Raising exceptions in Python

Question

I am reading from an e-book, here is my code:

import math

class QuadError( Exception ):
    pass

def quad(a,b,c):
    if a == 0:
        ex = QuadError( "Not Quadratic" )
        ex.coef = ( a, b, c )
        raise ex
    if b*b-4*a*c < 0:
        ex = QuadError( "No Real Roots" )
        ex.coef = ( a, b, c )
        raise ex
    x1= (-b+math.sqrt(b*b-4*a*c))/(2*a)
    x2= (-b-math.sqrt(b*b-4*a*c))/(2*a)
    return (x1,x2)

Although I understood the try... except thing inside a function, I can't understand this... I understand what it does, e.g I have used quad(4, 2, 4) that gives me an "No Real Roots" error, or the quad(0, b, c) However I can't understand how the program itself works... So,

if a == 0:

a varibale with the name: "ex", gets the value: QuadError( "Not Quadratic" ) so the programs searches for the Class QuadError that has a pass command in it???! Then why does it print the message??? I would expect something like...

class QuadError( Exception ):
    print Exception

Next thing I don't unserstand is the line:

ex.coef = ( a, b, c )

What's that??? is coef a command? Does it do something?

Thanks guys! :)

Are you familiar with classes in general, and inheritance in particular? It may be better to postpone learning about this until you learn about that. — Kevin
– Kevin, Commented Jan 29, 2015 at 18:32
I do know both! However, I read about them about 7 months ago, but I think I do remember... — midkin
– midkin, Commented Jan 29, 2015 at 18:33
QuadError(Exception) inherits from Exception, which means that it gets all the fancy goodies that come with the Exception class. Including the ability to act like any standard exception. the pass keyword is just a control flow statement of sorts that says, no more after me. and ex.coef is being assigned a tuple of a,b,c. — Mike McMahon
– Mike McMahon, Commented Jan 29, 2015 at 18:33
if you don't understand ex.coef = ( a, b, c ), you need to go back to basics. — acushner
– acushner, Commented Jan 29, 2015 at 18:37
I really don't... I studies classes some months ago, I didn't right now, but the book just has the above code in the Exceptions section. I 've got no idea what's wrong... — midkin
– midkin, Commented Jan 29, 2015 at 18:38

martineau · Accepted Answer · 2015-01-29 19:17:23Z

The following is a definition of a class derived from Exception, also known as a subclass of it. The body of it is empty (it adds no new behavior or attributes to the base class), so the statement pass is used because class bodies have to have something in them.

class QuadError(Exception):
    pass

In this part of the code:

ex = QuadError( "Not Quadratic" )
ex.coef = ( a, b, c )
raise ex

it's creating an instance of the new class and passing it a string which will become the exception's message attribute and be displayed if the exception is printed. The ex.coef assignment adds an attribute of that name with the value of which is tuple with the coefficients values. This isn't very useful by itself, however other code could try to access the added attribute and use the values somehow. There's no example of doing that in your code however.

In my option it might be better to add the coefficient values to the exceptions message itself so they would be printed along with one.

This is what I mean:

a, b, c = 0, 1, 2
try:
    ex = QuadError("Not Quadratic: coefficients a:{} b:{} c:{}".format(a,b,c))
    raise ex
except QuadError as qe:
    print(qe)

Output:

Not Quadratic: coefficients a:0 b:1 c:2

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