On Fri, 29 Aug 2014 21:41:30 -0400, Terry Reedy <tjre...@udel.edu> wrote:
>On 8/29/2014 4:40 PM, Seymore4Head wrote: >> What is the logic behind that? > >You should better ask, "How does that happen?". Attend carefully and >you should learn much. > >In expressions, python replaces names with the object the name is bound >to. In batch mode, a bare name is equivalent to 'pass'. In interactive >mode, a bare "name" is equivalent to "print(repr(name))". > > >>> s = 'abc' > >>> s >'abc' > >>> print(str(s)) >abc > >>> print(repr(s)) >'abc' > >This is a handy shortcut. "repr(name)" in turn, is also a shortcut. It >looks for the .__repr__ attribute of type(name) or one of the >superclasses. If there in no override in the inheritance tree, you see >the default from object: > > >>> object() ><object object at 0x00000000001B2C70> > >You have probably seen this pattern already. Now, what is the class of >the object bound to 'help' (and 'quit' and 'exit')? > > >>> type(help) ><class '_sitebuiltins._Helper'> > >>> type(quit) ><class '_sitebuiltins.Quitter'> > >>> type(exit) ><class '_sitebuiltins.Quitter'> > >Each of these classes has a .__repr__ method and a .__call__ method. >The first simply echoes a fixed string regardless of input. > > >>> type(help).__repr__(object()) >'Type help() for interactive help, or help(object) for help about object.' > >We can play the same game with python code. > > >>> class Helper: > def __repr__(self): > return 'Type myhelp() for interactive help, or myhelp(object) > for help >about object.' > def __call__(self, arg): > help(arg) > > >>> myhelp = Helper() > >>> myhelp >Type myhelp() for interactive help, or myhelp(object) for help about object. > >>> myhelp(Helper) >Help on class Helper in module __main__: > >class Helper(builtins.object) > | Methods defined here: > | > | __call__(self, arg) > | > | __repr__(self) > | > | ---------------------------------------------------------------------- > | Data descriptors defined here: > | > | __dict__ > | dictionary for instance variables (if defined) > | > | __weakref__ > | list of weak references to the object (if defined) > > >> Couldn't help do the same thing as help()? > >Couldn't python stab people in the back by violating its own rules ;-? > >(See the doc for what repr and hence .__repr__ are supposed to do.) > >You should now understand that the interpreter has *no special >knowledge* about help, exit, or quit. The special behavior is built >into their classes. That is how Python objects work in relation to a >few simple syntax rules, like "callable(args)" meaning 'call callable >with args'. > > > older versions of python would allow ? as help. > >If I ever knew that, I have forgotten. The interpreter must have >treated '?' as a special case for interactive input. I suspect help was >changed when quit and exit were added. I think that is more than I wanted to know. :) Thanks -- https://mail.python.org/mailman/listinfo/python-list
