Creating a Module

To create your own module you just need to create a .py file that includes your code. Generally, this will include functions.

Later we will look at including classes.

Modules can include both functions and variables.

1. Creating a Module

Create a file called calculator1.py.

Includes two functions and a variable:

# calculator1.py

# terrible approximation of pi
pi = 3.14

def add(x,y):
  return x + y

def mul(x,y):
  return x * y

Create another file in the same directory called main.py. We can then use calculator.py in our main.py via an import.

# main.py
import calculator1 as calc

print(calc.add(3,4)) # use the add function from calc
print(calc.mul(3,4)) # use the mul function from calc
print(calc.pi)       # use the pi variable 

For now, it's enough that the module is in the same folder as your main.py. Later we will discuss ways to organise your project.

2. if name == "__main__"

We have used this during this module already, but now it is time to explain why.

Now create two new files called calculator2.py and calculator3.py.

calculator2.py should contain the following code.

# calculator2.py
# terrible approximation of pi
pi = 3.14

def add(x,y):
  return x + y

def mul(x,y):
  return x * y

print(f"These are lines printed inside {__name__}")
print("For example we might like to test the add function")
print(add(3, 4))

calculator3.py should contain the following code.

# calculator3.py
# terrible approximation of pi
pi = 3.14

def add(x,y):
  return x + y

def mul(x,y):
  return x * y

if __name__ == "__main__":
  print(f"These are lines printed inside {__name__}")
  print("For example we might like to test the add function")
  print(add(3, 4))

The difference is that calculator3.py has this code contained within the if __name__ == "__main__" block.

__name__ is a special reserved name in Python which is used to keep track of the names of the files that are being run and imported.

The file that you run, normally main.py but it can be any file, takes on the name __main__.

Try running all of these files and look at the differences in the output.

Note if you want to run them manually from the terminal you can do by opening the terminal and typing ls. This will list the files in the current directory.

Type python main.py to run the main file.

Type python calculator2.py to run the calculator2.py file.

Type python calculator2.py to run the calculator3.py file.

You will not that __name__ for both calculator2.py and calculator3.py is __main__. This is because it is the starting (main) file being run by Python.

OK, back to if __name__ == "__main__".

What this does is tell python only to run that block of code if you are running calculator3.py.

Why do this?

Well, try importing calculator2.py and using it.

import calculator2 as calc

print(calc.add(3,4)) # use the add function from calc
print(calc.mul(3,4)) # use the mul function from calc
print(calc.pi)       # use the pi variable 

This will print out the following:

These are lines printed inside calculator2.py
For example, we might like to test the add function
7
7
12
3.14

Oh, dear! It printed out the code that is contained at the bottom of the file as well, really we just wanted to use the functions.

Now try it with calculator3.py:

import calculator3 as calc

print(calc.add(3,4)) # use the add function from calc
print(calc.mul(3,4)) # use the mul function from calc
print(calc.pi)       # use the pi variable 

This will print out the following:

7
12
3.14

Much better! Now it just imported the functions for us to use.

Try running calculator3.py again.

This will run the file and print out the following.

These are lines printed inside __main__
For example, we might like to test the add function
7

Notice now that we are running the file itself, so the if __name__ == "__main__" block runs!

=== TASK ===

Create a new file called hello.py

Your module should include two functions called hello() and goodbye() which take in a name and returns a string.

For example, hello() should operate as follows.

hello("Joe") # returns "Hello Joe"
print(hello("Joe")) # prints Hello Joe

And, goodbye() should operate as follows.

goodbye("Joe") # returns "Goodbye Joe"
print(goodbye("Joe")) # prints Goodbye Joe

I have included this code so that you can use in main.py to test your module.

# main.py
import hello

def main():
  print(hello.hello("Joe")) # prints Hello Joe
  print(hello.goodbye("Joe")) # prints Goodbye Joe

if __name__ == "__main__":
  main()

If you have done this correctly, the output of your program should be:

Hello Joe
Goodbye Joe