Snack, Vending Machine classes problem

Hello, all. I am new to programming (second semester) and it is not my strong suit. I have an assignment for class and here are the directions:
Name your source code file SweetTreats.py
a) Implement the classes Snack and VendingMachine according to the specifications given below.
b) Develop a main function to test each method in both classes.
c) Document your test cases in comments below each class.

Snack Class:
Variables
Description
isSweet
Boolean value, set to True if unspecified
name
The name of the snack, set to “ “ if unspecified
price
Holds the price of the snack, set to 1.00 if unspecified
Constructor

__init__(self, s, n, p)
Constructs a snack object with isSweet, name, price set to the
parameters s, n, p respectively.
Methods

__str__(self)
Returns a string representation of the snack with its name, price,
and isSweet values.

VendingMachine Class:
Variables
Description
snackList
A list of the snacks in the vending machine
location
A string which represents the location of the vending machine, set
to “NY” when unspecified.
Constructor

__init__(self, loc)
Constructs a vending machine object containing an empty list of
snacks and location set to the parameter loc.
Methods

addSnack(s)
Add snack s to the list of snacks.
getSaltySnack(p)
Return any salty snack of cost at most p, if one exists on the list.
Otherwise return None.
getCheapestSweet()
Return the cheapest sweet snack in the list. Return None if no
sweet snack is available.
getSweetCount()
Return the number of sweet snacks currently in the list.
display()
Displays the current list of snacks in the vending machine.

Here is my attempt at creating the snack class, any ideas as to what's wrong:
class Snack(object):

def __init__(self, s, n, p):
self.isSweet = s
self.name = n.str()
self.price = p

def isSweet(self, s):
if self.isSweet != None:
return self.isSweet
else:
self.isSweet = True
return self.isSweet

def getName(self, n):
if self.name != None:
return self.name
else:
self.name = ""
return self.name

def getPrice(self, p):
if self.price != None:
return self.price
else:
self.price = 1.00
return self.price

def __str__(self):
desc = str.("Snack: ", self.name, "costs ", self.price, "and is it sweet?", self.isSweet)
return desc

Thank you for the help, it is greatly needed and even more-so, appreciated!

Here's a better start for your Snack class. It needs more (one or two more) tests than I've provided. To preserve critical whitespace please use code tags when posting python. See link at my signature.

Thank you very much! I have a couple questions, though: Could you explain what's going on in the self.isSweet step? What does it mean that self.isSweet = not not s? Also, could you explain the last step in the __str__ method? I'm a little unsure about what all of those symbols mean...Thank you so much for the insight!

You can experiment in the python interpreter.
All python objects are either true or false.
Inputs are often bad. not not Object is the truth value of an object converted to Boolean.Since strings of length 0 are false and all other strings are true you see that what I've done is misleading.
(not not 'False') is True
To test isSweet I should use
isSweet in (False,True)
and raise an exception for bad data.

Your other question involves string formatting. The new style would read
'{}sweet {}, ${:.2f}'.format('not ','potaco thips',0.64)
Look up the % operator for strings. Strange, I can't find it either. Look here. You'll need to know
('I','am','a','tuple')

Hey, did you ever figure out the vending machine portion and would you mind posting it?