Question

I’m up to Exercise 41 in Learn Python the Hard Way, and I’m having a really hard time wrapping my brain around the fact that the entire thing hinges on a function running just because it’s been assigned as a value to a variable. I wrote up a little script to confirm that this is how it works, and it does:

def pants():
    print "Put on some pants!"

def shorts():
    print "And don't forget your underwear!"

zap = pants()
thing = shorts()

With the results being:

Put on some pants!
And don't forget your underwear! 

So obviously this happens, but I can’t understand why the language works that way — what the logic is behind the language that makes this a valuable way of operating. I think it’d be helpful for me moving forward to understand why this is, rather than just “that’s the way it works.”

For clarity: I’m asking (I guess) why the function is running, when all I’m doing is assigning it as a value for something. The print statements are just there so I can see that the function is indeed running.

It’s the fact that I’m not ever actually running

pants()
shorts()

that is confusing me.

To create a tortured analogy, if me-baking-cookies-at-home were “cookies()”, and I were to make cookies on Saturdays, I might eventually believe that

Saturday = cookies()

but just thinking “hey, Saturday is cookie day” is not the same as actually baking cookies… so why does just saying

Saturday = cookies()

actually bake the cookies, rather than just setting up Saturday with the variable “cookies()” for some later use?

Answer 1

When you use the parentheses () the function gets called. If you want to assign the function to the variable to reuse it you should remove there parentheses.

Example:

def pants():
    print "Put on some pants!"

def shorts():
    print "And don't forget your underwear!"

zap = pants
thing = shorts

And then when you want to call those functions:

zap()
thing()