Is there a way to decode numerical COM error-codes in pywin32

Question

Here is part of a stack-trace from a recent run of an unreliable application written in Python which controls another application written in Excel:

pywintype         


        

Answer 1

Specifically for pythoncom, the errors codes that result are more than cryptic. This is because pythoncom represents them internally as a 32bit signed integer, when the correct representation is a 32bit unsigned integer. As a result, the conversion that you end up seeing in the stack trace is incorrect.

In particular, your exception, according to pythoncom, is -2147352567, and your (for lack of a better word) Err.Number is -2146788248.

This however causes some issues when watching for specific errors, like below:

DISP_E_EXCEPTION = 0x80020009
#...
#except pywintypes.com_error as e:
#    print repr(e)
#    #pywintypes.com_error: (-2147352567, 'Exception occurred.', (0, None, None, None, 0, -2146788248), None)
#    hr = e.hresult

hr = -2147352567
if hr == DISP_E_EXCEPTION:
    pass #This never occurs
else:
    raise

To see why this has issues, lets look into these error codes:

>>> DISP_E_EXCEPTION = 0x80020009
>>> DISP_E_EXCEPTION
2147614729L
>>> my_hr = -2147352567
>>> my_hr == DISP_E_EXCEPTION
False

Again, this is because python sees the constant declared as positive, and pythoncom's incorrect declaration interpreted it as negative. Of course, the most obvious solution fails:

>>> hex(my_hr)
'-0x7ffdfff7'

The solution is to properly interpret the number. Luckily, pythoncom's representation is reversible. We need to interpret the negative number as a 32 bit signed integer, then interpret that as an unsigned integer:

def fix_com_hresult(hr):
    import struct
    return struct.unpack("L", struct.pack("l", hr))[0]

>>> DISP_E_EXCEPTION = 0x80020009
>>> my_hr = -2147352567
>>> my_hr == DISP_E_EXCEPTION
False
>>> fixed_hr = fix_com_hresult(my_hr)
>>> fixed_hr
2147614729L
>>> fixed_hr == DISP_E_EXCEPTION
True

So, putting it all together, you need to run fix_com_hresult() on that result from pythoncom, essentially all the time.

Since normally you need to do this when checking for exceptions, I created these functions:

def fix_com_exception(e):
    e.hresult = fix_com_hresult(e.hresult)
    e.args = [e.hresult] + list(e.args[1:])
    return e

def fix_com_hresult(hr):
    import struct
    return struct.unpack("L", struct.pack("l", hr))[0]

which can then be used how you expect:

DISP_E_EXCEPTION = 0x80020009
try:
    #failing call
except pywintypes.com_error as e:
    print repr(e)
    #pywintypes.com_error: (-2147352567, 'Exception occurred.', (0, None, None, None, 0, -2146788248), None)
    fix_com_exception(e)
    print repr(e)
    #pywintypes.com_error: (2147614729L, 'Exception occurred.', (0, None, None, None, 0, -2146788248), None)
    if e.hresult == DISP_E_EXCEPTION:
        print "Got expected failure"
    else:
        raise

I was unable to find a MSDN document listing all HRESULTs, but I found this: http://www.megos.ch/support/doserrors_e.txt

Also, since you have it, fix_com_hresult() should also be run on your extended error code (-2146788248), but as Euro Micelli said, it doesn't help you in this particular instance :)