TypeError: can only join an iterable in Python [Solved]

# TypeError: can only join an iterable in Python

The Python "TypeError: can only join an iterable" occurs when you pass a non-iterable value to the str.join() method, e.g. None from calling a built-in method that doesn't return anything.

To solve the error, make sure to pass an iterable to the join() method.

Here is an example of how the error occurs.

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fruits = ['kiwi', 'banana', 'apple']

new_list = fruits.reverse()
print(new_list)  # 👉️ None

# ⛔️ TypeError: can only join an iterable
my_str = ','.join(new_list) # 👈️ Passing None to join()
print(my_str)

The error is caused because the list.reverse() method mutates the original list in place and doesn't return anything (implicitly returns None).

# Make sure to pass an iterable to the str.join() method

To solve the error, we have to make sure to pass an iterable to the str.join() method, e.g. a list or a tuple.

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fruits = ['kiwi', 'banana', 'apple']

fruits.reverse()

my_str = ','.join(fruits)
print(my_str)  # 👉️ 'apple,banana,kiwi'

We passed the actual list to the str.join() method which solved the error.

# Most common sources of None in Python

If the value is None, the most common sources of None values are:

1. Having a function that doesn't return anything (returns None implicitly).
2. Explicitly setting a variable to None.
3. Assigning a variable to the result of calling a built-in function that doesn't return anything (e.g. reverse, sort, etc).
4. Having a function that only returns a value if a certain condition is met.

The str.join() method takes an iterable as an argument and returns a string which is the concatenation of the strings in the iterable.

# Functions that don't return anything return None

If the value you are passing to the join() method is None, make sure you aren't calling it with the result of a function that doesn't return anything.

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# 👇️ this function returns None
def get_list():
    print(['kiwi', 'banana', 'apple'])

# ⛔️ TypeError: can only join an iterable
my_str = ' '.join(get_list())

You can use a return statement to return a value from a function.

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def get_list():
    return ['kiwi', 'banana', 'apple']


my_str = ' '.join(get_list())

print(my_str)  # 👉️ "kiwi banana apple"

The function now returns a list, so we can safely pass the list to the join() method.

# Checking if a variable doesn't store None before calling join()

Use an if statement if you need to check whether a variable doesn't store a None value before joining it into a string.

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my_list = None

if my_list is not None:
    my_str = ' '.join(my_list)
    print(my_str)
else:
    # 👇️ this runs
    print('variable stores None')

Alternatively, you can provide an empty list as a fallback.

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my_list = None

if my_list is None:
    my_list = []

my_str = ','.join(my_list)

print(my_str)  # 👉️ ''

Make sure you aren't storing the result of calling one in a variable.

# A function only returning a value if a condition is met

Another common cause of the error is having a function that returns a value only if a condition is met.

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def get_list(a):
    if len(a) > 3:
        return a


# 👇️ None
my_list = get_list(['a', 'b'])

The if statement in the get_list function is only run if the passed in argument has a length greater than 3.

To solve the error, you either have to check if the function didn't return None, or return a default value if the condition is not met.

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def get_list(a):
    if len(a) > 3:
        return a

    return [] # 👈️ Return an empty list if the condition is not met


# 👇️ []
my_list = get_list(['a', 'b'])

Now the function is guaranteed to return a value regardless of whether the condition is met.