what causes AttributeError?

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

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 src.attribute_error
 2import unittest
 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

  • I make a directory for the project

    mkdir attribute_error

    the terminal goes back to the command line

  • I change directory to the project

    cd attribute_error

    the terminal shows I am in the attribute_error folder

    .../pumping_python/attribute_error

  • I make a directory for the source code

    mkdir src

    the terminal goes back to the command line

  • I make a Python file to hold the source code in the src directory

    touch src/attribute_error.py

    Note

    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

  • I make a directory for the tests

    mkdir tests

    the terminal goes back to the command line

  • I make the tests directory a Python package

    Danger

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

    touch tests/__init__.py

    Note

    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 a Python file for the tests in the tests directory

    touch tests/test_attribute_error.py

    Note

    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 the Integrated Development Environment (IDE) with the name of the program and the name of the file. That means if I type this in the terminal

    code tests/test_attribute_error.py

    Visual Studio Code opens test_attribute_error.py 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 requirements file for the Python packages I need, in the terminal

    echo "pytest" > requirements.txt

    the terminal goes back to the command line

  • I add pytest-watcher to the file

    echo "pytest-watcher" >> requirements.txt

    the terminal goes back to the command line

  • I setup the project with uv

    uv init

    the terminal shows

    Initialized project `attribute_error`

    then goes back to the command line

  • I remove main.py from the project because I do not use it

    rm main.py

    the terminal goes back to the command line

  • I install the Python packages I gave in the requirements file

    uv add --requirement requirements.txt

    the terminal shows it installed the Python packages

  • I use pytest-watcher to run the tests automatically

    uv run pytest-watcher . --now

    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_attribute_error.py::TestAttributeError::test_failure - AssertionError: True is not false
=========================== 1 failed in X.YZs ============================

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

     4class TestAttributeError(unittest.TestCase):
 5
 6    def test_failure(self):
 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

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

    • src.attribute_error.variable_00 points 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 raises AttributeError

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

    variable_00 is NOT an attribute of attribute_error.py in the src folder

  • 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)

  • I add the name to attribute_error.py

    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 in the src folder, I can use 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 add 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 in attribute_error.py

    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

Note

A variable in a module is an attribute of the module

test_attribute_error_w_functions

RED: make it fail

I add a new test to test_attribute_error.py

 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
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

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

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

     7def function_00():
 8    return None
 9
10
11def function_01():
12    return None
13
14
15def function_02():
16    return None

    the test passes

  • I add another failing 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

     7def function_00():
 8    return None
 9
10
11def function_01():
12    return None
13
14
15def function_02():
16    return None
17
18
19def function_03():
20    return None

    the test passes.

Note

test_attribute_error_w_class_attributes

I know that variables and functions in a module are attributes of the module.

A class in a module is also an attribute of the module, and variables in the class are attributes of the class.

RED: make it fail

I add a new test 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()
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

    19def function_03():
20    return None
21
22
23def AClass():
24    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 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
22
23
24# Exceptions seen

    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.

Note

test_attribute_error_w_class_methods

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

I also know that variables in a class are attributes of the class.

functions in a class are also attributes, they are called methods

RED: make it fail

  • I add a new test 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    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

    method_00 points to None and I cannot call None like a function

  • I make it a method with the def keyword

    23class AClass():
24
25    attribute_00 = None
26    attribute_01 = None
27    attribute_02 = None
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 in the src folder and I can call it with src.attribute_error.AClass.method_00()

REFACTOR: make it better

  • You know the “drill”, I add a new failing line to test_attribute_error.py

    25    def test_attribute_error_w_class_methods(self):
26        src.attribute_error.AClass.method_00()
27        src.attribute_error.AClass.method_01()

    the terminal shows AttributeError

    AttributeError: type object 'AClass' has no attribute 'method_01'. Did you mean: 'method_00'?

  • I add a definition for it in attribute_error.py

    30    def method_00():
31        return None
32
33    def method_01():
34        return None

    the test passes

  • I add another failing line to test_attribute_error.py

    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()

    the terminal shows AttributeError

    AttributeError: type object 'AClass' has no attribute 'method_02'. Did you mean: 'method_00'?

  • I add the method to AClass in attribute_error.py

    30    def method_00():
31        return None
32
33    def method_01():
34        return None
35
36    def method_02():
37        return None

    the test passes

  • I add the last line to test_attribute_error.py

    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

    the terminal shows AttributeError

    AttributeError: type object 'AClass' has no attribute 'method_03'. Did you mean: 'method_00'?

  • I add the method to AClass in attribute_error.py

    30class AClass():
31
32    attribute_00 = None
33    attribute_01 = None
34    attribute_02 = None
35    attribute_03 = None
36
37    def method_00():
38        return None
39
40    def method_01():
41        return None
42
43    def method_02():
44        return None
45
46    def method_03():
47        return None

    the test passes.

Note

close the project

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

  • I click in the terminal, then use q on the keyboard to leave the tests. The terminal goes back to the command line

  • 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 these 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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