Hi Mark, Thus spoketh Mark Summerfield <l...@qtrac.plus.com> unto us on Sun, 24 Jun 2012 08:42:33 +0100:
> Hi, > > Right now it is possible to do > > bv = BooleanVar() > bv.set(any_old_rubbish) > b = bv.get() # b is an int not a bool > > I've reported this as a bug and commented that I think that BooleanVar, > IntVar, FloatVar, and StringVar should honor the type in their names > (i.e., bool, int, float, str). > > If this matters to you could you look at the issue and maybe comment? > > http://bugs.python.org/issue15133 I've been there and spent my 2 cents ;) Now I just wanted to leave a comment here, too. First, the return value of get() is not neccessarily an int: >>> b = BooleanVar() >>> b.set('yes') >>> b.get() True >>> b.set('1') >>> b.get() True >>> b.set(1) >>> b.get() 1 >>> b.set(True) >>> b.get() 1 >>> b.set('true') >>> b.get() True At the first glance this seems somewhat erratic, nevertheless there is an obvious pattern: apparently all the strings that tcl accepts as boolean values cause get() to return True / False whereas the special values True / False / 0 / 1 passed to set() cause get() to return 1 / 0. This might be considered a bug, but it is not a BooleanVar speciality, but the behavior of tkapp.getboolean() and we encounter it everywhere in tkinter. The fact that values like 'yes' are accepted as boolean values does not seem to make sense if you think of the BooleanVar as a *Python* boolean object, but if you think of it as a Python representation of a *Tcl* boolean it makes a lot of sense. Then, your above observation that you can pass "any_old_rubbish" to set() and get() will in return give you an integer is not exactly correct: >>> b.set(1.5) >>> b.get() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python3.1/tkinter/__init__.py", line 316, in get return self._tk.getboolean(self._tk.globalgetvar(self._name)) TypeError: must be string, not float >>> b.set([]) >>> b.get() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python3.1/tkinter/__init__.py", line 316, in get return self._tk.getboolean(self._tk.globalgetvar(self._name)) _tkinter.TclError: expected boolean value but got "[]" and so on. I agree that it seems odd that you need to call get() to see the error. OTOH, the special thing about the BooleanVariable is that get() will always return a boolean (at least if we accept 0 / 1 as bool), no one promised that set () only accepts bool values: >>> b.set.__doc__ 'Set the variable to VALUE.' >>> b.get.__doc__ 'Return the value of the variable as a bool.' >>> b.__doc__ 'Value holder for boolean variables.' >>> The problem with set() is of course, that it must accept anything that Tcl might use as boolean, otherwise we might run into trouble if somewhere the return value of some tcl command is directly passed to set() . I think if we do not consider the BooleanVar to be a Python boolean object but rather a convenience object for handling Tcl boolean values these limitations won't hurt too much. In fact I have been using BooleanVars for many years now, and they worked so well that I never even noticed these oddities. Regards Michael .-.. .. ...- . .-.. --- -. --. .- -. -.. .--. .-. --- ... .--. . .-. War is never imperative. -- McCoy, "Balance of Terror", stardate 1709.2 _______________________________________________ Tkinter-discuss mailing list Tkinter-discuss@python.org http://mail.python.org/mailman/listinfo/tkinter-discuss
