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

Question

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.

Answer 1

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.

Answer 2

Exceptions never happen "for no apparent reason". There is always a reason and that reason needs to be fixed. Otherwise, your program will start to produce "random" data where "random" is at the mercy of the bug that you're hiding.

But of course, you need a solution for your problem. Here is my suggestion:

1. Create a wrapper class that implements all the methods that you need and delegates them to the real Excel instance.

2. Add a decorator before each method which wraps the method in a try except block and log the exception. Never swallow exceptions

Now the code works for your customer which buys you some time to find out the cause of the problem. My guess is that a) Excel doesn't produce a useful error message or b) the wrapper code swallows the real exception leaving you in the dark or c) the Excel method returns an error code (like "false" for "failed") and you need to call another Excel method to determine what the cause of the problem is.

[EDIT] Based on the comment below which boil down to "My boss doesn't care and there is nothing I can do": You're missing a crucial point: It's your bosses duty to make the decision but it your duty to give her a list of options along with pros/cons so that she can make a sound decision. Just sitting there saying "I can't do anything" will get you into the trouble that you're trying to avoid.

Example:

Solution 1: Ignore the errors

Pro: Least amount of work Con: There is a chance that the resulting data is wrong or random. If important business decisions are based on it, there is a high risk that those decisions will be wrong.

Solution 2: Log the errors

Pro: Little amount of work, users can start to use the results quickly, buys time to figure out the source of the problem Con: "If you can't fix it today, what makes you think you will have time to fix it tomorrow?" Also, it might take you a long time to find the source of the problem because you're no expert

Solution 3: Ask an expert

Find an expert in the field and help him/her have a look/improve the solution.

Pro: Will get a solution much more quickly than learning the ins and outs of COM yourself Con: Expensive but high chance of success. Will also find problems that we don't even know about.

...

I think you see the pattern. Bosses make wrong decisions because we (willingly) let them. Any boss in the world is happy for hard facts and input when they have to make a decision (well, those who don't shouldn't be bosses, so this is a surefire way to know when to start looking for a new job).

If you select solution #2, go for the wrapper approach. See the docs how to write a decorator (example from IBM). It's just a few minutes of work to wrap all the methods and it will give you something to work with.

The next step is to create a smaller example which sometimes fails and then post specific questions about Python, Excel and the COM wrapper here to figure out the reason for the problems.

[EDIT2] Here is some code that wraps the "dangerous" parts in a helper class and makes updating the styles more simple:

class BorderHelper(object):
    def __init__(self, excel):
        self.excel = excel

    def set( type, LineStyle = None, Weight = None, Color = None ):
        border = self.excel.Selection.Borders( type )

        try:
            if LineStyle is not None:
                border.LineStyle = LineStyle
        except:
            pass # Ignore if a style can't be set

        try:
            if Weight is not None:
                border.Weight = Weight
        except:
            pass # Ignore if a style can't be set

        try:
            if Color is not None:
                border.Color = Color
        except:
            pass # Ignore if a style can't be set

Usage:

    borders = BorderHelper( excel )

    borders.set( xlDiagonalDown, LineStyle = xlNone )
    borders.set( xlDiagonalUp, LineStyle = xlNone )
    borders.set( xlEdgeLeft, LineStyle = xlContinuous, Weight = xlThin, Color = xlAutomatic )
    ...

Answer 3

This just wraps functions calls, but you can extend it to handle attribute access as well, and to proxy the results of nested attribute accesses, finally just wrapping the __setattr__ in your try:except block.

It might be sensible to swallow only some specific exception types in your case (as @vsekhar says).

def onErrorResumeNext(wrapped):
    class Proxy(object):
        def __init__(self, fn):
            self.__fn = fn

        def __call__(self, *args, **kwargs):
            try:
                return self.__fn(*args, **kwargs)
            except:
                print "swallowed exception"

    class VBWrapper(object):
        def __init__(self, wrapped):
            self.wrapped = wrapped

        def __getattr__(self, name):
            return Proxy(eval('self.wrapped.'+name))

    return VBWrapper(wrapped)

Example:

exceptionProofBorders = onErrorResumeNext(excel.Selection.Borders)
exceptionProofBorders(xlDiagonalDown).LineStyle = xlNone
exceptionProofBorders(xlDiagonalup).LineStyle = xlNone