what causes AttributeError?

AttributeError is raised when I use a name that is NOT in an object that exists.

what is an attribute?

An attribute is a name for something that belongs to an object, for example, a human being has attributes like height, weight, sex and color, they are also known as properties.

preview

These are the tests I have by the end of the chapter

 1import unittest
 2import src.attribute_error
 3
 4
 5class TestAttributeError(unittest.TestCase):
 6
 7    def test_attribute_error_w_variables(self):
 8        src.attribute_error.variable_00
 9        src.attribute_error.variable_01
10        src.attribute_error.variable_02
11        src.attribute_error.variable_03
12
13    def test_attribute_error_w_functions(self):
14        src.attribute_error.function_00()
15        src.attribute_error.function_01()
16        src.attribute_error.function_02()
17        src.attribute_error.function_03()
18
19    def test_attribute_error_w_class_attributes(self):
20        src.attribute_error.AClass.attribute_00
21        src.attribute_error.AClass.attribute_01
22        src.attribute_error.AClass.attribute_02
23        src.attribute_error.AClass.attribute_03
24
25    def test_attribute_error_w_class_methods(self):
26        src.attribute_error.AClass.method_00()
27        src.attribute_error.AClass.method_01()
28        src.attribute_error.AClass.method_02()
29        src.attribute_error.AClass.method_03()
30
31
32# Exceptions seen
33# AssertionError
34# AttributeError
35# NameError
36# TypeError
37# SyntaxError

start the project

  • I name this project attribute_error

  • I open a terminal

  • then I make a directory for the project

    mkdir attribute_error

    the terminal goes back to the command line

    .../pumping_python

  • I change directory to the project

    cd attribute_error

    the terminal shows I am now in the attribute_error folder

    .../pumping_python/attribute_error

  • I make a folder for the source code

    mkdir src

    the terminal goes back to the command line

    .../pumping_python/attribute_error

  • I use touch to make an empty file for the program in the src folder

    touch src/attribute_error.py

    Attention

    on Windows without Windows Subsystem for Linux use New-Item src/attribute_error.py instead of touch src/attribute_error.py

    New-Item src/attribute_error.py

    the terminal goes back to the command line

    .../pumping_python/attribute_error

  • I make a directory for the tests

    mkdir tests

    the terminal goes back to the command line

  • I use touch to make an empty file in the tests folder to tell Python that it is a Python package

    Attention

    use 2 underscores (__) before and after init for __init__.py not _init_.py

    touch tests/__init__.py

    Attention

    on Windows without Windows Subsystem for Linux use New-Item tests/__init__.py instead of touch tests/__init__.py

    New-Item tests/__init__.py

    the terminal goes back to the command line

  • I make an empty file for the actual test

    touch tests/test_attribute_error.py

    Attention

    on Windows without Windows Subsystem for Linux use New-Item tests/test_attribute_error.py instead of touch tests/test_attribute_error.py

    New-Item tests/test_attribute_error.py

    the terminal goes back to the command line

  • I open test_attribute_error.py in the editor of the Integrated Development Environment (IDE)

    Tip

    I can open a file from the terminal in Visual Studio Code by typing code and the name of the file, for example, when I type this

    code tests/test_attribute_error.py

    test_attribute_error.py opens up in the editor

  • I add the first failing test to test_attribute_error.py

    1import unittest
2
3
4class TestAttributeError(unittest.TestCase):
5
6    def test_failure(self):
7        self.assertFalse(True)

  • I make a virtual environment in the terminal

    python3 -m venv .venv

    Attention

    on Windows without Windows Subsystem for Linux use python3 -m venv .venv instead of python3 -m venv .venv

    python -m venv .venv

    the terminal takes some time then goes back to the command line

  • I activate the virtual environment

    source .venv/bin/activate

    Attention

    on Windows without Windows Subsystem for Linux use .venv/bin/activate.ps1 NOT source .venv/bin/activate

    .venv/scripts/activate.ps1

    the terminal shows

    (.venv) .../pumping_python/attribute_error

  • I upgrade the Python package manager (pip) to the latest version

    python3 -m pip install --upgrade pip

    the terminal shows pip being uninstalled then installs the latest version or shows that it is already the latest version

  • I make a requirements.txt file for the Python programs my project needs

    echo "pytest-watch" > requirements.txt

    the terminal goes back to the command line

  • I use pip to install pytest-watch with the requirements file

    python3 -m pip install --requirement requirements.txt

    Attention

    on Windows without Windows Subsystem for Linux use python -m pip install --requirement requirements.txt instead of python3 -m pip install --requirement requirements.txt

    python -m pip install --requirement requirements.txt

    the terminal shows pip downloads and installs the Python programs that pytest-watch needs to run

  • I use pytest-watch to run the test

    pytest-watch

    the terminal shows

    ================================ FAILURES ================================
_______________________ TestAttributeError.test_failure _______________________

self = <tests.test_attribute_error.AttributeError testMethod=test_failure>

    def test_failure(self):
>       self.assertFalse(True)
E       AssertionError: True is not false

tests/test_attribute_error.py:7: AssertionError
========================= short test summary info ==========================
FAILED tests/test_functions.py::TestFunctions::test_failure - AssertionError: True is not false
=========================== 1 failed in X.YZs ============================

  • I hold ctrl (Windows/Linux) or option/command (MacOS) on the keyboard and use the mouse to click on tests/test_attribute_error.py:7 to open it in the editor

  • I add AssertionError to the list of Exceptions seen in test_functions.py

     7        self.assertFalse(True)
 8
 9
10# Exceptions seen
11# AssertionError

  • then I change True to False in the assertion

    7        self.assertFalse(False)

    the test passes

test_attribute_error_w_variables

RED: make it fail

  • I add an import statement at the top of test_attribute_error.py

    1import src.attribute_error
2import unittest

  • I change test_failure to test_attribute_error_w_variables

     5class TestAttributeError(unittest.TestCase):
 6
 7    def test_attribute_error_w_variables(self):
 8        src.attribute_error.variable_00
 9
10
11# Exceptions seen

    I think of src.attribute_error.variable_00 as an address

    • src is the src folder

    • .attribute_error points to attribute_error.py in the src folder

    • .variable_00 points to variable_00 in attribute_error.py which is in the src folder

    • src.attribute_error.variable_00 is pointing to variable_00 in attribute_error.py in the src folder

    Since attribute_error.py is empty, Python cannot find variable_00 inside it and the terminal shows AttributeError

    AttributeError: module 'src.attribute_error' has no attribute 'variable_00'

  • I add AttributeError to the list of Exceptions seen in test_attribute_error.py

    11# Exceptions seen
12# AssertionError
13# AttributeError

GREEN: make it pass

  • I open attribute_error.py from the src folder in the editor of my Integrated Development Environment (IDE), then I add a name

    1variable_00

    the terminal shows NameError

    NameError: name 'variable_00' is not defined

    NameError is raised when I use a name that is not defined in the file I am working in

  • I add NameError to the list of Exceptions seen in test_attribute_error.py

    11# Exceptions seen
12# AssertionError
13# AttributeError
14# NameError

  • I point variable_00 to None in attribute_error.py

    1variable_00 = None

    the test passes. variable_00 is now an attribute or property of attribute_error.py which is in the src folder and I can get to it with src.attribute_error.variable_00

REFACTOR: make it better

  • I do the same test a few more times as a drill in test_attribute_error.py

    7    def test_attribute_error_w_variables(self):
8        src.attribute_error.variable_00
9        src.attribute_error.variable_01

    the terminal shows AttributeError

    AttributeError: module 'src.attribute_error' has no attribute 'variable_01'. Did you mean: 'variable_00'?

  • I add the name to attribute_error.py

    1variable_00 = None
2variable_01

    the terminal shows NameError

    NameError: name 'variable_01' is not defined

    I point it to None to define it

    1variable_00 = None
2variable_01 = None

    the test passes

  • I another statement to test_attribute_error.py

     7    def test_attribute_error_w_variables(self):
 8        src.attribute_error.variable_00
 9        src.attribute_error.variable_01
10        src.attribute_error.variable_02

    the terminal shows AttributeError

    AttributeError: module 'src.attribute_error' has no attribute 'variable_02'. Did you mean: 'variable_00'?

  • I add the name and point it to None

    1variable_00 = None
2variable_01 = None
3variable_02 = None

    the test passes

  • one more line in test_attribute_error.py

     7    def test_attribute_error_w_variables(self):
 8        src.attribute_error.variable_00
 9        src.attribute_error.variable_01
10        src.attribute_error.variable_02
11        src.attribute_error.variable_03

    the terminal shows AttributeError

    AttributeError: module 'src.attribute_error' has no attribute 'variable_03'. Did you mean: 'variable_00'?

  • I add the attribute to attribute_error.py

    1variable_00 = None
2variable_01 = None
3variable_02 = None
4variable_03 = None

    the test passes

  • A variable in a class in a module is an attribute of class

test_attribute_error_w_functions

RED: make it fail

I add a new test to test_attribute_error.py

11        src.attribute_error.variable_03
12
13    def test_attribute_error_w_functions(self):
14        src.attribute_error.function_00()
15
16
17# Exceptions seen

the terminal shows AttributeError

AttributeError: module 'src.attribute_error' has no attribute 'function_00'

GREEN: make it pass

  • I add the name and point it to None in attribute_error.py

    4variable_03 = None
5
6
7function_00 = None

    the terminal shows TypeError

    TypeError: 'NoneType' object is not callable

  • I add TypeError to the list of Exceptions seen in test_attribute_error.py

    17# Exceptions seen
18# AssertionError
19# AttributeError
20# NameError
21# TypeError

  • I change the variable to a function in attribute_error.py

    4variable_03 = None
5
6
7def function_00():
8    return None

    the test passes. function_00 is now an attribute or property of attribute_error.py in the src folder. I can call it with src.attribute_error.function_00()

REFACTOR: make it better

  • time to do it as a drill, I add another call in test_attribute_error.py

    13    def test_attribute_error_w_functions(self):
14        src.attribute_error.function_00()
15        src.attribute_error.function_01()

    the terminal shows AttributeError

    AttributeError: module 'src.attribute_error' has no attribute 'function_01'. Did you mean: 'function_00'?

  • I add the function to attribute_error.py

     7def function_00():
 8    return None
 9
10
11def function_01():
12    return None

    the test passes

  • I add another line to test_attribute_error.py

    13    def test_attribute_error_w_functions(self):
14        src.attribute_error.function_00()
15        src.attribute_error.function_01()
16        src.attribute_error.function_02()

    the terminal shows AttributeError

    AttributeError: module 'src.attribute_error' has no attribute 'function_02'. Did you mean: 'function_00'?

  • I add a function for it in attribute_error.py

    11def function_01():
12    return None
13
14
15def function_02():
16    return None

    the test passes

  • I add another line in test_attribute_error.py

    13    def test_attribute_error_w_functions(self):
14        src.attribute_error.function_00()
15        src.attribute_error.function_01()
16        src.attribute_error.function_02()
17        src.attribute_error.function_03()
18
19
20# Exceptions seen

    the terminal shows AttributeError

    AttributeError: module 'src.attribute_error' has no attribute 'function_03'. Did you mean: 'function_00'?

  • I add the function to attribute_error.py

    15def function_02():
16    return None
17
18
19def function_03():
20    return None

    the test passes.

  • A function in a module is an attribute of the module

  • A variable in a class in a module is an attribute of class

test_attribute_error_w_class_attributes

I know that variables and functions defined in a module are attributes. variables defined inside a class are also attributes.

RED: make it fail

I add a new test to test_attribute_error.py

17        src.attribute_error.function_03()
18
19    def test_attribute_error_w_class_attributes(self):
20        src.attribute_error.AClass.attribute_00
21
22
23# Exceptions seen

the terminal shows AttributeError

AttributeError: module 'src.attribute_error' has no attribute 'AClass'

GREEN: make it pass

  • I add a function to attribute_error.py

    15def function_03():
16    return None
17
18
19def AClass():
20    return None

    the terminal shows AttributeError

    AttributeError: 'function' object has no attribute 'attribute_00'

  • I add a variable inside the function

    23def AClass():
24
25    attribute_00 = None
26    return None

    the terminal still shows the same Exception because I cannot get to a variable inside a function from outside the function

  • I use the class keyword instead of the def keyword to make AClass a class

    23class AClass():
24
25    attribute_00 = None
26    return None

    the terminal shows SyntaxError

    E    return None
E    ^^^^^^^^^^^
E  SyntaxError: 'return' outside function

    I cannot use a return statement outside a function

  • I add SyntaxError to the list of Exceptions seen in test_attribute_error.py

    23# Exceptions seen
24# AssertionError
25# AttributeError
26# NameError
27# TypeError
28# SyntaxError

  • I remove the return statement from AClass in attribute_error.py since it is no longer a function

    23class AClass():
24
25    attribute_00 = None

    the test passes. attribute_00 is now an attribute of the AClass class which is an attribute of attribute_error.py in which is in the src folder and I can get to it with src.attribute_error.AClass.attribute_00

REFACTOR: make it better

  • I add another failing line for practice to test_attribute_error.py

    19    def test_attribute_error_w_class_attributes(self):
20        src.attribute_error.AClass.attribute_00
21        src.attribute_error.AClass.attribute_01

    the terminal shows AttributeError

    AttributeError: type object 'AClass' has no attribute 'attribute_01'. Did you mean: 'attribute_00'?

  • I add the name to the class definition in attribute_error.py

    23class AClass():
24
25    attribute_00 = None
26    attribute_01 = None

    the test passes

  • I add another line to test_attribute_error.py

    19    def test_attribute_error_w_class_attributes(self):
20        src.attribute_error.AClass.attribute_00
21        src.attribute_error.AClass.attribute_01
22        src.attribute_error.AClass.attribute_02

    the terminal shows AttributeError

    AttributeError: type object 'AClass' has no attribute 'attribute_02'. Did you mean: 'attribute_00'?

  • I add the attribute to AClass in attribute_error.py

    23class AClass():
24
25    attribute_00 = None
26    attribute_01 = None
27    attribute_02 = None

    the test passes

  • I add another line to test_attribute_error.py

    19    def test_attribute_error_w_class_attributes(self):
20        src.attribute_error.AClass.attribute_00
21        src.attribute_error.AClass.attribute_01
22        src.attribute_error.AClass.attribute_02
23        src.attribute_error.AClass.attribute_03
24
25
26# Exceptions seen

    the terminal shows AttributeError

    AttributeError: type object 'AClass' has no attribute 'attribute_03'. Did you mean: 'attribute_00'?

  • I add the name to AClass in attribute_error.py

    23class AClass():
24
25    attribute_00 = None
26    attribute_01 = None
27    attribute_02 = None
28    attribute_03 = None

    the test passes

  • A variable in a class in a module is an attribute of class

  • A class in a module is an attribute of the module

  • A function in a module is an attribute of the module

  • A variable in a class in a module is an attribute of class

test_attribute_error_w_class_methods

I know that variables, functions and classes defined in a module are attributes.

I also know that variables defined inside a class are attributes. functions defined inside a class are also attributes, they are called methods

RED: make it fail

  • I add a new test to test_attribute_error.py

    23        src.attribute_error.AClass.attribute_03
24
25    def test_attribute_error_w_class_methods(self):
26        src.attribute_error.AClass.method_00()
27
28
29# Exceptions seen

    the terminal shows AttributeError

    AttributeError: type object 'AClass' has no attribute 'method_00'

GREEN: make it pass

  • I add the name to AClass and point it to None in attribute_error.py

    23class AClass():
24
25    attribute_00 = None
26    attribute_01 = None
27    attribute_02 = None
28    attribute_03 = None
29
30    method_00 = None

    the terminal shows TypeError

    TypeError: 'NoneType' object is not callable

  • I make it a method with the def keyword

    28    attribute_03 = None
29
30    def method_00():
31        return None

    the test passes. method_00 is now an attribute of AClass which is an attribute of attribute_error.py which is in the src folder and I can get to it with src.attribute_error.AClass.method_00()

REFACTOR: make it better

close the project

  • I close attribute_error.py and test_attribute_error.py in the editors

  • I click in the terminal and exit the tests with ctrl+c on the keyboard

  • I deactivate the virtual environment

    deactivate

    the terminal goes back to the command line, (.venv) is no longer on the left side

    .../pumping_python/attribute_error

  • I change directory to the parent of attribute_error

    cd ..

    the terminal shows

    .../pumping_python

    I am back in the pumping_python directory

review

I ran tests for AttributeError with

I also saw the following Exceptions

code from the chapter

Do you want to see all the CODE I typed in this chapter?

what is next?

you know

Would you like to test how to pass values from tests to functions with assert methods?

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