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.
Table of contents
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, 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.
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.
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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Have fun and happy researching!
Suf is a research scientist at Moogsoft, specializing in Natural Language Processing and Complex Networks. Previously he was a Postdoctoral Research Fellow in Data Science working on adaptations of cutting-edge physics analysis techniques to data-intensive problems in industry. In another life, he was an experimental particle physicist working on the ATLAS Experiment of the Large Hadron Collider. His passion is to share his experience as an academic moving into industry while continuing to pursue research. Find out more about the creator of the Research Scientist Pod here and sign up to the mailing list here!