How .join Works

Link to article

A simple built-in string method of Python, .join() allows you to combine strings passed through an iterable of strings, using a separator string as the delimiter.

>>> facts = ['Giorno', 'is', 'the', 'strongest', 'Jojo']
>>> " ".join(facts)
"Giorno is the strongest Jojo"
# You could also set the delimiter into it's own variable for more modularity
>>> separator = " "
>>> separator.join(facts)
"Giorno is the strongest Jojo"

You don’t have to use a whitespace either if you don’t want too. You could just as easily concatenate them like so;

>>> letters = ["R", "M", "P"]
>>> "".join(letters)
"RMP"
# Since you just need to pass in an iterable, strings themselves are technically iterables, so the following is valid 
>>> initials = "RMP"
>>> ",".join(initials)
"R,M,P"

CSV Output

A simple use case, but nevertheless important. If we have a list of strings that need to get formatted for a CSV, that becomes trivial with .join();

colors = ['red', 'blue', 'orange']
csv_line = ",".join(colors)

Now, you can use that new line to pack into a file. You can also use the methods outlined in this note.

URL Composition

Given a base URL, and a list of sub paths, we can create complete URLs;

base_url = "https://examplesite.com/"
subpaths = ["blog", "2025", "05", "join-strings"]

full_url = base-url + "/".join(subpaths)
print(full_url)
"https://examplesite.com/blog/2025/05/join-strings"

Converting Numbers

Obviously, being a string method, using .join() on non strings will throw an error. To avoid this, make sure to typecast any values passed in to strings;

countdown = [3, 2, 1]
"...".join(str(number) for number in countdown)
"3..2..1"

There is also a functional style approach, using map;

countdown = [3, 2, 1]
"...".join(map(str, countdown))
"3..2..1"

Practical Examples

One example straight from the article is that of a JSON log of events. Say we have the following;

{
	"2025-04-29 2:00": ["click", "add_to_cart", "purchases"],
	"2025-04-29 2:10": ["click", "page_view"],
	"2025-04-29 2:20": ["page_view", "click", "add_to_cart"]
}