str.join()

str.join(iterable)

Returns: str · Updated March 13, 2026 · String Methods

strings iteration joining

The .join() method concatenates elements from an iterable (such as a list, tuple, or generator) into a single string, inserting a separator between each element. It’s one of the most efficient ways to build strings from collections in Python.

Syntax

str.join(iterable)

Parameters

Parameter	Type	Default	Description
`iterable`	`iterable`	—	An iterable (list, tuple, set, generator) containing string elements to join

Returns: str — A single string with all elements joined by the separator.

Examples

Basic usage

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

# Join with a space separator
result = " ".join(words)
print(result)
# hello world python

# Join with a comma separator
result = ",".join(words)
print(result)
# hello,world,python

Joining tuples

fruits = ("apple", "banana", "cherry")

result = " - ".join(fruits)
print(result)
# apple - banana - cherry

Building file paths

path_parts = ["home", "user", "documents", "file.txt"]

# Using forward slash
result = "/".join(path_parts)
print(result)
# home/user/documents/file.txt

Practical CSV-like output

data = [
    ["name", "age", "city"],
    ["Alice", "30", "NYC"],
    ["Bob", "25", "LA"]
]

for row in data:
    print(",".join(row))
# name,age,city
# Alice,30,NYC
# Bob,25,LA

Using with dictionary keys

config = {"host": "localhost", "port": "8080"}

# Join dictionary keys
result = ", ".join(config.keys())
print(result)
# host, port

# Join dictionary values
result = ", ".join(config.values())
print(result)
# localhost, 8080

String building in loops

# Building a long string efficiently
words = ["the", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog"]

# Instead of: result = ""
#              for word in words: result += word + " "
# Use join (much more efficient):
result = " ".join(words) + " "
print(result)
# the quick brown fox jumps over the lazy dog

Common Patterns

Filtering and joining

files = ["report.pdf", "image.png", "data.csv", "script.py"]

# Join only files with specific extensions
pdfs = ", ".join(f for f in files if f.endswith(".pdf"))
print(pdfs)
# report.pdf

Reverse operation with split

text = "hello world python"

# Split into list
words = text.split(" ")
print(words)
# ['hello', 'world', 'python']

# Rejoin with different separator
result = "-".join(words)
print(result)
# hello-world-python

Creating formatted output

headers = ["Name", "Age", "Occupation"]
values = ["John", "28", "Developer"]

# Create a formatted table row
row = " | ".join(values)
print(row)
# John | 28 | Developer

# Create header separator
separator = "-" * len(row)
print(separator)
# -------------------

Errors

All elements in the iterable must be strings. Passing non-string elements raises a TypeError:

numbers = [1, 2, 3]
",".join(numbers)
# TypeError: sequence item 0: expected str instance, int found

# Convert to strings first:
result = ",".join(str(n) for n in numbers)
print(result)
# 1,2,3

Joining an empty iterable returns an empty string:

result = ",".join([])
print(result)
# (empty string)

result = ",".join("")
print(result)
# (empty string)

Performance

.join() is significantly faster than string concatenation in loops because it pre-allocates memory for the final string. This makes it the preferred method for building strings from multiple elements.

# Slow - creates many intermediate string objects
result = ""
for word in words:
    result += word + " "

# Fast - single allocation
result = " ".join(words)