Class hierarchy problem
I'm in need of help to solve this Python (ver. 3.3) problem: I have a hierarchy of classes (SubA, SubAB, SubB, ..., SubBCA, SubC,...), each of which is inheriting from a chain of superclasses with a common baseclass(Sup) on top. (So far, no problem) Now, I want to create instances of the correct subclasstype as decided by the common baseclass, like this: i = Sup(args_allowing_the_baseclass_to_deduce_correct_subclass) where i can be of any class except Sup itself (as decided by Sup) Now, the problem: How to design the __new__() and __init__() methods for the various classes in order to achieve what I want? (Some ten years I had the same problem (in a different context) and was helped by asking in this group. However, the solution has disappeared. Maybe the 2.x solution is not the same as in 3.x?) -- http://mail.python.org/mailman/listinfo/python-list
Re: Class hierarchy problem
On 06/08/2013 16:02, Chris Angelico wrote: My classhierarchy is like a multilevel tree where each non-leaf node (class) is given knowledge about its nearest subclasses and their 'capacities'. So, my idea is to let the 'upper' class recursively choose which of its nearest subclasses is the 'correct' one, until approaching a 'leaf' class from which the instance should be created. And, given my knowledge that a solution along the lines of this idea has been designed and was working, I'm still hopeful ... (or I'll have to investigate all those old backup-DVDs) [ responding on-list - I hope it was mere oversight that had this come privately to me alone ] This is code smell; this recursive search for the "right" class seems likely to be wrong. Can you have the classes perhaps register themselves in some way? On what basis is a superclass to determine that one of its subclasses should handle this request? ChrisA Consider a botanical classification system (somewhat analogous to my 'problem' as it effectively is related to classification of entities): A Domain should know about its Kingdoms, a Kingdom should know about its Phylums, ... a Genus should know about its Species. Of course it is possible to implement such a decision tree as a 'factory'. However, I would rather prefer to encapsulate those decisions at the class level where they 'belong'. BrJohan -- http://mail.python.org/mailman/listinfo/python-list
Python's re module and genealogy problem
For some genealogical purposes I consider using Python's re module. Rather many names can be spelled in a number of similar ways, and in order to match names even if they are spelled differently, I will build regular expressions, each of which is supposed to match a number of similar names. I guess that there will be a few hundred such regular expressions covering most popular names. Now, my problem: Is there a way to decide whether any two - or more - of those regular expressions will match the same string? Or, stated a little differently: Can it, for a pair of regular expressions be decided whether at least one string matching both of those regular expressions, can be constructed? If it is possible to make such a decision, then how? Anyone aware of an algorithm for this? -- https://mail.python.org/mailman/listinfo/python-list
Re: Python's re module and genealogy problem
On 11/06/2014 14:23, BrJohan wrote:
For some genealogical purposes I consider using Python's re module.
Rather many names can be spelled in a number of similar ways, and in
order to match names even if they are spelled differently, I will build
regular expressions, each of which is supposed to match a number of
similar names.
I guess that there will be a few hundred such regular expressions
covering most popular names.
Now, my problem: Is there a way to decide whether any two - or more - of
those regular expressions will match the same string?
Or, stated a little differently:
Can it, for a pair of regular expressions be decided whether at least
one string matching both of those regular expressions, can be constructed?
If it is possible to make such a decision, then how? Anyone aware of an
algorithm for this?
Thank you all for valuable input and interesting thoughts.
After having reconsidered my problem, it might be better to approach it
a little differently.
Either to state the regexps simply like:
"(Kristina)|(Christina)|(Cristine)|(Kristine)"
instead of "((K|(Ch))ristina)|([CK]ristine)"
Or to put the namevariants in some sequence of sets having elements like:
("Kristina", "Christina", "Cristine", "Kristine")
Matching is then just applying the 'in' operator.
I see two distinct advantages.
1. Readability and maintainability
2. Any namevariant occurring in just one regexp or set means no risk of
erroneous matching.
Comments?
--
https://mail.python.org/mailman/listinfo/python-list
Re: Python for EXIF-info-additions ?
Bruno Dilly wrote: > I think you can find what do you need into this repository, it's a > creative commons tool: > https://svn.berlios.de/svnroot/repos/cctools/publisher/branches/flickr-storage-branch/ > > take a look in the follow directory: > ccpublisher/jpeg/ > > I'm not sure if it's what you want, let me know. > > See you, > > Dilly > After glancing through the code I think it offers all - or most of - what I need. Thank you! /Bror J. -- http://mail.python.org/mailman/listinfo/python-list
Unpythonic? Impossible??
Assume having this class hierarchy: (in principle and without details) class A(object): class B1(A): class B2(A): class C1(A1): class C2(A1): class C3(B1): class C4(B2): each of those classes have an initializer __init__(self, data): and an overloaded __new__(cls, data): is it then possible to have this call: obj = A(data) return an instance of that particular class (e.g. class C3) in the hierarchy that - as decided by the __new__ functions - is the 'correct' one? A.__new__ could select between A, B1 and B2, while B1.__new__ could choose from B1, C3 and C4. I know how to use a class factory - and could work around using such a mechanism. However I am interested to know if I could let the classes do the work by themselves. /BJ -- http://mail.python.org/mailman/listinfo/python-list
Re: Unpythonic? Impossible??
"Scott David Daniels" <[EMAIL PROTECTED]> skrev i meddelandet news:[EMAIL PROTECTED] > BrJohan wrote: ... >> is it then possible to have this call: >> obj = A(data) >> return an instance of that particular class (e.g. class C3) in the >> hierarchy that - as decided by the __new__ functions - is the 'correct' >> one? >> >> A.__new__ could select between A, B1 and B2, while B1.__new__ could >> choose from B1, C3 and C4. >> >> I know how to use a class factory - and could work around using such a >> mechanism. However I am interested to know if I could let the classes do >> the work by themselves. > > Yes, it can be done. Yes, it is unclear (and hence UnPythonic). > The class factory _is_ the straightforward way to do this. The > following is the workaround (if you have to maintain A(...)): > > > class A(object): > def __new__(class_, *args, **kwargs): > if class_ is A: > if want_a_B1(*args, **kwargs): > return B1(*args, **kwargs) > elif want_a_B2(*args, **kwargs): > return B2(*args, **kwargs) > return object.__new__(class_) # Use *a,... except for object > > class B1(A): > def __new__(class_, *args, **kwargs): > if class_ is B1: > if want_a_B1(*args, **kwargs): > return B1(*args, **kwargs) > elif want_a_B2(*args, **kwargs): > return B2(*args, **kwargs) > return super(B1, class_).__new__(class_, *args, **kwargs) > > > --Scott David Daniels > [EMAIL PROTECTED] Agreed that the class factory method most often (maybe always) is the best one. For certain reasons, and in this particular case, I prefer the UnPythonic way. Sometimes it's good to have "more than one way to do it". It was the "return object.__new__(class_) " that I did not came to think of myself, that did it. Thank you for yor helpfulness. BrJohan -- http://mail.python.org/mailman/listinfo/python-list
Re: python and databases
On 2017-03-14 20:59, Xristos Xristoou wrote: I have a database in microsoft ACCESS with about 150 records.. if I want to get some data from this database using a query in python and i want to store in some variables in python that will do this ? to avoid the 150 if ...: Using the standard library the python? know easily put it an SQL query but i canot use additional library python. Some years ago I did some experiments using pyodbc and MS Access. Some success. -- https://mail.python.org/mailman/listinfo/python-list
