A class is basically an object which bundles together variables and methods (functions) for working with the object. We'll discuss classes following chapter 7 of
You should read all of chapter 7.
The Python tutorial also has a chapter on classes.
The book starts off with the special case of function classes. We have been using currying occassionally, so I thought it would be good to relate classes to currying.
Here is an example of function currying where we return an (affine) linear function $x \mapsto ax+b$. Here the variables $a$ and $b$ are parameters.
def linear_function(a, b):
def f(x):
return a*x + b
return f
f = linear_function(2, 1)
for x in range(2,5):
print("f({}) = {}".format(x, f(x)))
A more or less equivalent construction built for classes is the following.
class LinearFunction:
def __init__(self, a, b):
self.a = a
self.b = b
def __call__(self, x):
return self.a * x + self.b
Here we have defined a class LinearFunction
with two special methods:
__init__
which initializes a LinearFunction object.__call__
which is a special method that allows us to use function notation using the object. So if f
is a LinearFunction
we can use the notation f(x)
.The variable self
represents a particular LinearFunction
object. The __init__
method is called when a LinearFunction object is first constructed. We simply store the values of a
and b
in the LinearFunction object being built.
Then if f
is a LinearFunction
object it will have the parameters a
and b
stored in it. When we run f(x)
it executes the __call__
method. The __call__
method makes use of the values of a
and b
we have stored off to compute $ax+b$.
Here we illustrate the use of the LinearFunction
class:
f = LinearFunction(2, 1)
We constructed a LinearFunction
$f$ above. This calls the __init__
method with self
being the new object and with $a=2$ and $b=1$
The variables stored in a class object are its attributes. Our class has two attributes:
f.a
f.b
We can evaluate the __call__
method with the following:
f.__call__(2)
This executes the __call__
method with self
set to f
and with x
set to $2$. Because __call__
is a special method representing funtion application, we can also call it with:
f(2)
We can see our class works just as well as our linear_function
example using currying:
f = LinearFunction(2, 1)
for x in range(2,5):
print("f({}) = {}".format(x, f(x)))
A method is a function we can call involving from a class object. Most functions you create will not be special functions, so you can use pretty much whatever name you want. Below we add an integral
method which computes
$$\int_c^d f(x)~dx$$
if $f$ is a LinearFunction
.
class LinearFunction:
def __init__(self, a, b):
self.a = a
self.b = b
def __call__(self, x):
return self.a * x + self.b
def integral(self, c, d):
return 0.5*self.a*(d**2c**2) + self.b*(dc)
Here we demonstrate the use of our new method:
f = LinearFunction(1, 0) # The linear function x mapsto x
f.integral(0, 1) # The integral from 0 to 1
Recall that in the linear function $$x \mapsto ax+b,$$ the number $a$ represents the slope and $b$ represents the $y$intercept. We add methods for these.
class LinearFunction:
def __init__(self, a, b):
self.a = a
self.b = b
def __call__(self, x):
return self.a * x + self.b
def integral(self, c, d):
return 0.5*self.a*(d**2c**2) + self.b*(dc)
def slope(self):
return self.a
def y_intercept(self):
"return the yintercept"
return self.b
Here we demonstrate their use:
f = LinearFunction(3, 2)
f.slope()
f.y_intercept()
Often we'll want to create a class where the value represented is immutable, that is, it does not change. Allowing an object to change can often lead to unexpected results.
Our linear function can be changed because attributes can be set.
Here is an example of an issue that can arise, because we usually think of functions as things which do not change.
f = LinearFunction(3, 2)
print("f({}) = {}".format(1,f(1)))
# Now we change an attribute.
f.a = 3
print("f({}) = {}".format(1,f(1)))
We can access the methods and attributes available for an object in jupyter
by typing an object a period (e.g., f.
) and then pressing tab. Doing this you should get something like:
Note that we do not see the __init__
or __call__
methods. This is because their names begin with an underscore. So, we can hide the a
and b
attributes by giving these variables names that begin with an underscore. We'll use _a
and _b
. We make this modification to our class below:
class LinearFunction:
def __init__(self, a, b):
self._a = a
self._b = b
def __call__(self, x):
return self._a * x + self._b
def integral(self, c, d):
return 0.5*self._a*(d**2c**2) + self._b*(dc)
def slope(self):
return self._a
def y_intercept(self):
return self._b
f = LinearFunction(3, 2)
Now typing f.
and pressing tab yields:
The attributes have just been hidden. This is generally considered good enough for the purposes of creating an immutable object. While the object is not truly immutable, it means that the user is not likely to accidentally change the value of an attribute. The user can still do:
f._b = 3
It is nice to have a way to print an object to examine it. Currently we do not have a nice print out observe:
print(f)
To fix this we can create a __str__
method for converting our object to a human readable string. Here we add this method.
class LinearFunction:
def __init__(self, a, b):
self._a = a
self._b = b
def __call__(self, x):
return self._a * x + self._b
def integral(self, c, d):
return 0.5*self._a*(d**2c**2) + self._b*(dc)
def slope(self):
return self._a
def y_intercept(self):
return self._b
def __str__(self):
return "the linear function sending x to {}x + {}" \
.format(self._a, self._b)
f = LinearFunction(3, 4)
print(f)
Note that if we just leave a trailing f
at the end of a code block, we still get nonsense:
f
To fix this we add a __repr__
method. This is supposed to provide a complete representation of the object in a way that might be useful for the programmer (as opposed to __str__
which is supposed to be readable). Here we add such a method.
class LinearFunction:
def __init__(self, a, b):
self._a = a
self._b = b
def __call__(self, x):
return self._a * x + self._b
def integral(self, c, d):
return 0.5*self._a*(d**2c**2) + self._b*(dc)
def slope(self):
return self._a
def y_intercept(self):
return self._b
def __str__(self):
return "the linear function sending x to {}x + {}" \
.format(self._a, self._b)
def __repr__(self):
return "LinearFunction({}, {})".format(self._a, self._b)
f = LinearFunction(3, 4)
f
You can read more about these magic methods in this medium article. For a more technical description of the magic methods see the Python reference.
Recall that in jupyter
you can find out information about a function or class by typing the function name followed by a question mark and pressing tab. Our class and its methods have no docstring. Here we add docstrings to our function.
class LinearFunction:
"""
Represents a linear function from the real numbers to the real
numbers.
To construct the linear function mapping x to ax+b, use the
command `LinearFunction(a, b)`.
"""
def __init__(self, a, b):
"""Construct the linear function mapping x to ax+b."""
self._a = a
self._b = b
def __call__(self, x):
return self._a * x + self._b
def integral(self, c, d):
"""
Compute the integral of this function over the interval [c,d]
"""
return 0.5*self._a*(d**2c**2) + self._b*(dc)
def slope(self):
"""Return the slope of this line."""
return self._a
def y_intercept(self):
"""
Return the ycoordinate of the intersection of this line
with the yaxis. This is the same as evaluating the line
at zero.
"""
return self._b
def __str__(self):
return "the linear function sending x to {}x + {}" \
.format(self._a, self._b)
def __repr__(self):
return "LinearFunction({}, {})".format(self._a, self._b)
The docstrings are distinguished by triple parenthesis above. With this class definition, we can access documentation for the class by typing the following:
LinearFunction?
We can also access the documentation for the methods.
f = LinearFunction(0, 0)
f.slope?
f.y_intercept?
f.integral?
f.__init__?
This geeksforgeeks article has a good brief introduction to Python docstrings.
If $f:{\mathbb R} \to {\mathbb R}$ and $g:{\mathbb R} \to {\mathbb R}$ are two linear functions, then their sum
$$f+g:{\mathbb R} \to {\mathbb R}; \quad x \mapsto f(x)+g(x)$$
is also linear. We'd like to be able to add two linear functions by typing f+g
in Python. To do this we can define an __add__
function. We add a method like this below.
class LinearFunction:
"""
Represents a linear function from the real numbers to the real
numbers.
To construct the linear function mapping x to ax+b, use the
command `LinearFunction(a, b)`.
"""
def __init__(self, a, b):
"""Construct the linear function mapping x to ax+b."""
self._a = a
self._b = b
def __call__(self, x):
return self._a * x + self._b
def integral(self, c, d):
"""
Compute the integral of this function over the interval [c,d]
"""
return 0.5*self._a*(d**2c**2) + self._b*(dc)
def slope(self):
"""Return the slope of this line."""
return self._a
def y_intercept(self):
"""
Return the ycoordinate of the intersection of this line
with the yaxis. This is the same as evaluating the line
at zero.
"""
return self._b
def __str__(self):
return "the linear function sending x to {}x + {}" \
.format(self._a, self._b)
def __repr__(self):
return "LinearFunction({}, {})".format(self._a, self._b)
def __add__(self, other):
"""
Return the sum with another LinearFunction. Adding other
types will result in a TypeError.
"""
if type(other) == LinearFunction:
return LinearFunction(self._a + other._a, self._b + other._b)
raise TypeError("Only a LinearFunction can be added to a LinearFunction.")
Here we demonstrate how to carry out addition.
f = LinearFunction(1, 2)
g = LinearFunction(3, 4)
h = f+g # Results in a call to f.__add__(g)
print("The sum of {} and {} is {}.".format(f,g,h))
Note that our implementation only supports addition with a linear function. In principle we might support adding numbers as well. For example, $f+5$ is the function sending $x$ to $f(x)+5$. However, we set it up to give a TypeError.
f+5
By including __add__
the +=
operator is also useable. By default writing f += g
is the same as writing f = f+g
.
g += f
g
In multiplication (similar to addition) writing c * f
will result in c.__mul__(f)
. But if this fails, f.__rmul__
will be called. Here I am thinking of c
as a number so that
$$cf: x \mapsto c f(x).$$
Since $c$ is a number, it will not support multiplication by a LinearFunction
so instead we should implement __rmul__
to get scalar multiplication. Below we have added such an implementation.
class LinearFunction:
"""
Represents a linear function from the real numbers to the real
numbers.
To construct the linear function mapping x to ax+b, use the
command `LinearFunction(a, b)`.
"""
def __init__(self, a, b):
"""Construct the linear function mapping x to ax+b."""
self._a = a
self._b = b
def __call__(self, x):
return self._a * x + self._b
def integral(self, c, d):
"""
Compute the integral of this function over the interval [c,d]
"""
return 0.5*self._a*(d**2c**2) + self._b*(dc)
def slope(self):
"""Return the slope of this line."""
return self._a
def y_intercept(self):
"""
Return the ycoordinate of the intersection of this line
with the yaxis. This is the same as evaluating the line
at zero.
"""
return self._b
def __str__(self):
return "the linear function sending x to {}x + {}" \
.format(self._a, self._b)
def __repr__(self):
return "LinearFunction({}, {})".format(self._a, self._b)
def __add__(self, other):
"""
Return the sum with another LinearFunction. Adding other
types will result in a TypeError.
"""
if type(other) == LinearFunction:
return LinearFunction(self._a + other._a, self._b + other._b)
raise TypeError("Only a LinearFunction can be added to a LinearFunction.")
def __rmul__(self, c):
"""
Return this LinearFunction scaled by a multiplicative constant.
"""
return LinearFunction(c * self._a, c * self._b)
Here we demonstrate that it works:
f = LinearFunction(1, 2)
5 * f
Below, I included a list of magic methods that seem likely to be useful and mathematically relevant. This list is not comprehensive. For a comprehensive list see the Emulating numeric types from the Python reference.
Method  Operator  Note 

__add__(self, other) 
self+other 

__sub__(self, other) 
selfother 

__mul__(self, other) 
self*other 

__matmul__(self, other) 
self@other 
composition 
__truediv__(self, other) 
self/other 

__mod__(self, other) 
self%other 

__pow__(self, other[, modulo]) 
self**other 

__radd__(self, other) 
other+self 

__rsub__(self, other) 
otherself 

__rmul__(self, other) 
other*self 

__rmatmul__(self, other) 
other@self 

__rtruediv__(self, other) 
other/self 

__rmod__(self, other) 
other%self 

__rpow__(self, other[, modulo]) 
other**self 

__neg__(self) 
self 
negation 
__abs__(self) 
abs(self) 
absolute value 
__invert__(self) 
~self 
multiplicative inverse 