[EMAIL PROTECTED] wrote: >>A google search for biject.py and bijection.py >>produced no hits, so I suspect that this may not >>have been done before. > > > There are few (good too) implementations around, but they are called > bidict or bidirectional dicts. Sometimes I use this implementation, > with few changes: > http://www.radlogic.com.au/releases/two_way_dict.py
Thank you for this. You are quite right, a dictionary is a particular type of mapping. A mapping with an inverse is called (at least by me) a bijection. Therefore, as you say, bidict or something similar is correct for a bijection that is based on dictionaries. I had a look at the code in radlogic. There, the design philosophy is to add 'inverse operations' to a dictionary. Thus, it adds a method reversed_items. In my code, I used a different philosophy, which comes down to this. If a mapping is by design a bijection, then it should have an inverse method that gives the inverse mapping. This preserves the symmetry between a mapping and its inverse. (The inverse has an inverse, which is the original mapping.) Therefore, my semantics comes down to f, g = bidict() # Use the better name. assert f = g.inverse assert g = f.inverse and also f[a] = b if and only if g[b] = a By the way, it turns out that a bidict is not what my application needs. But I find it an interesting problem, and time spent on it I do not consider wasted. Best regards Jonathan -- http://mail.python.org/mailman/listinfo/python-list
