This is one of the most common errors we all faced at least once while working on a Python code. If you are facing a similar error then it is probably due to a for or while loop on an object.

def myfunction(data):
    for item in data:
        print(item)

In the above example, if data is None, we will get the specified error on the second line where we are iterating over data object. Basically this error means that the object we are trying to iterate over is NoneType i.e. is None. In simpler words, we are trying to run a for loop on a None object.

What is NoneType?

In python2, NoneType is the type of None.

# python2
>>> print(type(None))
<type 'NoneType'>    

In Python3 NoneType is the class of None

# python3
>>> print(type(None))
<class 'NoneType'>

When can this error occur?

As we saw, this error is reported when we try to iterate over a None object. All we have to find out why the object is None.

One reason could be the function returning data is setting the value of data to None. Another reason could be that we forgot to return anything at all.

For example,

def myfunction():
    a_list = [1,2,3]
    a_list.append(4)
    return a_list

returned_list = myfunction()
for item in returned_list:
    # do something with item

The above code will work just fine. 

Imagine, we forgot to return the a_list from myfunction. We will get the TypeError: 'NoneType' object is not iterable in the 6th line.

def myfunction():
    a_list = [1,2,3]
    a_list.append(4)
    # return a_list

returned_list = myfunction()
for item in returned_list:  # error will be reported in this line
    # do something with item

How to avoid this error?

One way to avoid this error is to check before iterating on an object if that object is None or not.

def myfunction():
    a_list = [1,2,3]
    a_list.append(4)
    # return a_list

returned_list = myfunction()
if returned_list is None:
    print("returned list is None")
else:
    for item in returned_list:
        # do something with item

Another way to handle this error is to write the for loop in try-except block.

def myfunction():
    a_list = [1,2,3]
    a_list.append(4)
    return a_list

returned_list = myfunction()
try:
    for item in returned_list:
        # do something with item
except Exception as e:
    # handle the exception accordingly

The third way is to explicitly assign an empty list to the variable if it is None.

def myfunction():
    a_list = [1,2,3]
    a_list.append(4)
    # return a_list

returned_list = myfunction()
if returned_list is None:
    returned_list = []
for item in returned_list:
    # do something with item

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