On Tue, Apr 6, 2010 at 7:51 PM, jdbosmaus <jayjoe...@yahoo.com> wrote: > Pretty new to Python, but I thought I understood what is meant by "an > assignment is a reference."
I recommend the effbot's treatment of the calling semantics: http://effbot.org/zone/call-by-object.htm ...But I don't think that's the issue here. > Until I tried to understand this. > Here is a (fragment of an) event handler for a group of three wxPython I believe the unintuitiveness here is due to quirks in the exact way wxPython wraps wxWindows, not Python itself. > [code] > def ScanModeHdlr(self, event): > esource = event.GetEventObject() #return button that was > pressed > > fontUn = esource.Font > fontSel = esource.Font > fontUn.Style = wx.NORMAL > fontSel.Style = wx.ITALIC > fontUn.PointSize = 9 > fontSel.PointSize = 12 > > btnlist = (self.TBN_NewScan, self.TBN_CheckScan, > self.TBN_ReScan) > > for x in btnlist: > if x is esource: > x.Font = fontSel > else: > x.Font = fontUn > [/code] > > Now, what bothers me is that in the 3rd and 4th lines, the RHS > "esource.Font" returns a wx.Font object that is a *copy* of the font > object of the button. <snip> >Then in > the for-loop we assign the font object to a LHS object that ... > semantically, looks exactly like the thing that returned a *copy* up > above. <snip> > I could understand if the setting-semantics in the for-loop needed to > be something like "x.SetFont(fontUn)", or even "x.SetFont = fontUn". > (for "could understand" read "would be much happier.") > I could also understand if the 3rd and 4th lines needed to look like > "fontUn = copy(esource.Font)". (which doesn't work; "copy" doesn't > know what to do.) In fact, I /suspect/ (I haven't used wxPython, so I can't be sure) both of what you describe is /exactly/ what's going on here. wxPython is /probably/ invoking the magic of properties (http://docs.python.org/library/functions.html#property) to make `esource.Font` return a copy and turn `esource.Font = fontUn` into a setter method call behind the scenes. > What I can't understand is how the semantics that actually works makes > sense in terms of Python's assignment conventions. Read about property() [see above link] and be enlightened. It basically lets accesses and assignments to an attribute trigger method calls; alternately, and probably more intuitively, you can look at it as allowing a getter-setter pair to be presented (syntactically) like a vanilla attribute. Hence, esource.Font /looks/ like an attribute, but doesn't quack [behave] like a normal one. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
