---

---

---

functions 3

Since I know how to use for loops, I can do better with the assertions of test_why_use_a_function

---

open the project

• I change directory to the functions folder

  cd functions
  

  the terminal shows I am in the functions folder

  .../pumping_python/functions (main)
  

• I use pytest-watcher to run the tests

  uv run pytest-watcher . --now
  

  the terminal shows

  rootdir: .../pumping_python/functions
  configfile: pyproject.toml
  collected 12 items
  
  tests/test_functions.py ............                          [100%]
  
  ======================= 12 passed in X.YZs =========================
  

• I hold ctrl on the keyboard and click on tests/test_functions.py to open it in the editor

---

a better way to test why use a function

---

RED: make it fail

---

• I add a for loop and `range object`_ to test_why_use_a_function

      def test_why_use_a_function(self):
          def add_x(x=3, y=0):
              return x + y
  
          x = 3
          y = 0
          self.assertEqual(add_x(y=0), 3)
  

• I use the variables in the assertion

          x = 3
          y = 0
          # self.assertEqual(add_x(y=0), 3)
          self.assertEqual(add_x(x, y), x+x)
          self.assertEqual(add_x(y=1), 4)
  

  the terminal shows AssertionError

  AssertionError: 3 != 6
  

---

GREEN: make it pass

---

I change the expectation

          x = 3
          y = 0
          # self.assertEqual(add_x(y=0), 3)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=1), 4)

the test passes

---

REFACTOR: make it better

---

• I remove the comment then do the same thing for the next assertion

          x = 3
          y = 0
          self.assertEqual(add_x(x, y), x+y)
          y = 1
          # self.assertEqual(add_x(y=1), 4)
          self.assertEqual(add_x(x, y), x+x)
          self.assertEqual(add_x(y=2), 5)
  

  the terminal shows AssertionError

  AssertionError: 4 != 6
  

• I change the expectation

          y = 1
          # self.assertEqual(add_x(y=1), 4)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=2), 5)
  

  the test passes. Wait a minute, self.assertEqual(add_x(x, y), x+y) is also a repetition!

• I remove the comment then add a variable for the next assertion

          y = 1
          self.assertEqual(add_x(x, y), x+y)
          y = 2
          # self.assertEqual(add_x(y=2), 5)
          self.assertEqual(add_x(x, y), y+y)
          self.assertEqual(add_x(y=3), 6)
  

  the terminal shows AssertionError

  AssertionError: 5 != 4
  

• I change the expectation to the right calculation

          y = 2
          # self.assertEqual(add_x(y=2), 5)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=3), 6)
  

  the test passes, this is the same as the add function from how to make a calculator

• I remove the comment, then add a variable for the next assertion

          y = 2
          self.assertEqual(add_x(x, y), x+y)
          y = 3
          # self.assertEqual(add_x(y=3), 6)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=4), 7)
  

  the test is still green because x and y are both 3

• on to the next one

          y = 3
          self.assertEqual(add_x(x, y), x+y)
          y = 4
          # self.assertEqual(add_x(y=4), 7)
          self.assertEqual(add_x(x, y), y+y)
          self.assertEqual(add_x(y=5), 8)
  

  the terminal shows AssertionError

  AssertionError: 7 != 8
  

• I change the expectation

          y = 4
          # self.assertEqual(add_x(y=4), 7)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=5), 8)
  

  the test passes

• I remove the comment, then add a variable

          y = 4
          self.assertEqual(add_x(x, y), x+y)
          y = 5
          # self.assertEqual(add_x(y=5), 8)
          self.assertEqual(add_x(x, x), x+y)
          self.assertEqual(add_x(y=6), 9)
  

  the terminal shows AssertionError

  AssertionError: 6 != 8
  

• I change the call

          y = 5
          # self.assertEqual(add_x(y=5), 8)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=6), 9)
  

  the test passes

• I remove the comment then add a variable

          y = 5
          self.assertEqual(add_x(x, y), x+y)
          y = 6
          # self.assertEqual(add_x(y=6), 9)
          self.assertEqual(add_x(y, y), x+y)
          self.assertEqual(add_x(y=7), 10)
  

  the terminal shows AssertionError

  AssertionError: 12 != 9
  

• I change the call

          y = 6
          # self.assertEqual(add_x(y=6), 9)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=7), 10)
  

  the test passes

• I remove the comment then add a variable

          y = 6
          self.assertEqual(add_x(x, y), x+y)
          y = 7
          # self.assertEqual(add_x(y=7), 10)
          self.assertEqual(add_x(x, y), y+y)
          self.assertEqual(add_x(y=8), 11)
  

  the terminal shows AssertionError

  AssertionError: 10 != 14
  

• I change the expectation

          y = 7
          # self.assertEqual(add_x(y=7), 10)
          self.assertEqual(add_x(x, y), x+y)
          self.assertEqual(add_x(y=8), 11)
  

  the test passes

• I remove the comment then add one more variable

          y = 7
          self.assertEqual(add_x(x, y), x+y)
          y = 8
          # self.assertEqual(add_x(y=8), 11)
          self.assertEqual(add_x(x, y), x+x)
  

  the terminal shows AssertionError

  AssertionError: 11 != 6
  

• I change the expectation

          y = 8
          # self.assertEqual(add_x(y=8), 11)
          self.assertEqual(add_x(x, y), x+y)
  

  the test passes

• I remove the comment then use the Rename Symbol feature to change the name of the add_x function to add

      def test_why_use_a_function(self):
          def add(x=3, y=0):
              return x + y
  
          x = 4
          y = 0
          self.assertEqual(add(x, y), x+y)
          y = 1
          self.assertEqual(add(x, y), x+y)
          y = 2
          self.assertEqual(add(x, y), x+y)
          y = 3
          self.assertEqual(add(x, y), x+y)
          y = 4
          self.assertEqual(add(x, y), x+y)
          y = 5
          self.assertEqual(add(x, y), x+y)
          y = 6
          self.assertEqual(add(x, y), x+y)
          y = 7
          self.assertEqual(add(x, y), x+y)
          y = 8
          self.assertEqual(add(x, y), x+y)
  
      def test_making_a_function_w_pass(self):
  

• I only have to make a change in one place if I want to test what happens when I add 4 to a number. I change x

      def test_why_use_a_function(self):
          def add(x=3, y=0):
              return x + y
  
          x = 4
          y = 0
          self.assertEqual(add(x, y), x+y)
  

  the test is still green

• I only have to make a change in one place if I want to test what happens when I add 5 to a number. I change x

      def test_why_use_a_function(self):
          def add(x=3, y=0):
              return x + y
  
          x = 5
          y = 0
          self.assertEqual(add_x(x, y), x+y)
  

  still green.

• I only have to make a change in one place if I want to test what happens when I add 6 to a number. I change x

      def test_why_use_a_function(self):
          def add(x=3, y=0):
              return x + y
  
          x = 6
          y = 0
          self.assertEqual(add_x(x, y), x+y)
  

  green. Much better than what I had before, the test passes for any number I try, but there has to be a better way that does not need all these lines of code

I can use a variable to remove duplication

---

close the project

• I close test_functions.py and functions.py in the editor

• I click in the terminal and use q on the keyboard to leave the tests and the terminal goes back to the command line

• I change directory to the parent of functions

  cd ..
  

  the terminal shows

  .../pumping_python
  

  I am back in the pumping_python directory

---

review

I ran tests to show that I can make functions with

• the def keyword

• positional arguments

• keyword arguments

• positional and keyword arguments

• default values

• can take any number of inputs

as a reminder

• I can use ‘*args’ when I do not know how many positional arguments the function has to take

• positional arguments are taken as a tuple

• positional arguments must come before keyword arguments

• I can use ‘**kwargs’ when I do not know how many keyword arguments the function

• keyword arguments are taken as a dictionary

• the identity function returns its input

• constant functions always return the same thing

• nothing runs after the return statement in a function

• functions return None by default

How many questions can you answer about functions?

---

code from the chapter

Do you want to see all the CODE I typed in this chapter?

---

what is next?

you know

• how to make a test driven development environment manually

• how to raise AssertionError with assert methods

• how to make functions

• what causes AttributeError

• how to pass values from tests to functions

• what is None and NOT None

• what is True and False in Python

• how to write programs that make decisions

• how to make a calculator

• how to use a variable

There is a problem, I have done these same steps for each of the chapters up till now

• I give the project a name

• I make a directory for the project

• I change directory to the project

• I make a directory for the source code

• I make a Python file to hold the source code in the ‘src’ folder

• I make a directory for the tests

• I make the ‘tests’ directory a Python package

• I make a Python file for the tests in the ‘tests’ directory

• I add the first failing test to the test file

• I make a requirements file for the Python packages I need

• I setup the project with uv

• I install the Python packages listed in the requirements file

• I run the tests automatically

• I open the test file in the editor from the terminal

• I make the test pass

• then I start working on the project

I think I know how to make a Python Test Driven Development environment. I am going to write a program that will do all the steps for making a project for me, so I never have to do those steps again.

Would you like to know how to make a Python Test Driven Development environment automatically?

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