Intercepting attributes in python

Question

I wasn't able to figure out what to search for this, so if it's already out there, I apologize in advance, and would be happy to just take a search reference to somewhere else.

Some background: I'm working on some extra features for a python Excel library and one thing I'd like to be able to do is apply a style to a range of cells in a single call. Right now, our code permits us to do something like ws[1][1].font.bold = True to bold cell A1. We also have a range operator that permits us to do something like ws.range("A1:B2").value = 1 to set all the values in that range to 1.

What I'd like to do now is add the ability to do ws.range("A1:B2").font.bold = True, but I can't figure out how. range("A1:B2") is an object that doesn't explicitly reference the cells (just the two corners to save on space), but does not explicitly have the .font or .value attribute, rather it's intercepted via a @property decorator to apply appropriately.

The thing is, if I add an @property decorator to create a font property, I can return a new font object, but I have no way of getting this font object after the bold attribute has been modified to apply it to all the cells in the range.

I was thinking I could yield the font, then add the apply logic after the yield, but that doesn't work as yield returns a generator. The only other idea I can think of is to create an explicit .font object, then in the Range class, override .font.__setattr__ to listen for changes, but then I can't figure out how to get the original range from within __setattr__ that needs to be applied without having to create a third intermediary class. Not only does that seem rather awkward, it also seems incredibly unpythonic.

Is something like this possible without having to result to rather esoteric methods? I hope my explanation was clear and again, if it's already out there, feel free to point me to an existing reference -- I just didn't know what to search for.

You're almost there with your last idea. Create a .font object that holds a reference to the original Range. Then override __setattr__() in the .font to update styles of all cells in the range. — millimoose
– millimoose, Commented Oct 9, 2013 at 0:11
In fact you could probably do this generically if you wanted, by having Range return the same "fake" property for any attribute name, and have it delegate all __setattr__ calls to the original. — millimoose
– millimoose, Commented Oct 9, 2013 at 0:13
Ah, okay, that seems like it might work, but there's not a less esoteric way to do it? I feel like that would be incredibly confusing to maintain. — kevmo314
– kevmo314, Commented Oct 9, 2013 at 0:14
I'm not sure how this is an esoteric thing. You want a write to the font's attributes to be applied to all cells in a range. So, you intercept writes to font's attributes, and apply them to all cells in the correct range. It's really the most straightforward way to do it - you don't need to detect modifications on the .font or anything, you just write back things immediately. — millimoose
– millimoose, Commented Oct 9, 2013 at 0:16

millimoose · Accepted Answer · 2013-10-09 15:23:45Z

1

To expand on my comment, you'd do something like:

class RangeFont(object):

    def __init__(self, range):
        self._range = range

    def __setattr__(self, name, value):
        if name.startswith('_'):
            super(RangeFont, self).__setattr__(name, value)
            return

        for cell in self._range: # assuming a range is iterable
            setattr(cell.font, name, value)


class Range(object):
    @property
    def font(self):
        return RangeFont(self)

A limitation of the above code is that it won't work for further levels of attributes, that is, you can't do ws.Range(...).font.foo.bar = 123. (That said you could extend this by having RangeFont.__getattr__ return a similar wrapper object.)

edited Oct 9, 2013 at 15:23

answered Oct 9, 2013 at 0:18

millimoose

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2 Comments

kevmo314 Over a year ago

okay, after giving up a couple times, I managed to implement this generically for deeper levels without having to make it too much more complicated: pastebin.com/raw.php?i=aAqmq554 :)

millimoose Over a year ago

@kevmo314 Oh, derp, I didn't remember you need to work around the setattr override in __init__. Anyway, glad you got what you need.

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