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update assert docs

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Benjamin Peterson 2011-06-29 10:52:39 -05:00
parent aa7f7a1c71
commit 4fcd346838
1 changed files with 12 additions and 12 deletions
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24
doc/assert.txt
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@ -196,30 +196,32 @@ Reporting details about a failing assertion is achieved either by rewriting
assert statements before they are run or re-evaluating the assert expression and
recording the intermediate values. Which technique is used depends on the
location of the assert, py.test's configuration, and Python version being used
to run py.test. However, for assert statements with a manually provided
message, i.e. ``assert expr, message``, no assertion introspection takes place and the manually provided message will be rendered in tracebacks.
to run py.test. Note that for assert statements with a manually provided
message, i.e. ``assert expr, message``, no assertion introspection takes place
and the manually provided message will be rendered in tracebacks.
By default, if the Python version is greater than or equal to 2.6, py.test
rewrites assert statements in test modules. Rewritten assert statements put
introspection information into the assertion failure message. py.test only
rewrites test modules directly discovered by its test collection process, so
asserts in supporting modules will not be rewritten.
asserts in supporting modules which are not themselves test modules will not be
rewritten.
.. note::
py.test rewrites test modules as it collects tests from them. It does this by
writing a new pyc file which Python loads when the test module is
imported. If the module has already been loaded (it is in sys.modules),
though, Python will not load the rewritten module. This means if a test
module imports another test module which has not already been rewritten, then
py.test will not be able to rewrite the second module.
py.test rewrites test modules on import. It does this by using an import hook
to write a new pyc files. Most of the time this works transparently. However,
if you are messing with import yourself, the import hook may interfere. If
this is the case, simply use ``--assertmode=reinterp`` or
``--assertmode=off``. Additionally, rewriting will fail silently if it cannot
write new pycs, i.e. in a read-only filesystem or a zipfile.
If an assert statement has not been rewritten or the Python version is less than
2.6, py.test falls back on assert reinterpretation. In assert reinterpretation,
py.test walks the frame of the function containing the assert statement to
discover sub-expression results of the failing assert statement. You can force
py.test to always use assertion reinterpretation by passing the
``--assertmode=old`` option.
``--assertmode=reinterp`` option.
Assert reinterpretation has a caveat not present with assert rewriting: If
evaluating the assert expression has side effects you may get a warning that the
@ -244,5 +246,3 @@ All assert introspection can be turned off by passing ``--assertmode=off``.
.. versionchanged:: 2.1
Introduce the ``--assertmode`` option. Deprecate ``--no-assert`` and
``--nomagic``.