在python中打印错误消息

Question

So for a point class I'm making I have this error class:

class Error(Exception):
    def __init__(self, message):
        self.message = message

I'm using this to raise an error and print out a message for certain errors. What I'm doing here is printing out an error message if either x or y (or both) are not float values:

def __init__(self, x, y):
    if not isinstance(x, float):
        raise Error("Parameter \"x\" illegal.")
    self.x = x
    if not isinstance(y, float):
        raise Error ("Parameter \"y\" illegal.")
    self.y = y

By raising and printing errors that way, the error messages I get look like this:

********** Point
*** constructor
caught: Parameter "x" illegal.
caught: Parameter "y" illegal.
0
***

But the error message is actually supposed to look like this:

********** Point
*** constructor
caught: Parameter "x" illegal.
caught: Parameter "y" illegal.
0 1
***

So why does the expected output print a 1 next to the 0? Other error messages also include:

*** rotate
caught: Parameter "a" illegal.
0
***

When I want it to look like:

*** rotate
caught: Parameter "a" illegal.
0 -1
***

So is there something I'm not printing out in my error message? Here's the code that prints out the error message:

print '*** constructor'
try:
    p0 = Point(1,1.0) # x illegal
except Error as e:
    print 'caught:', e.message
try:
    p0 = Point(1.0,'y') # y illegal
except Error as e:
    print 'caught:', e.message

print Point(0.0,1.0)

Here's my str method:

def __str__(self):
    return str(int(round(self.x)))

Answer 1

Default x and y to None:

x = None
y = None

Then in str method,

def __str__(self):
    str_x = ""
    if self.x is not None:
        str_x = str(int(round(self.x)))
    str_y = ""
    if self.y is not None:
        str_y = str(int(round(self.y)))
    return "%s %s" % (str_x, str_y)

Answer 2

Why not use python's built-in raise?

It will terminate the program and show an error message, just like it came out of the interpreter.

If you're picky about the class, define it like this:

class YourNameHere(Exception): pass

Whether you defined the class or not, raise it like this:

raise ExampleError("Parameter \"x\" illegal.","Parameter \"y\" illegal.")

It will show the following output:

Traceback (most recent call last):
  File "C:/testanswer.py", line 3, in <module>
    raise ExampleError("Parameter \"x\" illegal.","Parameter \"y\" illegal.")
ExampleError: ('Parameter "x" illegal.', 'Parameter "y" illegal.')

For single answers, there is no need to put it in a tuple.