views:

59

answers:

3

I'm writing per the following, in which I try to produce a decent error message when comparing two multiline blocks of Unicode text. The interior method that does the comparison raises an assertion, but the default explanation is useless to me

I need to add something to code such as this below:

def assert_long_strings_equal(one, other):
    lines_one = one.splitlines()
    lines_other = other.splitlines()
    for line1, line2 in zip(lines_one, lines_other):
        try:
            my_assert_equal(line1, line2)
        except AssertionError, error:
            # Add some information to the printed result of error??!
            raise

I cannot figure out how to change the printed error message in the assertionerror I catch. I always get AssertionError: u'something' != 'something else', which only shows the first line of the output.

How can I change the assertion message to print out whatever I want?

If it's relevant, I am using nose to run the test.

+1  A: 

You can pass the desired message when creating the exception.

raise AssertionError(line1 + ' != ' + line2)

Hope this helps.

zchtodd
That's not what the OP means; `AssertionError` is (generally) raised by the `assert` statement.
katrielalex
+4  A: 
assert expression, info

For instance,

>>> assert False, "Oopsie"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: Oopsie

From the docs:

Assert statements are a convenient way to insert debugging assertions into a program:

assert_stmt ::=  "assert" expression
["," expression] 

The simple form, assert expression, is equivalent to

if __debug__:
    if not expression:
        raise AssertionError 

The extended form

assert expression1, expression2

is equivalent to

if __debug__:
    if not expression1:
        raise AssertionError(expression2)

These equivalences assume that __debug__ and AssertionError refer to the built-in variables with those names. In the current implementation, the built-in variable __debug__ is True under normal circumstances, False when optimization is requested (command line option -O). The current code generator emits no code for an assert statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.

katrielalex
Note that you can also embed newlines in the `info` string expression to make them look nice when displayed.
martineau
Also note that you can add additional information to the string expression by using Python's string interpolation and newer string formating operations.
martineau
Raising an exception in general is not a problem. It's catching, modifying, and re-raising an expression, that I asked.
Andres Jaan Tack
@Andres -- you should edit the question to say that explicitly... I don't interpret the question as saying that.
katrielalex
Edited accordingly. :)
Andres Jaan Tack
+2  A: 

You want to take the caught exception, convert it to a string, combine it with some additional string info, and raise a new exception.

x = 3
y = 5
try:
    assert( x == y )
except AssertionError, e:
    raise( AssertionError( "Additional info. %s"%e ) )
Russell Borogove
why do people feel the need to show up an hour after the fact with an inferior, non-canonical answer?
aaronasterling
I didn't see any of the posted answers offer the general solution of how to add information to an exception and re-raise with both the original and additional information, which I thought was what the OP was asking.
Russell Borogove
Russel is right, this is the sort of thing I was looking for.
Andres Jaan Tack