Running a Python command

• Let’s say I ask you to run a python command print("Hello World"). How do you do it?
• You can run it in:
  • Python shell
  • Jupyter notebook
  • Python script
• You are familiar with Jupyter notebooks already, let’s look at the other two.

What’s a Python shell?

• A Python shell is an interactive environment where you can type and execute Python commands one at a time.
• You can start a Python shell by typing python or python3 in your terminal or command prompt.
• You’ll see a prompt that looks something like this: >>>
• Go ahead and write out print("Hello World") in the shell and hit enter.
• You should see Hello World printed out immediately.

Why have Python scripts if we have a Python shell?

• With a Python shell, you can only run one command at a time.
• With a Python script, you can write multiple lines of code and save them in a file with a .py extension, say my_script.py.
• You can then run the entire script at once by typing python my_script.py in your terminal or command prompt.
• This is useful for writing longer programs or automating tasks or for letting your other scripts use your code.

Defining and calling a function

• What’s the difference between defining a function and calling a function?
  • Defining a function creates the function
  • Calling a function executes your defined function
• The code that is inside the function only runs when you call the function, not when you define it.

def greet(name):
    print(f"Hello, {name}!")

• To actually see the greeting, we need to call the function like this:

greet("Elliot")
greet("E. T.")

Hello, Elliot!
Hello, E. T.!

Return values

• When you define a function, the return statement is used to give the output of the function.
• When you call the function, you can capture the returned value in a variable.

def add(a, b):
    return a + b

• So, if you want to capture the output of the add function, you can do it like this:

result = add(3, 5)
print(result)

8

Multiple return values

• What if you want multiple outputs from a function?

def get_coordinates():
    x = 10
    y = 20
    return x, y

• You can capture the multiple return values in separate variables like this:

x_coord, y_coord = get_coordinates()
print(x_coord, y_coord)

10 20

• We have examples of t, y outputs in previous slides. This is how that works.

What happens when we import a module?

• When you import a module, Python executes all the code in that module.

• This means that any functions, classes, or variables defined in the module become available for use

• Does this mean that all the functions defined in the module are called every time we import it?

  • No, only the functions that are called in the module are executed. (remember the function definition vs function call discussion)

Code outside functions in a module (a bit advanced)

• Let’s say you need to call a function in a script, but you don’t want the other code in the script to run when you import it.
• You can use the if __name__ == "__main__": construct to achieve this.
• This is a bit advanced, so we will not go into details now, but just know that it is possible to have code in a script that only runs when the script is executed directly, and not when it is imported as a module.