This error means that python is trying to iterate over a None object. With Python, you can only iterate over an object if that object has a value. This is because iterable objects only have a next item which can be accessed if their value is not equal to None. If you try to iterate over a None object, you encounter the TypeError: 'NoneType' object is not iterable error.
For example,
list.sort() only change the list but return None.
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.
Technically, you can avoid the NoneType exception by checking if a value is equal to None before you iterate over that value.
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