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pythonexcelvbacomtry-catch

Python clean way to wrap individual statements in a try except block

I'm currently doing some Python automation of Excel with COM. It's fully functional, and does what I want, but I've discovered something surprising. Sometimes, some of the Excel commands I use will fail with an exception for no apparent reason. Other times, they will work.

In the VB equivalent code for what I'm doing, this problem is apparently considered normal, and is plastered over with a On Error Resume Next statement. Python does not have said statement, of course.

I can't wrap up the whole set in a try except loop, because it could "fail" halfway through and not complete properly. So, what would be a pythonic way to wrap several independent statements into a try except block? Specifically, something cleaner than:

try:
   statement
except:
   pass
try:
   statement
except:
   pass

The relevant code is the excel.Selection.Borders bit.

def addGridlines(self, infile, outfile):
    """convert csv to excel, and add gridlines"""
    # set constants for excel
    xlDiagonalDown = 5
    xlDiagonalUp = 6
    xlNone = -4142
    xlContinuous = 1
    xlThin = 2
    xlAutomatic = -4105
    xlEdgeLeft = 7
    xlEdgeTop = 8
    xlEdgeBottom = 9
    xlEdgeRight = 10
    xlInsideVertical = 11
    xlInsideHorizontal = 12
            # open file
    excel = win32com.client.Dispatch('Excel.Application')
    workbook = excel.Workbooks.Open(infile)
    worksheet = workbook.Worksheets(1)

    # select all cells
    worksheet.Range("A1").CurrentRegion.Select()
    # add gridlines, sometimes some of these fail, so we have to wrap each in a try catch block
    excel.Selection.Borders(xlDiagonalDown).LineStyle = xlNone
    excel.Selection.Borders(xlDiagonalUp).LineStyle = xlNone
    excel.Selection.Borders(xlDiagonalUp).LineStyle = xlNone
    excel.Selection.Borders(xlEdgeLeft).LineStyle = xlContinuous
    excel.Selection.Borders(xlEdgeLeft).Weight = xlThin
    excel.Selection.Borders(xlEdgeLeft).ColorIndex = xlAutomatic
    excel.Selection.Borders(xlEdgeTop).LineStyle = xlContinuous
    excel.Selection.Borders(xlEdgeTop).Weight = xlThin
    excel.Selection.Borders(xlEdgeTop).ColorIndex = xlAutomatic
    excel.Selection.Borders(xlEdgeBottom).LineStyle = xlContinuous
    excel.Selection.Borders(xlEdgeBottom).Weight = xlThin
    excel.Selection.Borders(xlEdgeBottom).ColorIndex = xlAutomatic
    excel.Selection.Borders(xlEdgeRight).LineStyle = xlContinuous
    excel.Selection.Borders(xlEdgeRight).Weight = xlThin
    excel.Selection.Borders(xlEdgeRight).ColorIndex = xlAutomatic
    excel.Selection.Borders(xlInsideVertical).LineStyle = xlContinuous
    excel.Selection.Borders(xlInsideVertical).Weight = xlThin
    excel.Selection.Borders(xlInsideVertical).ColorIndex = xlAutomatic
    excel.Selection.Borders(xlInsideHorizontal).LineStyle = xlContinuous
    excel.Selection.Borders(xlInsideHorizontal).Weight = xlThin
    excel.Selection.Borders(xlInsideHorizontal).ColorIndex = xlAutomatic
    # refit data into columns
    excel.Cells.Select()
    excel.Cells.EntireColumn.AutoFit()
    # save new file in excel format
    workbook.SaveAs(outfile, FileFormat=1)
    workbook.Close(False)
    excel.Quit()
    del excel

Update:

Perhaps a bit of explanation on the error bit is required. Two identical runs on my test machine, with identical code, on the same file, produce the same result. One run throws exceptions for every xlInsideVertical line. The other throws exceptions for every xlInsideHorizontal. Finally, a third run completes with no exceptions at all.

As far as I can tell Excel considers this normal behavior, because I'm cloning the VB code built by Excel's macro generator, not VB code produced by a person. This might be an erroneous assumption, of course.

It will function with each line wrapped in a try except block I just wanted something shorter and more obvious, because 20 lines wrapped in their own try catch loops is just asking for trouble later.

Update2:

This is a scrubbed CSV file for testing: gist file

Conclusion:

The answer provided by Vsekhar is perfect. It abstracts away the exception suppression, so that later, if and when I have time, I can actually deal with the exceptions as they occur. It also allows for logging the exceptions so they don't disappear, not stopping other exceptions, and is small enough to be easily manageable six months from now.

Solution

  • Consider abstracting away the suppression. And to Aaron's point, do not swallow exceptions generally.

    class Suppressor:
    def __init__(self, exception_type):
        self._exception_type = exception_type

    def __call__(self, expression):
        try:
            exec expression
        except self._exception_type as e:
            print 'Suppressor: suppressed exception %s with content \'%s\'' % (type(self._exception_type), e)
            # or log.msg('...')

    Then, note in the traceback of your current code exactly what exception is raised, and create a Suppressor for just that exception:

    s = Suppressor(excel.WhateverError) # TODO: put your exception type here
s('excel.Selection.Borders(xlDiagonalDown).LineStyle = xlNone')

    This way you get line-by-line execution (so your tracebacks will still be helpful), and you are suppressing only the exceptions you explicitly intended. Other exceptions propagate as usual.