voger <vogernewslett...@yahoo.gr> writes: > I have a really hard time understanding where methods are defined in > python classes.
Reading the rest of your message, I think you're saying that you can't locate *which* class defines a method > […] With python everything looks like magic. It can seem that way. Fortunately, most of the magic needs to be explicitly declared in the code, so it's a matter of knowing what to look for. In this message, I'll guide you in when you should expect magic, and where to look for it. > Methods and properties appearing out of nowhere and somehow easing the > task at hand. I am not complaining for their existence but I really > can't understand how and what is defined so I can use it. This is a valid complaint, and certainly some libraries can get rather too clever with the “magic behaviour”, leading to difficulty in understanding what is happening well enough to debug problems. > Enough ranting and let me give an example of what I mean. > > Let's have a look at the pygeocoder library located here: > http://code.xster.net/pygeocoder/src/8888c863f907f8a706545fa9b27959595f45f8c5/pygeolib.py?at=default The important part of that file is the ‘GeocoderResult’ class definition. It defines a lot of magic behaviour. In general, the magic often happens with “dunder”-named attributes; that is, attributes named with a leading and traling double-underscore. An attribute named “__foo__” is a strong indication that it will be treated specially and be invoked by the Python interpreter, *without* the programmer needing to invoke the attribute by name. Here is the class definition showing just the dunder-named attributes:: class GeocoderResult(collections.Iterator): … def __init__(self, data): … def __len__(self): … def __iter__(self): … def __getitem__(self, key): … def __unicode__(self): … if sys.version_info[0] >= 3: # Python 3 def __str__(self): return self.__unicode__() def __next__(self): return self.return_next() else: # Python 2 def __str__(self): return self.__unicode__().encode('utf8') def next(self): return self.return_next() … def __getattr__(self, name): … So already you should be on high alert: there is a large amount of “magic” in this class! Seemlingly-simple uses of this class and its instances will result in invocations of those custom methods, even without seeing the names used explicitly. > Some properties I can see them defined but some others like > sublocality or administrative_area_level_1 I don't see them defined > anywhere. The specific magic happening here is that, when you access an attribute on an object, Python will allow interception of that request by a custom ‘__getattr__’ method. As you can see, the ‘GeocoderResult’ class defines that method for its instances, so you will need to look there for custom behaviour when accessing attributes on instances of this class. <URL:http://docs.python.org/3/reference/datamodel.html#object.__getattr__> Similarly, the class defines ‘__getitem__’ to intercept ‘foo[bar]’, it defines ‘__len__’ to intercept ‘len(foo)’, it defines ‘__iter__’ to intercept attempts to iterate over an instance; and so on. Much magic! Personally, I'm suspicious whether this class really needs to be so awfully clever; it is certainly making the behaviour less intuitive, as demonstrated in your case. It smells of a poor design. > I hope I didn't bore you with this long text but this is really > confusing for me and I hope someone can help me understand these > things. One good decision made by the Python core developers early on was a convention of naming these special, magical-behaviour “hook” attributes in a way that they are clearly special and signal to the reader that Python will be using them implicitly. So the general advice is to understand that any “dunder”-named attribute needs to be researched, because it can be invoked *without* you necessarily being aware of that. Hope that helps! -- \ “Choose mnemonic identifiers. If you can't remember what | `\ mnemonic means, you've got a problem.” —Larry Wall | _o__) | Ben Finney _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
