TypeError: 'NoneType' object is not iterable in Python

Question

What does error TypeError: 'NoneType' object is not iterable mean?

I am getting it on this Python code:

def write_file(data, filename): # creates file and writes list to it   with open(filename, 'wb') as outfile:     writer = csv.writer(outfile)     for row in data: # ABOVE ERROR IS THROWN HERE       writer.writerow(row) 

Answer 1

It means the value of data is None.

Answer 2

Explanation of error: 'NoneType' object is not iterable

In python2, NoneType is the type of None. In Python3 NoneType is the class of None, for example:

>>> print(type(None))     #Python2 <type 'NoneType'>         #In Python2 the type of None is the 'NoneType' type.  >>> print(type(None))     #Python3 <class 'NoneType'>        #In Python3, the type of None is the 'NoneType' class. 

Iterating over a variable that has value None fails:

for a in None:     print("k")     #TypeError: 'NoneType' object is not iterable 

Python methods return NoneType if they don't return a value:

def foo():     print("k") a, b = foo()      #TypeError: 'NoneType' object is not iterable 

You need to check your looping constructs for NoneType like this:

a = None  print(a is None)              #prints True print(a is not None)          #prints False print(a == None)              #prints True print(a != None)              #prints False print(isinstance(a, object))  #prints True print(isinstance(a, str))     #prints False 

Guido says only use is to check for None because is is more robust to identity checking. Don't use equality operations because those can spit bubble-up implementationitis of their own. Python's Coding Style Guidelines - PEP-008

NoneTypes are Sneaky, and can sneak in from lambdas:

import sys b = lambda x : sys.stdout.write("k")  for a in b(10):      pass            #TypeError: 'NoneType' object is not iterable  

NoneType is not a valid keyword:

a = NoneType     #NameError: name 'NoneType' is not defined 

Concatenation of None and a string:

bar = "something" foo = None print foo + bar    #TypeError: cannot concatenate 'str' and 'NoneType' objects 

What's going on here?

Python's interpreter converted your code to pyc bytecode. The Python virtual machine processed the bytecode, it encountered a looping construct which said iterate over a variable containing None. The operation was performed by invoking the __iter__ method on the None.

None has no __iter__ method defined, so Python's virtual machine tells you what it sees: that NoneType has no __iter__ method.

This is why Python's duck-typing ideology is considered bad. The programmer does something completely reasonable with a variable and at runtime it gets contaminated by None, the python virtual machine attempts to soldier on, and pukes up a bunch of unrelated nonsense all over the carpet.

Java or C++ doesn't have these problems because such a program wouldn't be allowed to compile since you haven't defined what to do when None occurs. Python gives the programmer lots of rope to hang himself by allowing you to do lots of things that should cannot be expected to work under exceptional circumstances. Python is a yes-man, saying yes-sir when it out to be stopping you from harming yourself, like Java and C++ does.