The problem with duck typing

Please support our Python advertiser: Programming Forums - DaniWeb Sister Site
Thread Solved

•
•
Join Date: Jun 2008
Posts: 46
Reputation: Fuse is an unknown quantity at this point 
Solved Threads: 3
Fuse's Avatar
Fuse Fuse is offline Offline
Light Poster

The problem with duck typing

 
0
  #1
Jun 8th, 2008
I have a problem guys. It's due to duck typing. Now I expected to run into something like this sooner or later, but I can't help but feel there's a better solution.

Python Syntax (Toggle Plain Text) 
  1. import re
  2. def patternMatching(pattern, string):  
  3.     matchList = re.findall(pattern, string)
  4.     print '\n'.join(['%s' % v for v in matchList])

If you run that function as patternMatching(r'The (fox)', 'The fox'), it gives you 'fox', right? Because matchList is ['fox'].

But what if you run it as patternMatching(r'(The) (fox)', 'The fox')? You get an error. Because matchList is [('The','fox')].

Duck typing is messing with my types, so the print function won't work.

Traceback (most recent call last):
  File "<pyshell#52>", line 1, in <module>
    print '\n'.join(['%s' % v for v in matchList])
TypeError: not all arguments converted during string formatting

What's a programmer to do? My patch solution is to check the type of the first element and then do two cases. But that won't hold if I get something like [('hello','bye'),'hello']... which I'm not sure I ever will, but the point remains.
Mir's Fuselage

Because what's not to like about being killed by a toilet seat from Mir's atmospheric re-entry?
Reply With Quote Quick reply to this message  
•
•
Join Date: Jul 2005
Posts: 1,221
Reputation: bumsfeld will become famous soon enough bumsfeld will become famous soon enough 
Solved Threads: 138
bumsfeld's Avatar
bumsfeld bumsfeld is offline Offline
Nearly a Posting Virtuoso

Re: The problem with duck typing

 
0
  #2
Jun 8th, 2008
It is only a duck if it acts, looks and sounds like one. Processed duck is not a duck.
Should you find Irony, you can keep her!
Reply With Quote Quick reply to this message  
•
•
Join Date: Jun 2008
Posts: 46
Reputation: Fuse is an unknown quantity at this point 
Solved Threads: 3
Fuse's Avatar
Fuse Fuse is offline Offline
Light Poster

Re: The problem with duck typing

 
0
  #3
Jun 8th, 2008
Actually, it it is looking, acting and sounding like a duck. It's a 1-tuple, so it's still a tuple. Python seems to think a 1-tuple is better expressed simply as the element within the 1-tuple, however, which causes this problem.
Mir's Fuselage

Because what's not to like about being killed by a toilet seat from Mir's atmospheric re-entry?
Reply With Quote Quick reply to this message  
•
•
Join Date: Aug 2005
Posts: 1,546
Reputation: Ene Uran has a spectacular aura about Ene Uran has a spectacular aura about 
Solved Threads: 174
Ene Uran's Avatar
Ene Uran Ene Uran is offline Offline
Posting Virtuoso

Re: The problem with duck typing

 
0
  #4
Jun 8th, 2008
I think you have to write it this way:
patternMatching(r"The|fox", 'The little brown fox')
drink her pretty
Reply With Quote Quick reply to this message  
•
•
Join Date: Jun 2008
Posts: 128
Reputation: slate is an unknown quantity at this point 
Solved Threads: 31
slate slate is offline Offline
Junior Poster

Re: The problem with duck typing

 
0
  #5
Jun 8th, 2008
Help on function findall in module re:

findall(pattern, string, flags=0)
Return a list of all non-overlapping matches in the string.

If one or more groups are present in the pattern, return a
list of groups; this will be a list of tuples if the pattern
has more than one group.

Empty matches are included in the result.
Reply With Quote Quick reply to this message  
•
•
Join Date: Jun 2008
Posts: 46
Reputation: Fuse is an unknown quantity at this point 
Solved Threads: 3
Fuse's Avatar
Fuse Fuse is offline Offline
Light Poster

Re: The problem with duck typing

 
0
  #6
Jun 8th, 2008
•
•
•
•
Originally Posted by Ene Uran View Post
I think you have to write it this way:
patternMatching(r"The|fox", 'The little brown fox')
Thanks! That would normally work. Unfortunately, that's not possible. This programme is designed to accept a user's own regular expression, so I have to accept the full range of regular expression syntax. I've got this solution so far, but I figured surely something more elegant would exist.

Python Syntax (Toggle Plain Text) 
  1. def patternMatching(self, event):
  2.     matchList = re.findall(self.patternArea.GetValue(), self.inputArea.GetValue())
  3.     for n in range(0, len(matchList)):
  4.         if type(matchList[n]) == type(''):
  5.             matchList[n] = (matchList[n],)
  6.     outputString = ''
  7.     for tupleElement in matchList:
  8.         outputString += '\n'.join(['%s' % stringElement for stringElement in tupleElement])
  9.     self.outputArea.SetValue(outputString)

Slate: Yes, but that's counter-intuitive. If there's one group you'll be iterating over a string, but if there's more than one it will be a tuple. Kind of un-Pythonic, no? It would make much more sense for the case of one group to be a 1-tuple.
Last edited by Fuse; Jun 9th, 2008 at 12:06 am.
Mir's Fuselage

Because what's not to like about being killed by a toilet seat from Mir's atmospheric re-entry?
Reply With Quote Quick reply to this message  
•
•
Join Date: Jun 2008
Posts: 128
Reputation: slate is an unknown quantity at this point 
Solved Threads: 31
slate slate is offline Offline
Junior Poster

Re: The problem with duck typing

 
0
  #7
Jun 9th, 2008
The title of the forum is about duck typing. The problem at hand has nothing to do with it IMHO.

I agree. Re.findall function is not so well suited for taking a user input regular expression, because it changes the output type depending on the input regexp.

This design decission can be argued against. A pro argument would be, that most of the time the regular expression is programmed by hand, and most of the time there is no more than one group. I don't know if it is true however. An against is your use case.
Reply With Quote Quick reply to this message  
•
•
Join Date: Sep 2007
Posts: 33
Reputation: paddy3118 is an unknown quantity at this point 
Solved Threads: 8
paddy3118 paddy3118 is offline Offline
Light Poster

Re: The problem with duck typing

 
0
  #8
Jul 2nd, 2008
Try re.finditer as in:

python Syntax (Toggle Plain Text) 
  1. def patternMatching(pattern, string):
  2.   print '\n'.join(string[matchobj.start():matchobj.end()]
  3.                   for matchobj in re.finditer(pattern, string))

- Paddy.
Reply With Quote Quick reply to this message  
Reply

Quick Reply to Thread
This thread has been marked solved.
Perhaps start a new thread instead?
•
•
•
•
•
•
•
•
Message:
•
•
•
•



Similar Threads
Other Threads in the Python Forum


Views: 1062 | Replies: 7
Thread Tools Search this Thread



Tag cloud for Python
anti approximation array avogadro beginner builtin cipher clear client code color converter countpasswordentry cturtle curved def dictionary drive dynamic examples excel file float format frange ftp function gui homework import input java lapse library line lines linux list lists loop microcontroller mouse multiple mysqldb mysqlquery newb number numbers output parsing path port prime program programming projects py2exe pygame pymailer pyqt python random recursion recursive redirect script scrolledtext singleton socket sqlite ssh stderr string strings subprocess sum syntax table terminal text textarea thread threading time tkinter tlapse tuple tutorial twoup ubuntu unicode unix urllib urllib2 variable web-scrape wikipedia windows word wxpython
About Us | Contact Us | Advertise | DaniWeb | Acceptable Use Policy | RSS Feed

©2003 - 2009 DaniWeb® LLC