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davearonson's solution

to Two Fer in the Python Track

Published at Mar 20 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

Two-fer or 2-fer is short for two for one. One for you and one for me.

"One for X, one for me."

When X is a name or "you".

If the given name is "Alice", the result should be "One for Alice, one for me." If no name is given, the result should be "One for you, one for me."

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run the appropriate command below (why they are different):

  • Python 2.7: py.test two_fer_test.py
  • Python 3.4+: pytest two_fer_test.py

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version): python -m pytest two_fer_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/two-fer directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see Running the Tests.



Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import unittest

from two_fer import two_fer

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.2.0

class TwoFerTest(unittest.TestCase):
    def test_no_name_given(self):
        self.assertEqual(two_fer(), 'One for you, one for me.')

    def test_a_name_given(self):
        self.assertEqual(two_fer("Alice"), "One for Alice, one for me.")

    def test_another_name_given(self):
        self.assertEqual(two_fer("Bob"), "One for Bob, one for me.")

if __name__ == '__main__':


import pytest

from two_fer import two_fer

ONE_FOR_YOU = "One for you, one for me."

# first, normal cases

def test_no_name():
    assert two_fer() == ONE_FOR_YOU

def test_given_name():
    assert two_fer("Alice") == "One for Alice, one for me."

# now, likely errors -- a caller may think "no name" is an empty string,
# or None, rather than not passing an argument at all.

def test_given_name_with_spaces_around():
    assert two_fer("   Alice        ") == "One for Alice, one for me."

def test_empty_name():
    assert two_fer("") == ONE_FOR_YOU

def test_none_name():
    assert two_fer(None) == ONE_FOR_YOU

# and now, some things that some jerk might do deliberately:

def test_blank_name():
    assert two_fer(" ") == ONE_FOR_YOU

def test_several_blanks_name():
    assert two_fer("     ") == ONE_FOR_YOU

def test_newline_name():
    assert two_fer("\n") == ONE_FOR_YOU

def test_number_name():
    assert two_fer(17) == ONE_FOR_YOU

def test_object_name():
    assert two_fer(object()) == ONE_FOR_YOU

# Did NOT think to test with a DIFFERENT name, as Exercism's suite
# does, to ensure that it's not just keying off of whether the name is
# present.  We could even cover both ideas in one with a random name,
# or to go really overboard, a long stretch of property testing.



def two_fer(name=DEFAULT_NAME):
    if isinstance(name, str):
        name = name.strip()
        if name == "": name = DEFAULT_NAME
        # If it's not even a string, just go with the default
        # name.  We COULD skip the "else" and just go with
        # whatever the param's string equivalent is, but I'm
        # being deliberately paranoid, albeit not quite enough
        # to raise an error.  :-)  I'm doing these exercises as
        # part of teaching some juniors to code and test, and we
        # do security software, so all my entries in this series
        # will probably have much more bulletproofing than an
        # academic exercise would justify.
        name = DEFAULT_NAME
    return f"One for {name}, one for me."

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