How to change the message in a Python AssertionError?

Question

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.

Answer 1

Use e.args, e.message is deprecated.

try:
    assert False, "Hello!"
except AssertionError as e:
    e.args += ('some other', 'important', 'information', 42)
    raise

This preserves the original traceback. Its last part then looks like this:

AssertionError: ('Hello!', 'some other', 'important', 'information', 42)

Works in both Python 2.7 and Python 3.

Answer 2

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.

Answer 3

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 ) )

Answer 4

With this method I was able to edit the message and still have the stack trace (+any other information) visible. Also newlines are displayed right.

try:
   my_assert_equal(line1, line2)
except AssertionError as e:
   message = e.args[0]
   message += "\nThis appends the default message and can have newlines"
   e.args = (message,) #wrap it up in new tuple
   raise

Answer 5

You can pass the desired message when creating the exception.

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

Hope this helps.