If you are eager to learn some Python and do not know how to start, this post may give you some hints. I will develop a very simple Python package from scratch, exemplifying some Object-oriented Programming (OOP) techniques and concepts, and using a Test-Driven Development (TDD) approach.

The package will provide some classes to deal with binary numbers (see the Rationale section), but remember that it is just a toy project. Nothing in this package has been designed with performance in mind: it wants to be as clear as possible.


Binary numbers are rather easy to understand, even if becoming familiar with them requires some time. I expect you to have knowledge of the binary numeral system. If you need to review them just take a look at the Wikipedia entry or one of the countless resources on Internet.

The package we are going to write will provide a class that represents binary numbers (Binary) and a class that represents binary numbers with a given bit size (SizeBinary). They shall provide basic binary operations like logical (and, or, xor), arithmetic (addition, subtraction, multiplication, division), shifts and indexing.

A quick example of what the package shall do:

>>> b = Binary('0101110001')
>>> hex(b)
>>> int(b)
>>> b[0]
>>> b[9]
>>> b.SHR()

Python and bases

Binary system is just a representation of numbers with base 2, just like hexadecimal (base 16) and decimal (base 10). Python can already natively deal with different bases, even if internally numbers are always stored as binary integers (more precise as numbers with base 2<sup>30</sup> and each digit as a binary number). Let us check it

>>> a = 5
>>> a
>>> a = 0x5
>>> a
>>> a = 0b101
>>> a
>>> hex(0b101)
>>> bin(5)

As you can see Python understands some common bases out of the box, using the 0x prefix for hexadecimal numbers and the 0b for binary ones (and 0o) for octals). However the number is always printed in its base-10 form (5 in this case). This means however that a binary number cannot be indexed, since integers does not provide support for this operation

>>> 0b101[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not subscriptable

You can also use a different base when converting things to integers, through the base parameter

>>> a = int('101', base=2)
>>> a
>>> a = int('10', base=5)
>>> a

Test-driven development

Simple tasks are the best way to try and use new development methodologies, so this is a good occasion to start working with the so-called test-driven approach. Test-driven Development (TDD) basically means that the first thing you do when developing is to write some tests, that is programs that use what you are going to develop. The purpose of those programs is to test that your final product complies with a given behaviour. So they provide

  • Documentation for your API: they are examples of use of your package.
  • Regression checks: when you change the code to develop new features they shall not break the behaviour of the previous package versions.
  • TODO list: until all tests run successfully you have something still waiting to be implemented.

I suggest you to follow this post until we have some tests (section "Writing some tests" included), then write your own class, trying to make it pass all the tests. This way, actually developing something, you can really learn both TDD and Python. Then you can check your code against mine and perhaps provide a far better solution than the one found by me.

Keep in mind, however, that the usual TDD workflow consist in writing 1 test and then adjusting the code to pass it (and all the previous tests). You shouldn't add a bunch of tests in on go, which I will do here just for brevity's sake.

Development environment

Create and activate a Python virtual environment using your favourite tool. You can follow the official documentation or use something more structured like pyenv.

Once the virtual environment is activated, install Pytest with

$ pip install pytest

Then create a directory for the project and enter it. You are free to give it the name you prefer, and I will assume all the following commands are executed inside that directory.

First of all let's create two directories for code and tests

$ mkdir src
$ touch src/__init__.py
$ mkdir tests
$ touch tests/__init__.py

Finally, let us check that everything is working correctly. Since there are no tests yet, Pytest should terminate without errors.

$ pytest
===================== test session starts =====================
collected 0 items 

=======================  in 0.00 seconds ======================

where [...] will be filled with information about your execution environment.


The approach used by Pytest is very straightforward: to test your library you just have to write some functions that use it. Those functions shall run without raising any exception; if a test (a function) runs without raising an exception it passes, otherwise it fails. Let us start writing a very simple test to learn the basic syntax. Create the file tests/test_binary.py and write in it the following code

def test_first():

If you run pytest again you shall obtain this result

$ pytest
===================== test session starts =====================
collected 1 items 

tests/test_binary.py .

=================== 1 passed in 0.01 seconds ==================

if you prefer (as I do) you may use the -v verbose switch to get detailed information about what tests have been executed

$ pytest -v
===================== test session starts =====================
collected 1 items 

tests/test_binary.py::test_first PASSED

=================== 1 passed in 0.01 seconds ==================

By default Pytest looks for Python files whose name starts with test_, and this is why it processes our file tests/test_binary.py. For each file it runs all functions whose name, again, starts with test_, and this is why test_first() has been executed.

The latter does nothing, so it runs without raising any exception and the test passes. Let us try to raise an exception

def test_first():
    raise ValueError

which gives the following output

$ pytest -v
===================== test session starts =====================
collected 1 items 

tests/test_binary.py::test_first FAILED

=========================== FAILURES ==========================
__________________________ test_first _________________________

    def test_first():
>       raise ValueError
E       ValueError

tests/test_binary.py:2: ValueError
=================== 1 failed in 0.01 seconds ==================

To easily write tests that raise exceptions when failing we may use the assert Python statement, which shall be followed by an expression. If the expression returns a true value, assert does nothing, otherwise it raises an AssertionError exception. Let us do a quick check in the Python console

>>> assert True
>>> assert False
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
>>> assert 1 == 1
>>> assert 1 == 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>

So usually our tests will contain some code and one or more assertions. I prefer to have just one assertion for each test, except perhaps when testing the very same feature more than once (for example getting various elements from a list). This way you are immediately aware of what assertion raised the exception, that is you immediately know what feature does not work as expected.

Writing some tests

So now we will pretend we have already developed our Binary class and write some tests that check its behaviour. I will add tests to check several aspects of our code and describe what they do. You can find the full project at https://github.com/TheDigitalCatOnline/python-oop-tdd-example, and I will mention specific commits during the development.


from src.binary import Binary

def test_binary_init_int():
    binary = Binary(6)
    assert int(binary) == 6

This is our first real test. First of all we import the class from the file binary.py (which doesn't exists yet). The function test_binary_init_int() should initialise a Binary with an integer. The assertion checks that the newly created variable binary has a consistent integer representation, which is the number we used to initialize it.

We want to be able to initialise a Binary with a wide range of values: bit strings ('110'), binary strings ('0b110'), hexadecimal strings ('0x6'), hexadecimal values (0x6), lists of integers ([1,1,0]) and list of strings (['1','1','0']). The following tests check all those cases

def test_binary_init_bitstr():
    binary = Binary("110")
    assert int(binary) == 6

def test_binary_init_binstr():
    binary = Binary("0b110")
    assert int(binary) == 6

def test_binary_init_hexstr():
    binary = Binary("0x6")
    assert int(binary) == 6

def test_binary_init_hex():
    binary = Binary(0x6)
    assert int(binary) == 6

def test_binary_init_intseq():
    binary = Binary([1, 1, 0])
    assert int(binary) == 6

def test_binary_init_strseq():
    binary = Binary(["1", "1", "0"])
    assert int(binary) == 6

Finally, let us check that our Binary class cannot be initialised with a negative number. I decided to represent negative numbers through the two's complement technique, which however requires a predefined bit length. So for simple binaries I just discard negative numbers.

import pytest


def test_binary_init_negative():
    with pytest.raises(ValueError):

As you can see, now we have to check that our class raises an exception, but if we make the class raise it the test will fail. To let the test pass we shall check that the exception is raised but suppress it, and this can be done with pytest.raises, which is a suitable context manager.


I want to check that my binary numbers can be correctly converted to integers (through int()), binary strings (through bin()), hexadecimals (through hex()) and to strings (through str()). I want the string representation to be a plain sequence of zeros and ones, that is the binary string representation without the 0b prefix.

Some examples (check the full code for the whole set of tests)

def test_binary_int():
    binary = Binary(6)
    assert int(binary) == 6

def test_binary_str():
    binary = Binary(6)
    assert str(binary) == '110'

Writing the class

Trying to run the tests at this point just returns a big failure due to an import error, since the binary.py module does not exists yet.

$ pytest -v
===================== test session starts =====================
collected 0 items / 1 errors
============================ ERRORS ===========================
____________ ERROR collecting tests/test_binary.py ____________
tests/test_binary.py:3: in <module>
    from src.binary import Binary
E   ModuleNotFoundError: No module named 'src.binary'
=================== 1 error in 0.01 seconds ===================

Let us create the file binary.py and start creating the Binary class.

class Binary:

Now when you run Pytest the output shows that all tests are found and that all of them fail (I will just show the first one)

$ pytest -v
===================== test session starts =====================


collected 13 items 

tests/test_binary.py::test_binary_init_int FAILED       [  7%]
tests/test_binary.py::test_binary_init_negative FAILED  [ 15%]
tests/test_binary.py::test_binary_init_bitstr FAILED    [ 23%]
tests/test_binary.py::test_binary_init_binstr FAILED    [ 30%]
tests/test_binary.py::test_binary_init_hexstr FAILED    [ 38%]
tests/test_binary.py::test_binary_init_hex FAILED       [ 46%]
tests/test_binary.py::test_binary_init_intseq FAILED    [ 53%]
tests/test_binary.py::test_binary_init_strseq FAILED    [ 61%]
tests/test_binary.py::test_binary_eq FAILED             [ 69%]
tests/test_binary.py::test_binary_int FAILED            [ 76%]
tests/test_binary.py::test_binary_bin FAILED            [ 84%]
tests/test_binary.py::test_binary_str FAILED            [ 92%]
tests/test_binary.py::test_binary_hex FAILED            [100%]

============================ FAILURES ==========================
_____________________ test_binary_init_int _____________________

    def test_binary_init_int():
>       binary = Binary(6)
E       TypeError: Binary() takes no arguments

tests/test_binary.py:7: TypeError


=================== 13 failed in 0.03 seconds ==================

So now you can start writing code and use your test battery to check if it works as expected. Obviously "as expected" means that all the tests you have pass, but this does not imply you covered all cases. TDD is an iterative methodology: when you find a bug or a missing feature you first write a good test or a set of tests that address the matter and then produce some code that make the tests pass.

At this point you are warmly encouraged to write the code by yourself and to check your product with the given battery of tests. Create the file tests/test_binary.py and copy into it one test at a time. Read the test carefully to understand what you need to implement and start writing the class. When you think you are done with a part of it just run the tests and see if everything works well, then move on with the new one


My solution

The most complex part of the class is the initialization, since I want it to accept a wide range of data types. Basically we have to deal with sequences (strings and lists) or with plain values. The latter ones shall be convertible to an integer, otherwise trying to initialise a binary number with them makes no sense. Since binary numbers are just a representation of integers I decided to store the value in the class as an integer inside the self._value attribute. Pay attention that this decision means that all leading zeros will be stripped from the number, i.e., Binary('000101') is equal to Binary('101'). This will be important for indexing and slicing.

This is the code

from collections.abc import Sequence

class Binary:
    def __init__(self, value=0):
        if isinstance(value, Sequence):
            if len(value) > 2 and value[0:2] == "0b":
                self._value = int(value, base=2)
            elif len(value) > 2 and value[0:2] == "0x":
                self._value = int(value, base=16)
                self._value = int("".join([str(i) for i in value]), base=2)
                self._value = int(value)
                if self._value < 0:
                    raise ValueError("Binary cannot accept negative numbers")
            except ValueError:
                raise ValueError(f"Cannot convert value {value} to Binary")

    def __int__(self):
        return self._value

Running the tests I get 4 failed, 9 passed in 0.02 seconds, which is a good score. The tests that still fail are test_binary_eq, test_binary_bin, test_binary_str and test_binary_hex. Since I still wrote no code for the conversions those failures were expected.

Let us review the code I wrote. I make use of the module collections.abc to check if the input is a sequence of a plain value. If you do not know what Abstract Base Classes are, please check this post and the documentation of the module collections.abc.

Basically through isinstance(value, Sequence) we check that the incoming value behaves like a sequence, which is different from saying that it is a list, a string, or other sequences. The first case covers an incoming string in the form 0bXXXXX, which is converted to an integer through the int() function. The second case is the same but for hexadecimal strings in the form 0xXXXXX.

The third case covers a generic sequence of values that shall be individually convertible to 0 or 1. The code converts each element of the sequence to a string, joins them in a single string and converts it with base 2. This covers the case of a string of zeros and ones and the case of an iterable of integers, like a list for example.

If the incoming value is not a sequence it shall be convertible to an integer, which is exactly what the try part does. Here we also check if the value is negative and raise a suitable exception.

Finally, the __int__() method (one of the Python magic methods) is automatically called when we apply int() to our binary, just like we do in a lot of the tests. This method is basically the one responsible of providing a conversion to integer of a given class. In this case we just have to return the value we stored internally.

Please note that I didn't write this code in a single burst. I had to run the tests more than once to tune my code.

I already wrote the method that performs the conversion to an integer. Some tests however (namely test_binary_bin and test_binary_hex) still fail with the error message TypeError: 'Binary' object cannot be interpreted as an integer.

According to the official documentation, "If x is not a Python int object, it has to define an __index__() method that returns an integer." so this is what we are missing. As for __index__(), the documentation states that "In order to have a coherent integer type class, when __index__() is defined __int__() should also be defined, and both should return the same value."

So we just have to add

def __index__(self):
    return self.__int__()

inside the class, and we get two more successful tests.

To make test_binary_str pass we have to provide a magic method that converts the object into a string, which is

def __str__(self):
    return bin(self)[2:]

It makes use of the internal Python algorithm provided by bin() stripping the 0b prefix.

The last failing test is test_binary_eq which tests for equality between two Binary objects. Out of the box, Python compares objects on a very low level, just checking if the two references point to the same object in memory. To make it smarter we have to provide the __eq__() method

def __eq__(self, other):
    return int(self) == int(other)

And now all the tests run successfully.


Binary operations

Now it is time to add new features to our Binary class. As already said, the TDD methodology wants us to first write the tests, then to write the code. Our class is missing some basic arithmetic and binary operations. Some of the test we can add are

def test_binary_addition_int():
    assert Binary(4) + 1 == Binary(5)

def test_binary_addition_binary():
    assert Binary(4) + Binary(5) == Binary(9)

These check that adding both an integer and a Binary to a Binary works as expected.

def test_binary_division_int():
    assert Binary(20) / 4 == Binary(5)

def test_binary_division_rem_int():
    assert Binary(21) / 4 == Binary(5)

These check two different cases because division can produce a remainder, which is not considered here.

def test_binary_get_bit():
    binary = Binary('0101110001')
    assert binary[0] == '1'
    assert binary[5] == '1'

def test_binary_not():
    assert ~Binary('1101') == Binary('10')

def test_binary_and():
    assert Binary('1101') & Binary('1') == Binary('1')

def test_binary_shl_pos():
    assert Binary('1101') << 5 == Binary('110100000')

The function test_binary_get_bit tests indexing and is one of the few tests that contain more than one assertion. Please note that binary indexing, unlike standard sequence indexing in Python, starts from the rightmost element.

Bitwise and arithmetic operations are implemented using Python magic methods. Please check the official documentation for a complete list of operators and related methods.

The code that implements the required behaviour is

    def __and__(self, other):
        return Binary(self._value & Binary(other)._value)

    def __or__(self, other):
        return Binary(self._value | Binary(other)._value)

    def __xor__(self, other):
        return Binary(self._value ^ Binary(other)._value)

    def __lshift__(self, pos):
        return Binary(self._value << pos)

    def __rshift__(self, pos):
        return Binary(self._value >> pos)

    def __add__(self, other):
        return Binary(self._value + Binary(other)._value)

    def __sub__(self, other):
        return Binary(self._value - Binary(other)._value)

    def __mul__(self, other):
        return Binary(self._value * Binary(other)._value)

    def __truediv__(self, other):
        return Binary(int(self._value / Binary(other)._value))

    def __invert__(self):
        return Binary([abs(int(i) - 1) for i in str(self)])

The method __invert__() is called when performing a bitwise NOT operation (~) and is implemented avoiding negative numbers. A simple solution is to convert the Binary into a string and then reverse every digit (abs(int(i) - 1) returns 0 for '1' and 1 for '0').



I want Binary to support indexing, just like lists. The difference between lists and the Binary type is that for the latter indexes start from the rightmost element.

We can test this in a simple way with

def test_binary_get_bit():
    binary = Binary("0101110001")
    assert binary[0] == "1"
    assert binary[5] == "1"

The implementation of this behaviour is actually very simple , but as we will see later it contains some bugs.

    def __getitem__(self, item):
        return str(self)[-(item + 1)]



Just like lists, Binary should support slicing. Always keep in mind the reversed indexing, that can be exemplified by

>>> b = Binary('01101010')
>>> b[4:7]
<binary.Binary object at 0x...> (110)
>>> b[1:3]
<binary.Binary object at 0x...> (101)

This can be directly converted into a test

def test_binary_slice():
    assert Binary('01101010')[0:3] == Binary('10')
    assert Binary('01101010')[1:4] == Binary('101')
    assert Binary('01101010')[4:] == Binary('110')

This shows that my initial implementation of the method __index__() was too trivial. The output of this test shows that the method doesn't support slicing, raising the exception TypeError: unsupported operand type(s) for +: 'slice' and 'int'. Checking the documentation of __getitem__() we notice that it shall manage both integers and slice objects (this is the missing part) and that it shall raise IndexError for illegal indexes to make for loops work. So I immediately add a test for this rule and other tests to match the correct behaviour

def test_binary_negative_index():
    assert Binary("0101110001")[-1] == "1"
    assert Binary("0101110001")[-2] == "0"

def test_binary_illegal_index():
    with pytest.raises(IndexError):

def test_binary_inappropriate_type_index():
    with pytest.raises(TypeError):

def test_binary_for_loop():
    assert [int(i) for i in Binary("01101010")] == [0, 1, 0, 1, 0, 1, 1]

Remember that leading zeros are stripped, so getting the index number 7 shall fail, since the binary number has just 7 digits, even if the incoming string has 8 characters.

I also added a test for empty slices. For those, I arbitrarily decided that an empty slice should return Binary(0).

def test_empty_slice():
    assert Binary("01101010")[4:4] == Binary("0")

Instead of trying to reimplement the whole list slicing behaviour with reversed indexes, it is much simpler to make use of it. We can just take the string version of our Binary, slice its reversed version and return the result (reversed again). The result of the slice can be a single element, however, so we have to check it against the Sequence class

    def __getitem__(self, key):
        reversed_list = [int(i) for i in reversed(str(self))]
        sliced = reversed_list.__getitem__(key)
        if isinstance(sliced, Sequence):
            if len(sliced) > 0:
                return Binary([i for i in reversed(sliced)])
                return Binary(0)
            return Binary(sliced)

The first list comprehension returns a list of integers with the reversed version of the binary number (i.e. from Binary('01101') to [1, 0, 1, 1], remember that leading zeros are stripped). Then we delegate the slice to the list type, calling __getitem__(). The result of this call may be a sequence or a single element (integer), so we tell apart the two cases. In the first case we reverse the result again, in the second case we just return it. In both cases we create a Binary object. The check on the length of the list must be introduced because the slice may return no elements, but the Binary class does not accept an empty list.


Splitting binaries

The last feature I want to add is a function split() that divides the binary number in two binaries. The rightmost one shall have the given size in bits, while the leftmost just contains the remaining bits. The following tests exemplify the behaviour of split()

def test_binary_split_no_remainder():
    assert Binary('110').split(4) == (0, Binary('110'))

def test_binary_split_remainder():
    assert Binary('110').split(2) == (1, Binary('10'))

def test_binary_split_exact():
    assert Binary('100010110').split(9) == (0, Binary('100010110'))

def test_binary_split_leading_zeros():
    assert Binary('100010110').split(8) == (1, Binary('10110'))

The code that implements it leverages the slicing behaviour

def split(self, bits):
    return (self[bits:], self[:bits])



Final words

If you tried and write your own class before checking my solution I'm sure you experienced both some frustration when tests failed and a great joy when they finally passed. I'm also sure that you could appreciate the simplicity of TDD and perhaps understand why so many programmes adopt it.

In the next post I will guide you through the addition of the SizeBinary class, again following the TDD methodology.


2015-05-15 As suggested by Jacob Zimmerman the class lacks some methods to be a complete numeric class, most notably __radd__ and __rsub__. Indeed, my first goal was to show TDD so I did not add the whole series of reflected arithmetic operations. You will find all those methods here and try to implement them following the methodology shown in the post. Jacob also suggested to shorten the __str__() implementation, and I fixed it. Thanks Jacob!

2015-09-22 Christopher McCormack spotted an error about binary indexing: "starts from the leftmost element" should be "starts from the rightmost element". Now it has been fixed. Thanks Christopher!

2016-12-20 GitHub user dndln found an error in the 'Binary operations' section. The test_binary_get_bit() test included the assertion assert binary[9] == '0' which cannot be successful, since leading zeros are stripped, as stated in the previous section 'My Solution'. The attached code files were already correct. Thanks a lot for pointing it out!

2022-11-25 GitHub user rioj7 found several typos and corrected them. He also contributed a better explanation for the internal representation of decimal numbers. Thanks!


Feel free to reach me on Twitter if you have questions. The GitHub issues page is the best place to submit corrections.

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