Q about assignment and references

Chris Rebert clp2 at rebertia.com
Wed Apr 7 05:29:46 CEST 2010


On Tue, Apr 6, 2010 at 7:51 PM, jdbosmaus <jayjoeryu at 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



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