Python's screwed up exception hierarchy

Doing this in Python is bad bad bad:

try:
  # some code
except Exception, e:  # Bad
  log.error("Uncaught exception!", e)

Yet you need to do something like that, typically in the event loop of an application server, or when one library is calling into another library and needs to make sure that no exception escapes from the call, or that all exceptions are re-packaged in another type of exception.

The reason the above is bad is that Python badly screwed up their standard exception hierarchy.

    __builtin__.object
        BaseException
            Exception
                StandardError
                    ArithmeticError
                    AssertionError
                    AttributeError
                    BufferError
                    EOFError
                    EnvironmentError
                    ImportError
                    LookupError
                    MemoryError
                    NameError
                        UnboundLocalError
                    ReferenceError
                    RuntimeError
                        NotImplementedError
                    SyntaxError
                        IndentationError
                            TabError
                    SystemError
                    TypeError
                    ValueError

Meaning, if you try to catch all Exceptions, you're also hiding real problems like syntax errors (!!), typoed imports, etc.  But then what are you gonna do?  Even if you wrote something silly such as:

try:
  # some code
except (ArithmeticError, ..., ValueError), e:
  log.error("Uncaught exception!", e)

You still wouldn't catch the many cases where people define new types of exceptions that inherit directly from Exception. So it looks like your only option is to catch Exception and then filter out things you really don't want to catch, e.g.:

try:
  # some code
except Exception, e:
  if isinstance(e, (AssertionError, ImportError, NameError, SyntaxError, SystemError)):
    raise
  log.error("Uncaught exception!", e)

But then nobody does this. And pylint still complains.

Unfortunately it looks like Python 3.0 didn't fix the problem :( – they only moved SystemExit, KeyboardInterrupt, and GeneratorExit to be subclasses of BaseException but that's all.

They should have introduced another separate level of hierarchy for those errors that you generally don't want to catch because they are programming errors or internal errors (i.e. bugs) in the underlying Python runtime.

thread.error: can't start new thread

Today I was puzzled for a short while by a machine on which Python seemed to be acting up:

$ python
Python 2.5.2 (r252:60911, Jan 20 2010, 23:14:04) 
[GCC 4.2.4 (Ubuntu 4.2.4-1ubuntu3)] on linux2
>>> import thread
>>> thread.start_new(lambda: None, ())
Traceback (most recent call last):
 File "", line 1, in 
thread.error: can't start new thread

The machine had 8G of RAM free and was running only about 200 processes (~500 threads total). So, WTF? While running the command above, I straced Python and here's what I saw:

mmap(NULL, 17592186048512, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x40, -1, 0) = -1 ENOMEM (Cannot allocate memory)
mmap(NULL, 17592186048512, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = -1 ENOMEM (Cannot allocate memory)
write(2, "Traceback (most recent call last"..., 35) = 35

The second argument to mmap is the amount of memory you want to allocate, and yes that's 16GB. I checked /etc/security/limits.conf and I saw that the problem came from there. Someone put this line:

*       -       stack   17179869184  # 16GB

Now when Python creates a thread, it first allocates* memory for the stack with mmap before calling clone to start the thread. If you're also puzzled by this can't start new thread error, make sure it's not a ulimit problem!

* Of course mmap doesn't actually allocate the memory, it just "reserves" it. Actual memory isn't allocated until you use it, but in this case the OS was refusing the "reservation request" because it was more than the maximum amount of RAM that a process is allowed to have according to other ulimits