5.2.3.4 What About Exceptions?

Python 2.2

previous page next page

5.2.3.4 What About Exceptions?

No problem, provided that the traceback is the only output produced by the example: just paste in the traceback.^5.1Since tracebacks contain details that are likely to change rapidly (for example, exact file paths and line numbers), this is one case where doctest works hard to be flexible in what it accepts.

Simple example:

>>> [1, 2, 3].remove(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: list.remove(x): x not in list

That doctest succeeds if ValueError is raised, with the "list.remove(x): x not in list" detail as shown.

The expected output for an exception must start with a traceback header, which may be either of the following two lines, indented the same as the first line of the example:

Traceback (most recent call last):
Traceback (innermost last):

The traceback header is followed by an optional traceback stack, whose contents are ignored by doctest. The traceback stack is typically omitted, or copied verbatim from an interactive session.

The traceback stack is followed by the most interesting part: the line(s) containing the exception type and detail. This is usually the last line of a traceback, but can extend across multiple lines if the exception has a multi-line detail:

>>> raise ValueError('multi\n    line\ndetail')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: multi
    line
detail

The last three lines (starting with ValueError) are compared against the exception's type and detail, and the rest are ignored.

Best practice is to omit the traceback stack, unless it adds significant documentation value to the example. So the last example is probably better as:

>>> raise ValueError('multi\n    line\ndetail')
Traceback (most recent call last):
    ...
ValueError: multi
    line
detail

Note that tracebacks are treated very specially. In particular, in the rewritten example, the use of "..." is independent of doctest's ELLIPSIS option. The ellipsis in that example could be left out, or could just as well be three (or three hundred) commas or digits, or an indented transcript of a Monty Python skit.

Some details you should read once, but won't need to remember:

Doctest can't guess whether your expected output came from an exception traceback or from ordinary printing. So, e.g., an example that expects "ValueError: 42 is prime" will pass whether ValueError is actually raised or if the example merely prints that traceback text. In practice, ordinary output rarely begins with a traceback header line, so this doesn't create real problems.
Each line of the traceback stack (if present) must be indented further than the first line of the example, or start with a non-alphanumeric character. The first line following the traceback header indented the same and starting with an alphanumeric is taken to be the start of the exception detail. Of course this does the right thing for genuine tracebacks.
When the IGNORE_EXCEPTION_DETAIL doctest option is is specified, everything following the leftmost colon is ignored.
The interactive shell omits the traceback header line for some SyntaxErrors. But doctest uses the traceback header line to distinguish exceptions from non-exceptions. So in the rare case where you need to test a SyntaxError that omits the traceback header, you will need to manually add the traceback header line to your test example.
For some SyntaxErrors, Python displays the character position of the syntax error, using a ^ marker:
```
>>> 1 1
  File "<stdin>", line 1
    1 1
      ^
SyntaxError: invalid syntax
```
Since the lines showing the position of the error come before the exception type and detail, they are not checked by doctest. For example, the following test would pass, even though it puts the ^ marker in the wrong location:
```
>>> 1 1
Traceback (most recent call last):
  File "<stdin>", line 1
    1 1
    ^
SyntaxError: invalid syntax
```

Changed in version 2.4: The ability to handle a multi-line exception detail, and the IGNORE_EXCEPTION_DETAIL doctest option, were added.

Footnotes

... traceback.^5.1: Examples containing both expected output and an exception are not supported. Trying to guess where one ends and the other begins is too error-prone, and that also makes for a confusing test.

See About this document... for information on suggesting changes.

previous page start next page