Python has many magic functions that will allow your classes to do all kinds of fun stuff. You may want your object to act like a dictionary, or a list, you may want to make sure str() prints your object a certain way, or you may want to override operators. You can tell your object how to act with the "in" clause and how to deconstruct when using the "with" clause. There are so many other magic methods as well.
Here's a simple example of using magic functions
class OppositeMath():
def __init__(self, num):
self.num = num
def __eq__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return self.num != num
def __gt__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return self.num <= num
def __ge__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return self.num < num
def __lt__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return self.num >= num
def __le__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return self.num > num
def __add__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return OppositeMath(self.num - num)
def __sub__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return OppositeMath(self.num + num)
def __mul__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return OppositeMath(self.num / num)
def __div__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return OppositeMath(self.num * num)
def __truediv__(self, num):
return self.__div__(num)
def __str__(self):
return str(self.num)
def __int__(self):
return int(self.num)
def __long__(self):
return long(self.num)
def __float__(self):
return float(self.num)
a = OppositeMath(2)
b = OppositeMath(4)
print(str(a + b)) # -2
print(str(a - b)) # 6
print(str(a * b)) # .5
print(str(a / b)) # 8
print(str(a > b)) # True
print(str(a < b)) # False
print(str(a == b)) # True
print(str(a >= b)) # True
print(str(a <= b)) # False
You can test this example with repl.it
But look at what we can do now, we can do math backwards! Obviously there are several other mathematical functions that should probably be implemented as well like square root and such, but this is a nice start.
So let's look at __add__.
def __add__(self, num):
if isinstance(num, OppositeMath):
num = num.num
return OppositeMath(self.num - num)
When a + b is run a will be the self that comes into the __add__ method and b will be num. So the left hand side must always be an OppositeMath. I check to see if the right hand side is an OppositeMath, and if it is I do math on the OppositeMath's num variable. I also return an OppositeMath object because we may want to do more opposite math after returning the value! Every other magic function is pretty similar for this example. I also implement str, float, int, and long to tell the interpreter what to do if someone tries to cast an OppositeMath to those types.
OppositeMath itself isn't very useful, but you could do something like implement an event dispatcher that uses the magic functions iadd and isub to take advantage of += and -=. You could have those operators add and remove events.
Here's a good resource for more magic methods: http://www.rafekettler.com/magicmethods.html.
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