Re: [Tutor] Question about Dictionaries
On Tue, 17 Aug 2010 09:12:39 am Huy Ton That wrote: > What do you mean by subclass? It's a fundamental term from object-oriented programming. If you have a class that defines certain data and behaviour, you can create a *subclass* that inherits the same data and behaviour, except for specific examples which are over-ridden or over-loaded. A simple example: class MyInt(int): # creates a subclass of int def __str__(self): return "My Integer %d" % self MyInts are exactly the same as built-in ints except their string form is different. >>> n = MyInt(42) >>> print n My Integer 42 -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Question about Dictionaries
On 8/16/2010 4:12 PM Huy Ton That said...
What do you mean by subclass?
If you need repeated access such that iterating over a large dict frequently
impacts performance, you could subclass dict and maintain a second index
allowing instant access to the keys associated with a specific value.
HTH,
Emile
Something along these lines:
class myDict(dict):
def __init__(self):
self.altKeys = {}
def __setitem__(self,ky,val):
self.altKeys[val]=ky
return dict.__setitem__(self, ky,val)
def lookup(self,ky):
return self.altKeys[ky]
a = myDict()
a[1] = 111
a[2] = 222
a[3] = 333
a[3]
a.lookup(333)
Emile
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Re: [Tutor] Question about Dictionaries
On Tue, 17 Aug 2010 03:44:33 am Chorn, Guillaume wrote: > Hi All, > > I know that I can look up the value for a particular key in a > dictionary, but can I look up the key associated with a particular > value? I understand that this could be problematic from the > standpoint of multiple keys having the same value, but even then I > feel like Python could just return a list of keys with that value. There is no built-in way of doing so, but it's easy to write your own. def reverse_lookup(d, target, first_only=False): found = [] for key, value in d.items(): if value == target: if first_only: return key found.append(key) return found Or if you want a one-liner, use a list-comp: [k for (k,v) in d.items() if v == target] -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Question about Dictionaries
Thanks Dave. Actually I figured out a relatively simple solution to my specific problem--since the dictionary I made was the result of zipping together two lists, I simply made a second dictionary with the lists switched around after the zip command and used that. It worked in my case because all key-value pairings were unique... -Original Message- From: Dave Angel [mailto:da...@ieee.org] Sent: Monday, August 16, 2010 4:03 PM To: Chorn, Guillaume Cc: tutor@python.org Subject: Re: [Tutor] Question about Dictionaries Chorn, Guillaume wrote: > Hi All, > > I know that I can look up the value for a particular key in a > dictionary, but can I look up the key associated with a particular > value? I understand that this could be problematic from the standpoint > of multiple keys having the same value, but even then I feel like Python > could just return a list of keys with that value. > > Thanks! > > Guillaume > > Notice: This e-mail message, together with any attachments, contains > information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, > New Jersey, USA 08889), and/or its affiliates Direct contact information > for affiliates is available at > http://www.merck.com/contact/contacts.html) that may be confidential, > proprietary copyrighted and/or legally privileged. It is intended solely > for the use of the individual or entity named on this message. If you are > not the intended recipient, and have received this message in error, > please notify us immediately by reply e-mail and then delete it from > your system. > > As long as you don't mind it being (relatively) slow, you could do something like (untested): [key for key, item in mydict if item == value] DaveA Notice: This e-mail message, together with any attachments, contains information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, New Jersey, USA 08889), and/or its affiliates Direct contact information for affiliates is available at http://www.merck.com/contact/contacts.html) that may be confidential, proprietary copyrighted and/or legally privileged. It is intended solely for the use of the individual or entity named on this message. If you are not the intended recipient, and have received this message in error, please notify us immediately by reply e-mail and then delete it from your system. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Question about Dictionaries
"Chorn, Guillaume" wrote
dictionary, but can I look up the key associated with a particular
value?
Not directly but its not hard to do bearing in mind you will
get a collection back.:
d = {1:10,2:20,3:10,4:20}
val = 10
ks = [k for k in d if d[k] == val]
ks
[1, 3]
You need to trawl the dictionary whatever you do
(unless you get very sophisticated and build a double
dictionary class, but then insertions get slow because
you have to update both versions...)
HTH,
--
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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Re: [Tutor] Question about Dictionaries
What do you mean by subclass? On Aug 16, 2010 3:26 PM, "Emile van Sebille" wrote: On 8/16/2010 10:44 AM Chorn, Guillaume said... > > Hi All, > > I know that I can look up the value for a particular key in a > dictionary, but can... Yes. But you'll need to implement it. There are likely modules out there that'll do this, but it'd take more time to look up and evaluate than to simply write and implement exactly what you need. > I understand that this could be problematic from the standpoint > of multiple keys having the sa... So, in untested code you'd have a function something like: result = [] for ky, val in dict.items(): if val == targetval: result.append(ky) return result If you need repeated access such that iterating over a large dict frequently impacts performance, you could subclass dict and maintain a second index allowing instant access to the keys associated with a specific value. HTH, Emile ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Question about Dictionaries
Chorn, Guillaume wrote: Hi All, I know that I can look up the value for a particular key in a dictionary, but can I look up the key associated with a particular value? I understand that this could be problematic from the standpoint of multiple keys having the same value, but even then I feel like Python could just return a list of keys with that value. Thanks! Guillaume Notice: This e-mail message, together with any attachments, contains information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, New Jersey, USA 08889), and/or its affiliates Direct contact information for affiliates is available at http://www.merck.com/contact/contacts.html) that may be confidential, proprietary copyrighted and/or legally privileged. It is intended solely for the use of the individual or entity named on this message. If you are not the intended recipient, and have received this message in error, please notify us immediately by reply e-mail and then delete it from your system. As long as you don't mind it being (relatively) slow, you could do something like (untested): [key for key, item in mydict if item == value] DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Question about Dictionaries
> I know that I can look up the value for a particular key in a > dictionary, but can I look up the key associated with a particular > value? I am using bidict in one of my projects: http://pypi.python.org/pypi/bidict/0.1.1 It's probably a bit more complex than what I need, but the parts I am using are working fine, and the design of the extra features is kind of interesting. I think something like this should be in the stdlib, but there is some controversy about how exactly it should work. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Question about Dictionaries
On 8/16/2010 10:44 AM Chorn, Guillaume said... Hi All, I know that I can look up the value for a particular key in a dictionary, but can I look up the key associated with a particular value? Yes. But you'll need to implement it. There are likely modules out there that'll do this, but it'd take more time to look up and evaluate than to simply write and implement exactly what you need. I understand that this could be problematic from the standpoint of multiple keys having the same value, but even then I feel like Python could just return a list of keys with that value. So, in untested code you'd have a function something like: result = [] for ky, val in dict.items(): if val == targetval: result.append(ky) return result If you need repeated access such that iterating over a large dict frequently impacts performance, you could subclass dict and maintain a second index allowing instant access to the keys associated with a specific value. HTH, Emile ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
