Q and A for Chapter 3.4 – 3.14

CS 104Victor Norman

Creating new functions

Q: Can you create new functions with new names, or just use the one they show in the book?

A: You can create any function you want with any legal name (legal identifier, like for variables). You must define the function before calling it.

Creating new functions

def aFunc(param1, param2): # code indented 4 spaces that does stuff print param1 + param2

def yell(param1): # param1 must be str print param1.upper()

aFunc(3, 4)aFunc(“Hello”, “world”)hi = “Greetings, earthling.”yell(hi)

String functions

Q: Why create functions that work with strings? Functions to compute math stuff are obvious.A: Lots of computing is with strings. def prependTime(outStr): # code here to get the time print time + “: “ + outStr

Function Parameters

Q: Can functions take more than 1 or 2 parameters?A: Yes: a function definition specifies how many parameters it takes. The function call must provide that many arguments.(Reminder: a function call evaluates its arguments before calling the function, where the parameters point to those values within the function code.)

Functions2

Q: Can you define a function inside a function?

A: You can, but you better not until you’ve been working with python for a few years. And, besides, that inner function definition goes away when the outer function call returns, so it isn’t very useful.

Stack frames! Help!

Q: I need help with this stack frames business. Does python actually do this?

A: Python does this internally, to keep track of what code is calling what code, etc. You don’t see this when it is running.Check this out.

Local Variables

Q: I don’t understand local variables.

A: (That’s not a question… :-)A local variable in a function is a name that refers to a value, but is created when the function code is run, and is deleted when the function returns.(A parameter is a local variable that is initialized to refer to the value passed in by the corresponding argument.)

References to functions

Q: What happens when you call a function without ()? Like this: supposed print_twice was defined, but you “call” it this way:print_twice(without the ())A: This is just a reference to the function print_twice. It is a reference to where the function is defined in memory. It is not a function call.

void functions?

Q: What is the point of a void function that doesn’t return something?A: Functions that don’t return things usually:• print something, or,• alter some object that is passed in.

So, they can be very useful.

from someModule import *

Q: Why not always just import everything from a module? So, that you don’t have to do module.function or module.variable every time?A: You will pollute the namespace by doing it this way. But, that’s OK… for now… (We’ll talk about this much much later.)