Python function has access to global variables, and doesn't need passed arguments

Question

When teaching functions in Python I get a little annoyed that the code below actually prints out the greeting. I know the preferred way would be to pass in the name, but, for students, if it works then why do something more complex. I want to teach them how to pass in arguments but if they mess up and forget it will still work. Thoughts about how to approach situations like this without being convoluted or contrived?

def greeting():
    print("Hello " + name)


name = input("Enter your name: ")
greeting()

Answer 1

Your example demonstrates how to declare and use a function with a parameter, but not why they should do so.

Actually, there are no reason (for them) why they should bother with a new concept (function) instead of writing

name = input("Enter your name: ")
print("Hello " + name)

which can be done with the programming elements they already know. And is both shorter and simpler,

So, how to sell functions with parameters to beginners?

• they avoid code duplication (the example should contain several calls)
• they are called with different values (so parameters is the simplet way to transmit information)
• they implement a decomposition of tasks into subtasks (the sooner they practice the procedural approach, the better)

Here is an example:

def draw_line(width, end, middle):
    line = end + middle*(width-2) + end;
    print(line)

def draw_rect(height, width):
   draw_line(width, "+", "-")
   for  line in range(height-2):
      draw_line(width, "|", " ")
   draw_line(width, "+", "-")

def draw_square(side):
    draw_rect(side, side)

def main(args):
    draw_square(3)
    draw_square(5)
    draw_square(7)

Actually, I use Processing with a group of students. Most of them total beginners. The first motivation for functions is decomposition.

After they draw a simple figure with a few predefined function calls, as fill() to select a fill color, rect() and ellipse(), they are taught to split the work into user-defined functions

void draw() {
    draw_sky();
    draw_beach();
    draw_sunset();
}

and parameters are then used for similar parts occurring at different places

void draw_seabird(int x, int y) {
   ....
}

Answer 2

I teach Python after teaching students C, so my context might be slightly different. That said, my students get used to program execution beginning at main() from day one. You could take this as an opportunity to introduce the same approach in Python with the following:

def greeting(name):
    print("Hello, " + name)

def main():
    greeting("tazboy")

if __name__ == "__main__":
    main()

If a student doesn't have a main function defined, they will see NameError: name 'main' is not defined.

I will admit that the boilerplate at the bottom is about as opaque as you can get. Even so, boilerplate code is something they will encounter in many other programming languages. I correlate it for my students to the MLA heading they write at the top of any essay. It's something that just has to be there, and it has a clear and important purpose.

Answer 3

I would think that any use of the same function with another parameter, stored in another variable, would solve the problem. For example, if they are pair programming:

def greeter(name):
  print("Hello, " + name)

def pair_greeter(name_one, name_two):
  print(name_one + " and " + name_two + " are a great team!")

typist_name = input("Who's the typist?")
greeter(typist_name)
observer_name = input("Who's the observer?")
greeter(observer_name)
pair_greeter(typist_name, observer_name)

Answer 4

This answer is just to add an aspect to the existing answers:

Don't blame the code given too much: It is a very good example if you want to explain how the python interpreter actually works!

You could show the code and ask the students, whether name really is available in greeting() since greeting is defined at a point in time where the interpreter does no know anything abount name. Then show a step-by-step walkthrough in a visual debugger.

Afterwards, you can explain why this is a really bad idea, e.g. by using different functions asking for names etc.