Like any architect, seeing people use the system and what they put out on it is where the pleasure of building comes from. I was debugging some bits the other day and happened upon this movie made by the College of Fine Arts (COFA Digital Media student John Turello). It’s a wonderful movie I can – and I am sure you can - relate to:

You can also watch Monkey Spoon on UNSWTV or on YouTube.

If you’re using python with Cocoa, you might run into problem with method signatures. Because python is type-less, sometimes you have to explicitly set the method signatures for your object’s methods. If you are getting wacky errors for seemingly no reason this might be why. The errors this type of problem generate look something like:

does not implement protocol NSDraggingDestination: the signature for method performDragOperation: is c12@0:4@8 instead of B12@0:4@8

You can fix this by explicitly setting the method’s signature using the @objc.signature decorator. For example:

import objc

...

@objc.signature('B12@0:4@8')
def prepareForDragOperation_(self, sender):
        #print "prepareForDragOperation_"
        return True

You probably have used a decorators when creating methods for @objc.IBAction.

But the question for me became, how the heck do you define the signature? That is some alien looking code. Well, those things are called “Type Encodings” (so you have a search term), and you can read about them in the Apple documentation.

Here is the current link, but for some crazy reason Apple seems to be moving around their documentation and breaking links all over the place. Plus the back button doesn’t work on their new documentation format – I am not amused. You should also note that the documentation is for the iPhone which I am assuming is the same for the desktop (you can obviously only use python on the desktop).

Here is a copy of the table that is on that page

It goes on to define “Objective-C method encodings”

Even with that information though, it is a bit complicated to understand or write these signatures by hand. For example, the post’s title: ‘v@:@@o^@’ appears to be a method that returns void (or an object?) and takes three parameters. First a normal object, the second an out object and the third is a pointer to an object.

Kind of an interesting gotcha for a type-less high level language.

I hit this error using python on Mac OS X Leopard while writing a Cocoa based application and using minidom. I was doing a simple httplib.HTTPConnection request to a server and getting back XML in the body of the document. I was pulling the XML out using the response.read() method.

The server was setting the mime type and encoding correctly, and a type() call against the result showed it as a valid python string (<type ’str’>) and it was well formed XML. However, for some reason whenever I would feed it to minidom.parseString() Cocoa (or the PyObjC bridge or whatever) would die with the error “TypeError: Unsupported format string type”

I tried making the string an NSString thinking it was some Mac OS X implementation problem (various versions of NSString.alloc().initWithString_(str_response) setting encodings, copies of objects, etc), but it always caused the same error.

In the end, I simply gave up on the minidom library and used ElementTree instead (from xml.etree import ElementTree). ElementTree.fromstring(str_response) worked out of the box.

I hope this saves someone some time. It had me stumped for several hours since there is no other information beyond “Unsupported format string type” with the stack trace of:

File "/System/Library/Frameworks/Python.framework/Versions/2.5/Extras/lib/python/PyObjCTools/AppHelper.py", line 235, in runEventLoop

I am still loving python though :)

Update

After working with ElementTree a bit more, I’ve discovered that the real culprit to this difficult-to-figure-out error message was likely due to the try/except statement around the method call. For example it was doing this:

try:
    crazy.thing()
except Exception, e:
    rsp = NSRunAlertPanel(
        u"Bad Things", 
        e, 
        u"Let Me Try Again", 
        u"Quit", 
        None)

Any exception in the crazy.thing method will be masked by the “TypeError: Unsupported format string type” error when it tries to display e. To fix it I simply casted e to a string:

try:
    crazy.thing()
except Exception, e:
    rsp = NSRunAlertPanel(
        u"Bad Things", 
        str(e), 
        u"Let Me Try Again", 
        u"Quit", 
        None)

So the real cause of my minidom problem is unknown since I’ve moved on to ElementTree. I am a bit disappointed in the error reporting with python Cocoa, but other than a few elusive and confusing errors it is sweet, and fun development platform.