> For the complete documentation index, see [llms.txt](https://orsenthil.gitbook.io/learn-python-with-tests/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://orsenthil.gitbook.io/learn-python-with-tests/master.md).

# Learn Python With Tests

This book introduces test driven development in Python. We follow a template while introducing the features of Python.

* Write tests first.
* Exercise tests.
* Fix tests.
* Refactor
* And Repeat

## Inspiration

This book is inspired from the work <https://github.com/quii/learn-go-with-tests/> which introduces Go programming using similar pattern. After reading the Go book, I felt that I could dwell into real-world go projects confidently. I wanted a similar book for Python and I decided to write the current one.

## Boiler plate code.

We want to learn Python without much overhead and dependencies. I wanted to make sure that section of the code can be copied to a filename and run with the python interpreter.

If the repetition annoys you, please excuse me, and skip ahead.

```python
# filename: test_hello.py

if __name__ == '__main__':
    pass
```

* `#` is the start of the comment.
* `#filename` indicates the filename for the snippet.
* The `if __name__ == '__main__'` is a special secret that makes sure that python interpreter will execute the

  whatever comes after this line, only when invoked using `python` interpreter. We typically write the name of the

  tests after this to demonstrate it.&#x20;

## Hello, World

This first chapter can be studied with only python interpreter installed on your system. Please install the latest release of Python from <https://www.python.org/downloads/> before we get started.

On my local system, the interpreter that I use is

```
$ ./python 
Python 3.8.1 (default, Jan 10 2020, 06:25:32) 
[GCC 9.2.1 20191008] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
```

### Write tests first - 1

```python
# filename: test_hello.py

def test_hello() -> None:
    expected = "Hello, World!"
    actual = get_hello()
    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)


if __name__ == '__main__':
    test_hello()
```

### Exercise tests - 1

* Run the tests

```
$ python test_hello.py 
Traceback (most recent call last):
  File "test_hello.py", line 11, in <module>
    test_hello()
  File "test_hello.py", line 6, in test_hello
    actual = get_hello()
NameError: name 'get_hello' is not defined
```

The `NameError` indicates we used a term that is not defined. We have not written our `get_hello` function yet , so let us write it.

### Fix tests - 1

Let us write the `get_hello` method.

```python
def get_hello() -> None:
    pass


def test_hello() -> None:
    expected = "Hello, World!"
    actual = get_hello()
    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)


if __name__ == '__main__':
    test_hello()
```

### Exercise tests - 2

```
$ python test_hello.py 
Traceback (most recent call last):
  File "test_hello.py", line 15, in <module>
    test_hello()
  File "test_hello.py", line 11, in test_hello
    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)
AssertionError: None != Hello, World!
```

Now, we see a new error, `AssertionError`, which indicates our expected output and actual output were not matching.

### Fix tests - 2

Let us make sure that `get_hello` returns the string "Hello, World"

```python
def get_hello() -> str:
    return "Hello, World!"
```

The full test code will be

```python
# filename: test_hello.py


def get_hello() -> str:
    return "Hello, World!"


def test_hello() -> None:
    expected = "Hello, World!"
    actual = get_hello()
    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)


if __name__ == '__main__':
    test_hello()
```

Running this will give no output indicating that our test was successful.

```
$ python test_hello.py
```

### Refactor

This was a very simple introduction to writing a test and exercising it. We don't have anything to refactor. Rest of the book will present examples that will explain the need for refactoring the code after it is written.

## Chapters

* [Setup](/learn-python-with-tests/setup.md)


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