Learn Python With Tests

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Learn Python With Tests
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.
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# filename: test_hello.py
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​
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if __name__ == '__main__':
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    pass
Copied!
# 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. 
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
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$ ./python 
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Python 3.8.1 (default, Jan 10 2020, 06:25:32) 
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[GCC 9.2.1 20191008] on linux
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Type "help", "copyright", "credits" or "license" for more information.
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>>>
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Write tests first - 1
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# filename: test_hello.py
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​
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def test_hello() -> None:
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    expected = "Hello, World!"
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    actual = get_hello()
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    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)
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​
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​
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if __name__ == '__main__':
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    test_hello()
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Exercise tests - 1
Run the tests
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$ python test_hello.py 
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Traceback (most recent call last):
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  File "test_hello.py", line 11, in <module>
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    test_hello()
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  File "test_hello.py", line 6, in test_hello
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    actual = get_hello()
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NameError: name 'get_hello' is not defined
Copied!
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.
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def get_hello() -> None:
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    pass
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​
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​
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def test_hello() -> None:
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    expected = "Hello, World!"
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    actual = get_hello()
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    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)
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​
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​
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if __name__ == '__main__':
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    test_hello()
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Exercise tests - 2
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$ python test_hello.py 
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Traceback (most recent call last):
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  File "test_hello.py", line 15, in <module>
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    test_hello()
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  File "test_hello.py", line 11, in test_hello
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    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)
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AssertionError: None != Hello, World!
Copied!
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"
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def get_hello() -> str:
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    return "Hello, World!"
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The full test code will be
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# filename: test_hello.py
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​
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​
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def get_hello() -> str:
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    return "Hello, World!"
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​
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​
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def test_hello() -> None:
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    expected = "Hello, World!"
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    actual = get_hello()
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    assert actual == expected, "{actual} != {expected}".format(actual=actual, expected=expected)
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​
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​
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if __name__ == '__main__':
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    test_hello()
Copied!
Running this will give no output indicating that our test was successful.
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$ python test_hello.py
Copied!
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​
Setup
Last modified 2yr ago
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Contents
Inspiration
Boiler plate code.
Hello, World
Write tests first - 1