Re: key - key pairs
Man, this is not my week! Another bug in my posted code! The posted version of SymmetricDict fails when adding an entry in which the key equals the value. First bug is in __setitem__ in which the insertion is done twice, which is wasteful but benign. The second bug is in __delitem__, which throws an exception when we try to delete the back-pointing entry - which was already deleted since there is only one entry. Another cautionary tale on the value of testing... Here it the improved SymmetricDict code. -- Paul class SymmetricDict(dict): def __delitem__(self,k): v = self[k] super(SymmetricDict,self).__delitem__(k) if not v==k: super(SymmetricDict,self).__delitem__(v) def __setitem__(self,k,v): if k in self: del self[k] if v in self: del self[v] super(SymmetricDict,self).__setitem__(k,v) if not v==k: super(SymmetricDict,self).__setitem__(v,k) -- http://mail.python.org/mailman/listinfo/python-list
key - key pairs
Hello, is there in python a kind of dictionary that supports key - key pairs? I need a dictionary in which I can access a certain element using two different keys, both unique. For example: I've a dictionary with strings and times. Sometimes I have the string and I want to have the time, other time I've the time and I want the string. It is important that one of the keys supports the min/max builtin function. Thanks, Florian -- http://mail.python.org/mailman/listinfo/python-list
Re: key - key pairs
is there in python a kind of dictionary that supports key - key pairs? I need a dictionary in which I can access a certain element using two different keys, both unique. A Python dictionary needs a unique key, so a pair of keys is still one unique key, but probably it is some kind of misunderstanding here, because a dictionary is not a database which needs a key to quickly find an entry, so it can have as many keys as required. What about two dictionaries where each has as a value a key to the target dictionary with the actual values? For example: I've a dictionary with strings and times. Sometimes I have the string and I want to have the time, other time I've the time and I want the string. It is important that one of the keys supports the min/max builtin function. From this example it seems, that what is needed is a two-way dictionary. I don't know about a special kind of dictionary for this, so maybe someone else knows about such. I can only recommend to use two dictionaries, where the second one is created out of the first one, so that they key, value pair is reversed. If you need some code for the latter, let me know. Claudio Florian Lindner [EMAIL PROTECTED] schrieb im Newsbeitrag news:[EMAIL PROTECTED] Hello, is there in python a kind of dictionary that supports key - key pairs? I need a dictionary in which I can access a certain element using two different keys, both unique. For example: I've a dictionary with strings and times. Sometimes I have the string and I want to have the time, other time I've the time and I want the string. It is important that one of the keys supports the min/max builtin function. Thanks, Florian -- http://mail.python.org/mailman/listinfo/python-list
Re: key - key pairs
Just an example of dictionary object in python.
PythonWin 2.4.1 (#65, Mar 30 2005, 09:33:37) [MSC v.1310 32 bit
(Intel)] on win32.
Portions Copyright 1994-2004 Mark Hammond ([EMAIL PROTECTED]) -
see 'Help/About PythonWin' for further copyright information.
d = {}
d[key1] = value1
d[key2] = value2
d[key3] = value3
d[key3] = value3
for i, j in d.items():
... print i, j
...
key3 value3
key2 value2
key1 value1
if d.has_key(key1):
... print d[key1]
... else:
... print no key
...
value1
if d.has_key(not in dict):
... print d[key1]
... else:
... print no key
...
no key
HTH
Sameer
--
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Re: key - key pairs
On Thursday 23 June 2005 02:40 pm, Florian Lindner wrote: is there in python a kind of dictionary that supports key - key pairs? I need a dictionary in which I can access a certain element using two different keys, both unique. For example: I've a dictionary with strings and times. Sometimes I have the string and I want to have the time, other time I've the time and I want the string. It is important that one of the keys supports the min/max builtin function. Well, really, you're always using one or the other as the key and the other as the value. Furthermore, it is not in the general case assured that you can do this --- the keys may not really be 1:1. If you are content to restrict yourself to the 1:1 case, you can construct an inverse dictionary from the first dictionary like this: time2string = dict([ (b,a) for a,b in string2time.items() ]) Note that if string2time has duplicate values, this will arbitrarily pick one (in a consistent, but implementation dependent way) to use as the key in the inverse mapping. -- Terry Hancock ( hancock at anansispaceworks.com ) Anansi Spaceworks http://www.anansispaceworks.com -- http://mail.python.org/mailman/listinfo/python-list
Re: key - key pairs
Hi All-- Terry Hancock wrote: On Thursday 23 June 2005 02:40 pm, Florian Lindner wrote: is there in python a kind of dictionary that supports key - key pairs? I need a dictionary in which I can access a certain element using two different keys, both unique. For example: I've a dictionary with strings and times. Sometimes I have the string and I want to have the time, other time I've the time and I want the string. It is important that one of the keys supports the min/max builtin function. Well, really, you're always using one or the other as the key and the other as the value. Furthermore, it is not in the general case assured that you can do this --- the keys may not really be 1:1. If you are content to restrict yourself to the 1:1 case, you can construct an inverse dictionary from the first dictionary like this: time2string = dict([ (b,a) for a,b in string2time.items() ]) Note that if string2time has duplicate values, this will arbitrarily pick one (in a consistent, but implementation dependent way) to use as the key in the inverse mapping. Well, Florian said, using two different keys, both unique; if that is true, then a single key maps to a single value vice versa. Easiest way, it seems to me, would be to subclass dict and provide get/set that always insert the value as a key. So that dict[string]=time also means dict[time]=string. Only one dict required then. Or am I missing something? Metta, Ivan -- Ivan Van Laningham God N Locomotive Works http://www.andi-holmes.com/ http://www.foretec.com/python/workshops/1998-11/proceedings.html Army Signal Corps: Cu Chi, Class of '70 Author: Teach Yourself Python in 24 Hours -- http://mail.python.org/mailman/listinfo/python-list
Re: key - key pairs
Ivan Van Laningham wrote: Well, Florian said, using two different keys, both unique; if that is true, then a single key maps to a single value vice versa. Of course you are right. I got caught up in the problem I imagined (the pair being unique). ... subclass dict and provide get/set that always insert the value as a key. So dict[string]=time also means dict[time]=string. Or am I missing something? Yup, getting to min and max. I presume that will be key-dependent. --Scott David Daniels [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
Re: key - key pairs
No need to update __getitem__, since the modified __setitem__ drops in the reverse values. But __delitem__ needs overriding, and some special guard needs to be added to __setitem__ to prevent orphaning any old value:key entries. -- Paul Here's one possible solution: class SymmetricDict(dict): def __delitem__(self,x): v = self[x] super(SymmetricDict,self).__delitem__(x) super(SymmetricDict,self).__delitem__(v) def __setitem__(self,k,v): if k in self: del self[k] if v in self: del self[v] super(SymmetricDict,self).__setitem__(k,v) super(SymmetricDict,self).__setitem__(v,k) sd = SymmetricDict() sd[A] = 1 print sd[A] print sd[1] sd[A] = 2 print sd[A] print sd[2] print sd[1] prints: 1 A 2 A Traceback (most recent call last): File symmetricDict.py, line 25, in ? print sd[1] KeyError: 1 -- http://mail.python.org/mailman/listinfo/python-list
