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

User · Answer

Just continue the loop when you get None Exception,

example:

   a = None
   if a is None:
       continue
   else:
       print("do something")

This can be any iterable coming from DB or an excel file.

User · Answer

Code  for row in data  Error message  TypeError   NoneType  object is not iterable  Which object is it complaining about  Choice of two  row and data  In for row in data  which needs to be iterable  Only data   What s the problem with data  Its type is NoneType  Only None has type NoneType  So data is None   You can verify this in an IDE  or by inserting e g  print  data is   repr data  before the for statement  and re-running   Think about what you need to do next   How should  no data  be represented  Do we write an empty file  Do we raise an exception or log a warning or keep silent

User · Answer

Another thing that can produce this error is when you are setting something equal to the return from a function  but forgot to actually return anything   Example   def foo dict of dicts       for key  row in dict of dicts items            for key  inner row in row items                Do SomeThing      Whoops  forgot to return all my stuff  return1  return2  return3   foo dict of dicts    This is a little bit of a tough error to spot because the error can also be produced if the row variable happens to be None on one of the iterations  The way to spot it is that the trace fails on the last line and not inside the function   If your only returning one variable from a function  I am not sure if the error would be produced    I suspect error   NoneType  object is not iterable in Python  in this case is actually implying  Hey  I m trying to iterate over the return values to assign them to these three variables in order but I m only getting None to iterate over

User · Answer

It means the value of data is None

User · Answer

For me it was a case of having my Groovy hat on instead of the Python 3 one  Forgot the return keyword at the end of a def function  Had not been coding Python 3 in earnest for a couple of months  Was thinking last statement evaluated in routine was being returned per the Groovy  or Rust  way  Took a few iterations  looking at the stack trace  inserting try      except TypeError      block debugging stepping thru code to figure out what was wrong  The solution for the message certainly did not make the error jump out at me

User · Answer

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    gt  gt  gt  print type None        Python2  lt type  NoneType  gt           In Python2 the type of None is the  NoneType  type    gt  gt  gt  print type None        Python3  lt class  NoneType  gt          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

User · Answer

You re calling write file with arguments like this   write file foo  bar    But you haven t defined  foo  correctly  or you have a typo in your code so that it s creating a new empty variable and passing it in

User · Answer

It means that the data variable is passing None  which is type NoneType   its equivalent for nothing  So it can t be iterable as a list  as you are trying to do

[python] TypeError: 'NoneType' object is not iterable in Python

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