[Tutor] n00b question: dictionaries and functions.

[Bob Gailer]

Jesse wrote:
> Hey, this should be an easy question for you guys. I'm writing my 
> first program (which Bob, Alan, and Danny have already helped me 
> with--thanks, guys!), and I'm trying to create a simple command-line 
> interface. I have a good portion of the program's "core functions" 
> already written, and I want to create a dictionary of functions. When 
> the program starts up, a global variable named command will be 
> assigned the value of whatever the user types:
>  
> command = raw_input("Cmd > ")
>  
> If command is equivalent to a key in the dictionary of functions, that 
> key's function will be called. Here's an example that I wrote for the 
> sake of isolating the problem:
>  
>
> def add():
>     x = float(raw_input("Enter a number: "))
>     y = float(raw_input("And a second number: "))
>     print x + y
> def subtract():
>     x = float(raw_input("Enter a number: "))
>     y = float(raw_input("And a second number: "))
>     print x - y
>    
>
> commands = {"add": add(), "subtract": subtract()}
>
>
>  
> Now, before I could even get to writing the while loop that would take 
> a command and call the function associated with that command in the 
> commands dictionary, I ran this bit of code and, to my dismay, both 
> add() and subtract() were called. So I tried the following:
>  
> def add(x, y):
>     x = float(raw_input("Enter a numer: "))
>     y = float(raw_input("And a second number: "))
> add = add()
>  
> When I ran this, add() was called. I don't understand why, though. 
> Surely I didn't call add(), I merely stored the function call in the 
> name add. I would expect the following code to call add:
>  
> def add(x, y):
>     x = float(raw_input("Enter a numer: "))
>     y = float(raw_input("And a second number: "))
> add = add()
> add
>  
> Can someone clear up my misunderstanding here? I don't want to end up 
> writing a long while loop of conditional statements just to effect a 
> command-line interface.
"stored the function call" NO - this mixing 2 things. What you want is 
to store a reference to the function, then call the function thru its 
reference in response to user input. add is a reference to the function. 
add() calls (runs) the function.

commands = {"add": add, "subtract": subtract} # stores function references
command = raw_input("Cmd > ")
if command in commands:
    commands[command]() # retrieve a function reference and calls it.