[pywin32-bugs] [ pywin32-Bugs-2052253 ] Problem calling WMI method

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Bugs item #2052253, was opened at 2008-08-14 19:06
Message generated for change (Comment added) made by rupole
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Ramon Garcia (ramong)
Assigned to: Nobody/Anonymous (nobody)
Summary: Problem calling WMI method

Initial Comment:
An exception is issued if a WMI method call is attempted. The following code shows it.

	wmiservice = Dispatch("WbemScripting.SWbemLocator")
	wmi = wmiservice.ConnectServer("localhost", "root\cimv2")
	moniker = r"winmgmts:" \
		"{impersonationLevel=impersonate,(Security,Backup)}!\\\\" + \
		computer + r"\root\cimv2"
	wmi = GetObject(moniker)
	log_files = wmi.ExecQuery \
		("SELECT * FROM Win32_NTEventLogFile")
	for log_file in log_files:
		base = re.sub("event$", "", log_file.FileName)
		backup_filename = base + u".evt"
		log_file.BackupEventlog(backup_filename)

The following exception is raised

  File "c:\python25\lib\site-packages\win32com\client\dynamic.py", line 495, in __getattr__
    raise pythoncom.com_error, details
com_error: (-2147352567, 'Ocurri\xf3 una excepci\xf3n.', (0, u'SWbemObjectEx', u'Invalid parameter ', None, 0, -2147217400), None)


We traced this problem to WMI issing an unexecpected return code when it is attempted to get a property that is a method. The function __getattr__ in class CDispatch tries to resolve an attribute request as an attribute first, and then, if an error is raisen by COM, as a method:

if details[0] in ERRORS_BAD_CONTEXT:
    # May be a method.
    self._olerepr_.mapFuncs[attr] = retEntry
    return self._make_method_(attr)
raise pythoncom.com_error, details

But the list of such errors seems not be enough. WMI is returning error code winerror.DISP_E_EXCEPTION in this case. We workarounded the problem adding winerror.DISP_E_EXCEPTION to ERRORS_BAD_CONTEXT in dynamic.py, but perhaps there is a more specific solution.

Best regards

Ramon Garcia
ra...@cn...
Comisión Nacional de Mercado de Valores

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>Comment By: Roger Upole (rupole)
Date: 2008-09-03 19:49

Message:
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The main problem is that there are any number of reasons why
that particular error may be returned, and any number of error codes
that might be returned when an object does not receive required
parameters.  No matter how specific you attempt to make it, it will
still be too general.  The best that can be done is to make a specific
wrapper just for WMI objects.

Also, as demonstrated WMI does not give you a way to actually
retrieve a method object thru normal Invoke semantics.  This is why it
provides Methods_ and ExecMethod_ for dynamic languages such as
Python.  For example: log_file.Methods_('BackupEventlog')

Tim Golden's WMI module uses these special methods to hide the
details so that you can use natural syntax.
I highly recommend it.



----------------------------------------------------------------------

Comment By: Ramon Garcia (ramong)
Date: 2008-09-03 19:03

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Originator: YES

Thanks for looking at this issue.

In the interest of easy of use I think that pythoncom should behave as
close as posible to windows Wscript interpreter, because it is what the
script developer expects. object.method(arg) would work like under
WScript.

The solution that we are using, adding winerror.DISP_E_EXCEPTION to
ERRORS_BAD_CONTEXT is somewhat quick and dirty, but works for us. What I
would do would be to add a more specific check, like checking that if
winerror.DISP_E_EXCEPTION and the errorcode -2147217400 inside the wrapped
exception object, and if that happens then make object.method return an
object that can be called. The only risk that I can see in our method is
the risk of breaking working code. Using a more specific solution, it
should not have any risk.

(Excuse me if there are any mistakes in this comment. I don't have the
machine at hand, I and writting from what I can remember).


----------------------------------------------------------------------

Comment By: Roger Upole (rupole)
Date: 2008-09-03 17:05

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I think the underlying problem here is that WMI does not make a distiction
between
asking for a method and executing it. Both of these produce the error you
describe:

log_file._oleobj_.Invoke(id, 0, pythoncom.DISPATCH_METHOD, True)
log_file._oleobj_.Invoke(id, 0, pythoncom.DISPATCH_PROPERTYGET, True)

And both succeed if you pass a filename parameter as an extra arg.
This indicates that the error is not produced by attempting to get the
attribute,
but rather from actually calling the method without a filename parameter.

You may want to look at using Tim Golden's WMI module, which provides a
much
easier to use wrapper for these objects.
http://timgolden.me.uk/python/wmi.html




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