How Can I Find a List of All Exceptions That a Given Library Function Throws in Python?

Question

Sorry for the long title, but it seems most descriptive for my question.

Basically, I'm having a difficult time finding exception information in the official python documentation. For example, in one program I'm currently writing, I'm using the shutil libary's move function:

from shutil import move
move('somefile.txt', '/tmp/somefile.txt')

That works fine, as long as I have write access to /tmp/, there is enough diskspace, and if all other requirements are satisfied.

However, when writing generic code, it is often difficult to guarantee those factors, so one usually uses exceptions:

from shutil import move
try:
    move('somefile.txt', '/tmp/somefile.txt')
except:
    print 'Move failed for some reason.'

I'd like to actually catch the appropriate exceptions thrown instead of just catching everything , but I simply can't find a list of exceptions thrown for most python modules. Is there a way for me to see which exceptions a given function can throw, and why? This way I can make appropriate cases for each exception, eg:

from shutil import move
try:
    move('somefile.txt', '/tmp/somefile.txt')
except PermissionDenied:
    print 'No permission.'
except DestinationDoesNotExist:
    print "/tmp/ doesn't exist"
except NoDiskSpace:
    print 'No diskspace available.'

Answer points go to whoever can either link me to some relevant documentation that I've somehow overlooked in the official docs, or provide a sure-fire way to figure out exactly which exceptions are thrown by which functions, and why.

Thanks!

UPDATE : It seems from the answers given that there really isn't a 100% straight-forward way to figure out which errors are thrown by specific functions. With meta programming, it seems that I can figure out simple cases and list some exceptions, but this is not a particularly useful or convenient method.

I'd like to think that eventually there will be some standard for defining which exceptions are raised by each python function, and that this information will be included in the official documentation. Until then I think I will just allow those exceptions to pass through and error out for my users as it seems like the most sane thing to do.

Answer 1

To amplify Messa, catch what you expect are failure modes that you know how to recover from. Ian Bicking wrote an article that addresses some of the overarching principles as does Eli Bendersky's note .

The problem with the sample code is that it is not handling errors, just prettifying them and discarding them. Your code does not "know" what to do with a NameError and there isn't much it should do other than pass it up, look at Bicking's re-raise if you feel you must add detail.

IOError and OSError are reasonably "expectable" for a shutil.move but not necessarily handleable. And the caller of your function wanted it to move a file and may itself break if that "contract" that Eli writes of is broken.

Catch what you can fix, adorn and re-raise what you expect but can't fix, and let the caller deal with what you didn't expect, even if the code that "deals" is seven levels up the stack in main .

Answer 2

Python doesn't have a mechanism right now for declaring which exceptions are thrown, unlike (for example) Java. (In Java you have to define exactly which exceptions are thrown by what, and if one of your utility methods needs to throw another exception then you need to add it to all of the methods which call it which gets boring quickly!)

So if you want to discover exactly which exceptions are thrown by any given bit of python then you need to examine the documentation and the source.

However python has a really good exception hierarchy.

If you study the exception hierarchy below you'll see that the error superclass you want to catch is called StandardError - this should catch all the errors that might be generated in normal operations. Turning the error into into a string will give a reasonable idea to the user as to what went wrong, so I'd suggest your code above should look like

from shutil import move
try:
    move('somefile.txt', '/tmp/somefile.txt')
except StandardError, e:
    print 'Move failed: %s' % e

Exception hierarchy

BaseException
|---Exception
|---|---StandardError
|---|---|---ArithmeticError
|---|---|---|---FloatingPointError
|---|---|---|---OverflowError
|---|---|---|---ZeroDivisionError
|---|---|---AssertionError
|---|---|---AttributeError
|---|---|---BufferError
|---|---|---EOFError
|---|---|---EnvironmentError
|---|---|---|---IOError
|---|---|---|---OSError
|---|---|---ImportError
|---|---|---LookupError
|---|---|---|---IndexError
|---|---|---|---KeyError
|---|---|---MemoryError
|---|---|---NameError
|---|---|---|---UnboundLocalError
|---|---|---ReferenceError
|---|---|---RuntimeError
|---|---|---|---NotImplementedError
|---|---|---SyntaxError
|---|---|---|---IndentationError
|---|---|---|---|---TabError
|---|---|---SystemError
|---|---|---TypeError
|---|---|---ValueError
|---|---|---|---UnicodeError
|---|---|---|---|---UnicodeDecodeError
|---|---|---|---|---UnicodeEncodeError
|---|---|---|---|---UnicodeTranslateError
|---|---StopIteration
|---|---Warning
|---|---|---BytesWarning
|---|---|---DeprecationWarning
|---|---|---FutureWarning
|---|---|---ImportWarning
|---|---|---PendingDeprecationWarning
|---|---|---RuntimeWarning
|---|---|---SyntaxWarning
|---|---|---UnicodeWarning
|---|---|---UserWarning
|---GeneratorExit
|---KeyboardInterrupt
|---SystemExit

This also means that when defining your own exceptions you should base them off StandardError not Exception.

Base class for all standard Python exceptions that do not represent
interpreter exiting.

Answer 3

Yes, you can (for simple cases), but you need a bit of meta-programming. Like the other answers have said, a function does not declare that it throws a particular error type, so you need to look at the module and see what exception types it defines, or what exception types it raises. You can either try to grok the documentation or leverage the Python API to do this.

To first find which exception types a module defines, just write a simple script to go through each object in the module dictionary module.__dict__ and see if it ends in the word "Error" or if it is a subclass of Exception:

def listexns(mod):
    """Saved as: http://gist.github.com/402861
    """
    module = __import__(mod)
    exns = []
    for name in module.__dict__:
        if (issubclass(module.__dict__[name], Exception) or
            name.endswith('Error')):
            exns.append(name)
    for name in exns:
        print '%s.%s is an exception type' % (str(mod), name)
    return

If I run this on your example of shutils I get this:

$ python listexn.py shutil
Looking for exception types in module: shutil
shutil.Error is an exception type
shutil.WindowsError is an exception type
$

That tells you which error types are defined, but not which ones are thrown. To achieve the latter, we need to walk over the abstract syntax tree generated when the Python interpreter parses the module, and look for every raise statement, then save a list of names which are raised. The code for this is a little long, so first I'll state the output:

$ python listexn-raised.py /usr/lib/python2.6/shutil.py
Looking for exception types in: /usr/lib/python2.6/shutil.py
/usr/lib/python2.6/shutil.py:OSError is an exception type
/usr/lib/python2.6/shutil.py:Error is an exception type
$ 

So, now we know that shutil.py defines the error types Error and WindowsError and raises the exception types OSError and Error . If we want to be a bit more complete, we could write another method to check every except clause to also see which exceptions shutil handles.

Here's the code to walk over the AST, it just uses the compiler.visitor interface to create a walker which implements the "visitor pattern" from the Gang of Four book:

class ExceptionFinder(visitor.ASTVisitor):
    """List all exceptions raised by a module. 
    Saved as: http://gist.github.com/402869
    """

    def __init__(self, filename):
        visitor.ASTVisitor.__init__(self)
        self.filename = filename
        self.exns = set()
        return

    def __visitName(self, node):
        """Should not be called by generic visit, otherwise every name
        will be reported as an exception type.
        """
        self.exns.add(node.name)
        return

    def __visitCallFunc(self, node):
        """Should not be called by generic visit, otherwise every name
        will be reported as an exception type.
        """
        self.__visitName(node.node)
        return

    def visitRaise(self, node):
        """Visit a raise statement.
        Cheat the default dispatcher.
        """
        if issubclass(node.expr1, compiler.ast.Name):
            self.__visitName(node.expr1)
        elif isinstance(node.expr1, compiler.ast.CallFunc):
            self.__visitCallFunc(node.expr1)
        return

Answer 4

As these operations usually use libc functions and operating system calls, mostly you get IOError or OSError with an errno number; these errors are listed in man pages of that libc/OS calls.

I know this is possibly not a complete answer, it would be good to have all exceptions listed in documentation...