Blog Post #116: The Pythonic Way to Combine Strings: The .join() Method

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A very common task in programming is to take a list of strings and combine them into a single string, often with a separator between each item. For example, you might have a list of tags that you want to display as a single, comma-separated string.

While you could do this with a for loop and the + operator, Python provides a much cleaner, faster, and more “Pythonic” way: the .join() string method. In this post, you’ll learn why .join() is the superior choice.

The ‘Obvious’ Way (And Its Problems)

A beginner’s first instinct is often to build the string piece by piece inside a loop, concatenating with + as they go.

words = ["hello", "world", "this", "is", "python"]
sentence = ""

for word in words:
    sentence += word + " "

# We have to manually remove the extra space at the end
sentence = sentence.strip()

print(sentence) # Output: hello world this is python

This approach has a few problems:

  • It’s inefficient: In Python, strings are immutable. Each += operation doesn’t just add to the existing string; it actually creates a brand new string in memory. In a loop with thousands of items, this is very slow and memory-intensive.
  • It’s clumsy: Notice we had to add a space after each word. This usually results in an extra, unwanted separator at the end of the string that we then have to manually remove (using a method like .strip()).

The Pythonic Solution: separator.join(list_of_strings)

The .join() method is the correct and highly optimized tool for this job. It’s a string method that takes one argument: a list (or any iterable) of strings.

The syntax can seem a little backward at first: you don’t call .join() on the list; you call it on the separator string you want to use as “glue.”

words = ["hello", "world", "this", "is", "python"]

# Use a space as the separator string
sentence = " ".join(words)
print(sentence)

# Use a comma and a space as the separator
csv_string = ", ".join(words)
print(csv_string)

The output is clean and correct:

hello world this is python
hello, world, this, is, python

Let’s appreciate the benefits of this approach:

  • Efficient: .join() is highly optimized in Python to perform this single task very quickly, creating only the final string in memory.
  • Readable: It’s a single, expressive line that clearly states your goal.
  • Correct: It intelligently places the separator between the elements, so you never get an extra one at the start or end.

A Common Gotcha: Joining Non-String Items

The .join() method has one strict rule: the iterable it operates on must contain only strings. If you have a list with numbers or other data types, it will raise a TypeError.

data = ["item1", 25, "item3", 99.9]

# This will cause a TypeError!
# result = ", ".join(data)

The solution is to convert every item in the list to a string before joining. A concise way to do this is with a generator expression (which we’ll cover in more detail later).

data = ["item1", 25, "item3", 99.9]

# This converts each item to a string before joining
result = ", ".join(str(item) for item in data)
print(result) # Output: item1, 25, item3, 99.9

What’s Next?

You’ve now learned the Pythonic idiom for joining a list of strings. When you need to build a string from an iterable, resist the urge to use a for loop and +. Reach for the efficient and elegant .join() method instead.

We’re continuing our journey into writing cleaner and more expressive code. In our next post, we’ll look at a beautiful Pythonic shortcut for writing complex logical checks. In Post #117, we will explore chained comparisons.

Author

Debjeet Bhowmik

Experienced Cloud & DevOps Engineer with hands-on experience in AWS, GCP, Terraform, Ansible, ELK, Docker, Git, GitLab, Python, PowerShell, Shell, and theoretical knowledge on Azure, Kubernetes & Jenkins. In my free time, I write blogs on ckdbtech.com

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