Since d4eb02b6aac9 py_compile.compile() fails to raise exceptions when writing of target file fails.

$ cd /tmp
$ touch test.py
$ mkdir dir
$ chmod a-w dir
mode of ‘dir’ changed from 0755 (rwxr-xr-x) to 0555 (r-xr-xr-x)
$ python3.3 -c 'import py_compile; py_compile.compile("test.py", cfile="/tmp/dir/test.pyc")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib64/python3.3/py_compile.py", line 141, in compile
    with open(cfile, 'wb') as fc:
PermissionError: [Errno 13] Permission denied: '/tmp/dir/test.pyc'
$ python3.4 -c 'import py_compile; py_compile.compile("test.py", cfile="/tmp/dir/test.pyc")'
$ python3.3 -c 'import py_compile; py_compile.compile("test.py", cfile="/tmp/dir")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib64/python3.3/py_compile.py", line 141, in compile
    with open(cfile, 'wb') as fc:
IsADirectoryError: [Errno 21] Is a directory: '/tmp/dir'
$ python3.4 -c 'import py_compile; py_compile.compile("test.py", cfile="/tmp/dir")'
$ ls -l dir
total 0
$

It's because of this try/except to silence failed bytecode creation: http://hg.python.org/cpython/file/d404d33a999c/Lib/importlib/_bootstrap.py#l1105

I hate the py_compile module.

I figured out what I have to do to make this work properly again to avoid the exception from being swallowed. Roughly:

    # XXX calculate mode (_cache_bytecode)
    # XXX create subdirectories as necessary (set_data)
    # XXX write file (_write_atomic)
    # Above replaces loader._cache_bytecode(file, cfile, bytecode)

That will bypass the try/except block causing the issues.

New changeset 04aaeae6ee7b by Brett Cannon in branch 'default':
Issue #17244: Don't mask exceptions raised during the creation of
http://hg.python.org/cpython/rev/04aaeae6ee7b

Test broken on Windows:

======================================================================
FAIL: test_exceptions_propagate (test.test_py_compile.PyCompileTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\test_py_compile.py", line 66, in test_exceptions_propagate
    py_compile.compile(self.source_path, self.pyc_path)
AssertionError: OSError not raised

http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/1850/steps/test/logs/stdio

Does os.chmod(self.directory, stat.S_IREAD) not work on directories on Windows?

Essentially: no. The permissions system in Windows is very different 
from that of Unix. The CRT attempts to mimic it, but for things like 
read-onlyness, it does so by setting the (old-style DOS) attributes. 
These are only just meaningful for files, and are meaningless for 
directories. (Or, strictly, have a different meaning since ISTR the 
Explorer shell uses the readonly bit to indicate that a directory has 
some special significance).

Thanks for the info, Tim. I'll add a skip decorator to not run under Windows then.

New changeset 0803a28dca3c by Brett Cannon in branch 'default':
Issue #17244: Windows doesn't let you set permissions on directories.
http://hg.python.org/cpython/rev/0803a28dca3c