(filtered_list)
# Output: "banana,cherry"

Using filter()

The filter() function can also be used to join lists based on conditions. Unlike list comprehensions, filter() is often more readable when dealing with complex conditions. Here's how to use filter() to join only the even numbers from a list:

my_list = [1, 2, 3, 4, 5]
even_list = filter(lambda x: x % 2 == 0, my_list)
result = ','.join(map(str, even_list))
# Output: "2,4"


Performance Considerations

When dealing with large lists or time-sensitive applications, performance is a key factor to consider. Different methods for joining lists can have varying impacts on performance. Below, we examine the speed and time complexity of some common methods.

Speed Comparison Between Methods

To demonstrate the speed of each method, let's use Python's built-in timeit library to measure the execution time.

import timeit

# Using `+` operator
def using_plus_operator():
    my_list = ['a'] * 1000
    result = ''
    for item in my_list:
        result += item

time_for_plus = timeit.timeit(using_plus_operator, number=10000)
print(f"Time taken using `+` operator: {time_for_plus}")

# Using .join()
def using_join():
    my_list = ['a'] * 1000
    result = ''.join(my_list)

time_for_join = timeit.timeit(using_join, number=10000)
print(f"Time taken using `.join()`: {time_for_join}")

# Using list comprehension
def using_list_comprehension():
    my_list = [str(x) for x in range(1000)]
    result = ','.join(my_list)

time_for_list_comprehension = timeit.timeit(using_list_comprehension, number=10000)
print(f"Time taken using list comprehension: {time_for_list_comprehension}")

When executed, this code will print out the time taken for each method. You'll generally find that .join() is faster than using the + operator, especially as the list grows in size.

Time Complexity

  • Using + operator: The time complexity is O(n2)
  • Using .join(): The time complexity is O(n)
  • Using list comprehension: The time complexity depends on the filtering logic but is generally O(n) for simple conditions.


Common Pitfalls and Mistakes

Even though joining lists in Python is relatively straightforward, there are some common pitfalls that both beginners and experienced programmers might encounter.

Type Errors

When using methods like str.join(), remember that it expects all elements in the iterable to be strings. If your list contains non-string data types, you'll encounter a TypeError.

# This will throw a TypeError
numbers = [1, 2, 3]
result = ''.join(numbers)

To avoid this, make sure to convert all elements to strings before joining them.

# This will work
numbers = [1, 2, 3]
result = ''.join(map(str, numbers))

Separator Mistakes

When using custom separators, make sure that the separator is appropriate for the data you are joining. For example, using a comma for a list that already contains commas could lead to confusion.

# Might cause confusion
words = ['apple', 'banana', 'cherry, mango']
result = ', '.join(words)  # Output: "apple, banana, cherry, mango"

Joining Large Lists

While Python's .join() method is efficient, joining very large lists can still consume a lot of memory and time. If you're working with extremely large lists, consider using generators or working with chunks of the list at a time.

# Instead of this
large_list = ['a'] * 10**6
result = ''.join(large_list)

# Consider doing this
from itertools import islice

def chunked_join(large_list, chunk_size=10000):
    result = ''
    iterator = iter(large_list)
    while True:
        chunk = list(islice(iterator, chunk_size))
        if not chunk:
        result += ''.join(chunk)
    return result

result = chunked_join(large_list)


Frequently Asked Questions

In this section, we'll address some frequently asked questions and common misconceptions regarding the joining of lists in Python.

Can I join lists using the + operator?

While you can concatenate lists using the + operator, this is not the same as joining. The + operator combines two lists into a new one but doesn't merge elements within the lists into single elements.

Is str.join() the most efficient way to join lists of strings?

Yes, the str.join() method is generally the most efficient way to join lists of strings in Python, both in terms of speed and memory usage.

Can I join lists of integers directly?

No, you need to convert the integers to strings first. You can use the map() function for this.

What is the difference between joining and flattening a list?

Joining a list merges the elements of the list into a single element, usually a string. Flattening a list involves removing one level of nesting from a nested list. It doesn't combine elements but makes the list "shallower."

Can I use custom separators in str.join()?

Yes, you can specify a custom string as a separator which will be placed between the elements in the final string.


Examples and Best Practices

In this section, we'll cover some good practices to follow and pitfalls to avoid when joining lists in Python.

In this section, we'll cover some good practices to follow and pitfalls to avoid when joining lists in Python.

Avoid Using + for String Accumulation

While it may be tempting to use + to concatenate strings in a loop, it's inefficient because strings are immutable and this creates unnecessary copies.

Bad Example:

joined_str = ""
for s in list_of_strings:
    joined_str += s


joined_str = "".join(list_of_strings)

Avoid Manual Type Conversion

Manually converting each element to a string using a loop is inefficient.

Bad Example:

numbers = [1, 2, 3]
joined_str = ''
for num in numbers:
    joined_str += str(num)


joined_str = ''.join(map(str, numbers))

Avoid Using str() on Lists for Joining

Using str() on a list gives you a string that looks like the repr() output of the list, which is often not what you want.

Bad Example:

list_to_join = [1, 2, 3]
joined_str = str(list_to_join)
# joined_str will be "[1, 2, 3]"


joined_str = ''.join(map(str, list_to_join))
# joined_str will be "123"

Avoid Ignoring Element Types

If you have mixed types in a list, be sure to handle them appropriately, preferably by converting everything to strings.

Bad Example:

mixed_list = ['a', 1, 'b']
joined_str = ''.join(mixed_list)  # TypeError


joined_str = ''.join(map(str, mixed_list))



In this comprehensive guide, we've explored various ways to join lists in Python, along with best practices, common mistakes, and performance considerations. The aim has been to provide insights that are helpful for both beginners and experienced developers.

  • Method Matters: The method you choose for joining lists depends on what you're trying to accomplish. If it's a simple concatenation, str.join() or itertools.chain() may be your go-to. For more complex conditions, list comprehensions and filter() may be more appropriate.
  • Performance Concerns: If you're dealing with large lists or need to perform the operation repeatedly, consider the time complexity of your chosen method.
  • Type Safety: Always keep track of the data types in your list. Most joining methods expect strings, so make sure to convert numbers or other types to strings as needed.
  • Nested Lists: For joining nested lists, you might need to flatten the list first. Tools like itertools.chain() can be handy in such cases.
  • Conditions: If you want to join based on some conditions, using list comprehensions or the filter() function can be an efficient way to do so.
  • Avoid Common Mistakes: Steer clear of common pitfalls such as using the + operator for joining strings in a loop, or ignoring the types of elements in your list.


When to Use Which Method for Joining Lists

  • Simple Joins: Use ''.join(list) for basic needs.
  • Custom Separators: For a custom separator between elements, go with str.join().
  • Nested Lists: Utilize itertools.chain() for joining nested lists.
  • Conditional Joins: Use list comprehensions or filter() for joins based on conditions.


Additional Resources

For further reading and more complex examples, the Official Python Documentation offers in-depth explanations and examples.


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Bashir Alam

He is a Computer Science graduate from the University of Central Asia, currently employed as a full-time Machine Learning Engineer at uExel. His expertise lies in OCR, text extraction, data preprocessing, and predictive models. You can reach out to him on his Linkedin or check his projects on GitHub page.

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