py.test tries to rewrite every module that it collects as a test module. Assertion rewriting uses a PEP 302 import hook to capture test modules for rewriting. I'm happy to report doing this was easier than I expected. Most of the code in the import hook I had to write was dealing with detecting test modules rather than supporting import's extremely complicated API. Rewriting has a non-zero cost during test collection, so py.test compiles rewritten modules to bytecode and caches them in the PEP 3147 PYC repository,
__pycache__
. One major thing I did have to account for was the possibility that multiple py.test processes would be writing PYC files. (This is a very real possibility when the xdist plugin is being used. Therefore, py.test uses only atomic operations on the rewritten PYC file. Windows, lacking atomic rename, was a pain here.I'm now going to demonstrate what py.test's rewriting phase does to a test module. Let's dive in with a failing test for a (broken) function that is supposed to create empty files:
import os
def make_empty_file(name):
with open(name, "w") as fp:
fp.write("hello")
def test_make_empty_file():
name = "/tmp/empty_test"
make_empty_file(name)
with open(name, "r") as fp:
assert not fp.read()
This test nicely demonstrates the problem with py.test's old assertion method mentioned in the first paragraph. If we force the old assertion interpretation mode with
--assert=reinterp
, we see:def test_make_empty_file():
name = "/tmp/empty_test"
make_empty_file(name)
with open(name, "r") as fp:
> assert not fp.read()
E AssertionError: (assertion failed, but when it was re-run for printing intermediate values, it did not fail. Suggestions: compute assert expression before the assert or use --no-assert)
test_empty_file.py:11: AssertionError
The problem is that assert statement has the side-effect of reading the file. When py.test reinterprets the assert statement, it uses the same file object, now at EOF, and
read()
returns an empty string. py.test's new rewriting mode fixes this by scanning the assert for introspection information before executing the test. Running py.test with assertion rewriting enabled gives the desired result:def test_make_empty_file():
name = "/tmp/empty_test"
make_empty_file(name)
with open(name, "r") as fp:
> assert not fp.read()
E assert not 'hello'
E + where 'hello' =
E + where
test_empty_file.py:11: AssertionError
So what magic has py.test worked to display such nice debugging information? This is what Python is actually executing:
def test_make_empty_file():
name = '/tmp/empty_test'
make_empty_file(name)
with open(name, 'r') as fp:
@py_assert1 = fp.read
@py_assert3 = @py_assert1()
@py_assert5 = (not @py_assert3)
if (not @py_assert5):
@py_format6 = ('assert not %(py4)s\n{%(py4)s = %(py2)s\n{%(py2)s = %(py0)s.read\n}()\n}' %
{'py0': (@pytest_ar._saferepr(fp) if ('fp' in @py_builtins.locals() is not @py_builtins.globals()) else 'fp'),
'py2': @pytest_ar._saferepr(@py_assert1),
'py4': @pytest_ar._saferepr(@py_assert3)})
raise AssertionError(@pytest_ar._format_explanation(@py_format6))
del @py_assert5, @py_assert1, @py_assert3
name = '/tmp/empty_test'
make_empty_file(name)
with open(name, 'r') as fp:
@py_assert1 = fp.read
@py_assert3 = @py_assert1()
@py_assert5 = (not @py_assert3)
if (not @py_assert5):
@py_format6 = ('assert not %(py4)s\n{%(py4)s = %(py2)s\n{%(py2)s = %(py0)s.read\n}()\n}' %
{'py0': (@pytest_ar._saferepr(fp) if ('fp' in @py_builtins.locals() is not @py_builtins.globals()) else 'fp'),
'py2': @pytest_ar._saferepr(@py_assert1),
'py4': @pytest_ar._saferepr(@py_assert3)})
raise AssertionError(@pytest_ar._format_explanation(@py_format6))
del @py_assert5, @py_assert1, @py_assert3
As you can see, it's not going to be winning any awards for beautiful Python! (Ideally, though, you'll never have to see or think about it.) Examining the rewritten code, we see a lot of internal variables starting with "@" have been created. The "@", invalid in Python identifiers, is to make sure internal names don't conflict with any user-defined names which might be in the scope. In the first four written lines under the with statement, the test of the assert statement has been expanded into its component subexpressions. This allows py.test to display the values of subexpressions should the assertion fail. If the assertion fails, the if statement in the fifth line of rewriting evaluates to
True
and a AssertionError
will be raised. Under the if statement is the real mess. This is where the helpful error message is generated. The line starting with @py_format6
is simply does string formatting (with %
) on a template generated from the structure of the assert statement. This template is filled in with the intermediate values of the expressions collected above. @py_builtins
is the builtins module, used in case the test is shadowing builtins the rewriting code uses. The @pytest_ar
variable is a special module of assertion formatting helpers. For example, @pytest_ar._saferepr
is like builtin repr
but gracefully handles long reprs and __repr__
methods that raise exceptions. A non-obvious trick in the format dict is the expression @pytest_ar._saferepr(fp) if ('fp' in @py_builtins.locals() is not @py_builtins.globals()) else 'fp'
. This checks whether fp
is a local variable or not and customizes the display accordingly. After the initial formatting, the helper function _format_explanation
is called. This function produces the indentation and "+" you see in the error message. Finally, we note that if the assertion doesn't fail, py.test cleans up after itself by deleting temporary variables.The example above is a fairly tame (and luckily also typical) assertion. Rewriting gets more "exciting" when boolean operations and comparisons enter because they require short circuit evaluation, which complicates both the expansion of expressions and formatting (think lots of nested ifs).
In conclusion, py.test's new assertion rewriting fixes some long standing issues with assertion introspection and continues py.test's long tradition of excellent debugging support. (There are now three(!) assertion introspection methods in py.test: two reinterpretation implementations as well as rewriting) I just hope I haven't scared you completely off py.test! :)
93 comments:
How easy do you think it would be to abstract out the assertion rewriting and reuse it with other test frameworks?
Could it even be done as a standalone library?
@Michael The assertion stuff is not very dependent on py.test internals, so it should be easy to separate them out if you wanted to. In fact, until recently, assertion introspection code was in the pylib, a separate library than py.test.
Nice.
Though I strongly discourage people from using the assert statement in most non-trivial cases in Python due to its syntax oddity and the common error of using ()s that changes the assert into an assert of an always true tuple.
@gps In recent versions of Python, the compiler will give you a warning if you make a vacuous assert statement.
$ python3
Python 3.1.3 (r313:86834, Feb 27 2011, 22:48:24)
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> assert (3, "msg")
:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?
People interested in this should also check out "bindann":
http://tahoe-lafs.org/trac/bindann
It inspects the stack and the source code to extract the values that are relevant to the failing exception. Very cool! I feel like it should have gotten a lot more attention than it did when it was first created.
Disclosure: my brother invented it while staying rent free in my house, and he is super awesome and I love him, so I might be a tad bit biased.
@Zooko sounds promising, but where is it gone? The link is broken.
Interesting also your own quote from September 14, 2007 in commit 8fd98624:
don't try to use bindann
It causes a mysterious misbehavior in Python import which causes the previous patch to fail (the patch to not run trial tests if dependencies can't be imported)
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