The join() takes all items in an iterable and joins them into one string. If you attempt to pass a non-iterable object to the join() method, you will raise the error: Python TypeError: can only join an iterable.

You can solve this by ensuring that you do not assign the result of any method that performs in-place to a variable with the same name as the iterable you want to join. If you do this, you will pass a None object to the join() method, which is non-iterable.

This tutorial will go through the error in detail and an example to learn how to solve it.


TypeError: can only join an iterable

TypeError tells us that we are trying to perform an illegal operation for a specific Python data type.

There are data types like NoneType that are non-iterable. As the error message suggests, join() is an illegal operation for non-iterable objects.

Example: Sorting a List of Strings

Let’s look at an example where we define a list of names we want to join. However, we want to sort the names in ascending order using the sort() method before joining. The code would look as follows:

names = ["Bill", "Zeus", "Griffith", "John", "Daniel", "Andrew"]

names = sort(names)

names = names.sort()

sorted_names = ' '.join(names)

print(sorted_names)

Let’s run the code to see the result:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
sorted_names = ' '.join(names)

TypeError: can only join an iterable

Our program throws the error because the sort() method performs in-place sorting and returns a None value. If we assign the output of names.sort() to names, the variable will be None when we pass it to the join() method. None is a non-iterable data type, and join() is illegal on non-iterable objects.

Solution

To solve the problem, we must keep the names variable as a list, not a None object. We do not need to assign the output of the names.sort() to a variable, and we can call the method and sort the list in place. Let’s look at the revised code:

names = ["Bill", "Zeus", "Griffith", "John", "Daniel", "Andrew"]

names.sort()

sorted_names = ' '.join(names)

print(sorted_names)

Let’s run the code to see what happens:

Andrew Bill Daniel Griffith John Zeus

Our code successfully sorts the list of names, joins them with an empty-space separator, and prints the result to the console.

Summary

Congratulations on reading to the end of this tutorial! If you attempt to pass a non-iterable object like a None to the <span class="crayon-inline lang:python decode:true">join()</span> method, you will raise the error: Python TypeError: can only join an iterable. This error commonly occurs when you use a function that performs and assigns the result to the variable you want to join. If a function performs in place and returns a None value, you do not need to assign it to a variable, for example, sort(). To solve this error, ensure you are using the function without assignment.

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