Your constructor sets self.prevNode to None, and later you try to access node.prevNode.label, which is like trying to access None.label. None doesn't have any attributes, so trying to access any will give you an AttributeError.

This happens because the scipy module doesn't have any attribute named sparse. That attribute only gets defined when you import scipy.sparse.

Submodules don't automatically get imported when you just import scipy; you need to import them explicitly. The same holds for most packages, although a package can choose to import its own submodules if it wants to. (For example, if scipy/__init__.py included a statement import scipy.sparse, then the sparse submodule would be imported whenever you import scipy.)

And what does that error generally mean?

An Attribute in Python is a name belonging to an object - a method or a variable. An AttributeError means that the program tried to use an attribute of an object, but the object did not have the requested attribute.

For instance, string objects have the 'upper' attribute, which is a method that returns the uppercase version of the string. You could write a method that uses it like this:

def get_upper(my_string):
  return my_string.upper()

However, note that there's nothing in that method to ensure that you have to give it a string. You could pass in a file object, or a number. Neither of those have the 'upper' attribute, and Python will raise an Attribute Error.

As for why you're seeing it in this instance, you haven't provided enough detail for us to work it out. Add the full error message to your question.

You have to set if first before you use it:

class Ver2ProjectWithTTS(object):

    def __init__(self):
        # first set it
        self.list_merge = []
        self.read_json_file()
        self.read_xml_file()
        self.say_something()

Anyway don't do any advanced logic in constructors, it's not a good practice. Make a method instead:

class Ver2ProjectWithTTS(object):

    def __init__(self):
        # first set it
        self.list_merge = []

    def do_the_job(self):
        self.read_json_file()
        self.read_xml_file()
        self.say_something()

    ...

instance = Ver2ProjectWithTTS()
instance.do_the_job()

AttributeError typically identifies the missing attribute. e.g.:

class Foo:
    def __init__(self):
        self.a = 1

f = Foo()
print(f.a)
print(f.b)

When I run that, I see:

$ python foo.py
1
Traceback (most recent call last):
  File "foo.py", line 10, in <module>
    print(f.b)
AttributeError: Foo instance has no attribute 'b'

That's pretty explicit. If you're not seeing something like that, please post the exact error you're seeing.

EDIT

If you need to force the printing of an exception (for whatever reason), you can do this:

import traceback

try:
    # call function that gets AttributeError
except AttributeError:
    traceback.print_exc()

That should give you the full error message and traceback associated with the exception.

As the argument of loadTestsFromTestCase, you're trying to access Boy.BoyTest, i.e., the BoyTest attribute of class object Boy, which just doesn't exist, as the error msg is telling you. Why don't you just use BoyTest there instead?

You are giving the object.__getattribute__() handler a signal that the attribute doesn't exist by raising an AttributeError. The defined behaviour is to then call __getattr__ instead. The exception is lost, it was handled by __getattribute__. From the documentation:

Called unconditionally to implement attribute accesses for instances of the class. If the class also defines __getattr__(), the latter will not be called unless __getattribute__() either calls it explicitly or raises an AttributeError.

If you don't want __getattribute__ to handle the exception, you need to move your __getattr__ behaviour over to a custom __getattribute__ method instead:

class A(object):
    @property
    def test(self):
        raise AttributeError('message which should not be lost')
        return 'this would never return'

    def __getattribute__(self, name):
        try:
            value = super(A, self).__getattribute__(name)
        except AttributeError as ae:
            # chance to handle the attribute differently
            # if not, re-raise the exception
            raise ae

Note that the hasattr() function behaves the same way; it'll return False when an exception is raised when trying to access the attribute.

self.entbx is created by create_widget(). You are calling reveal() -- which requires self.entbx -- before you've called create_widget():

self.reveal()
self.create_widget()

Your indentation is goofed, and you've mixed tabs and spaces. Run the script with python -tt to verify.

Attributes are properties of functions or classes or modules, and if a property is not found then it raises attribute error.

NameError are related to variables.

>>> x=2
>>> y=3
>>> z    #z is not defined so NameError

Traceback (most recent call last):
  File "<pyshell#136>", line 1, in <module>
    z
NameError: name 'z' is not defined

>>> def f():pass

>>> f.x=2 #define an attribue of f
>>> f.x
2
>>> f.y   #f has no attribute named y

Traceback (most recent call last):
  File "<pyshell#141>", line 1, in <module>
    f.y
AttributeError: 'function' object has no attribute 'y'

>>> import math   #a module  

>>> math.sin(90) #sin() is an attribute of math
0.8939966636005579

>>> math.cosx(90)  #but cosx() is not an attribute of math

Traceback (most recent call last):
  File "<pyshell#145>", line 1, in <module>
    math.cosx(90)
AttributeError: 'module' object has no attribute 'cosx'

You can check if required message is in exception string.

except AttributeError as e:
    if "'product.product' object has no attribute 'order_line'" not in str(e):
        raise

But this is not recommended as you shouldn't be checking for attributes based of messages which can change in future.

Better approach would be to check if the attribute is present using hasattr

if hasattr(product.product, 'order_line'):
   # Do your stuff