objectoriented -?- functional

[Armin]

loving Java (oo)

Don't mind, weirder things have happened

http://wiki.muenster.org/index.php/Schwan

LOL!!

as well as SML (fun) I use to practice both of them
separately.
Now, with Python I would like to combine 'oo.extend()' with 'functional
map':

Python 2.4.4 (#2, Oct 22 2008, 19:52:44)
[GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

def reverse_(list):

... """list.reverse() returns None; reverse_ returns the reversed
list"""
... list.reverse()
... return list
...

ll = [[11, 'a'], [33, 'b']]
l = ll[:] # make a copy !
l = map(reverse_, l[:]) # make a copy ?
ll.extend(l)
print("ll=", ll)

('ll=', [['a', 11], ['b', 33], ['a', 11], ['b', 33]])

But I expected to get ...
('ll=', [[11, 22], [33, 44], [22, 11], [44, 33]])
... how would that elegantly be achieved with Python ?

Sorry, I cannot infer the pattern. How would you do that at all?

Peter

If you want to reimplement list.extend() (as I understand?) you will have to
subclass it and change the behaviour.

 

[Walther Neuper]

Hi,

loving Java (oo) as well as SML (fun) I use to practice both of them
separately.
Now, with Python I would like to combine 'oo.extend()' with 'functional
map':

Python 2.4.4 (#2, Oct 22 2008, 19:52:44)
[GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)] on linux2
Type "help", "copyright", "credits" or "license" for more information..... """list.reverse() returns None; reverse_ returns the reversed
list"""
.... list.reverse()
.... return list
....

>>> ll = [[11, 'a'], [33, 'b']]
>>> l = ll[:] # make a copy !
>>> l = map(reverse_, l[:]) # make a copy ?
>>> ll.extend(l)
>>> print("ll=", ll)

('ll=', [['a', 11], ['b', 33], ['a', 11], ['b', 33]])

But I expected to get ...
('ll=', [[11, 22], [33, 44], [22, 11], [44, 33]])
.... how would that elegantly be achieved with Python ?

Cheers, Walther

--
------------------------------------------------------------------------
Walther Neuper Mailto: (e-mail address removed)
Institute for Software Technology Tel: +43-(0)316/873-5728
University of Technology Fax: +43-(0)316/873-5706
Graz, Austria Home: www.ist.tugraz.at/neuper
------------------------------------------------------------------------

 

[Peter Otten]

loving Java (oo)

Don't mind, weirder things have happened

http://wiki.muenster.org/index.php/Schwan

as well as SML (fun) I use to practice both of them
separately.
Now, with Python I would like to combine 'oo.extend()' with 'functional
map':

Python 2.4.4 (#2, Oct 22 2008, 19:52:44)
[GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)] on linux2
Type "help", "copyright", "credits" or "license" for more information.... """list.reverse() returns None; reverse_ returns the reversed
list"""
... list.reverse()
... return list
...

ll = [[11, 'a'], [33, 'b']]
l = ll[:] # make a copy !
l = map(reverse_, l[:]) # make a copy ?
ll.extend(l)
print("ll=", ll)

('ll=', [['a', 11], ['b', 33], ['a', 11], ['b', 33]])

But I expected to get ...
('ll=', [[11, 22], [33, 44], [22, 11], [44, 33]])
... how would that elegantly be achieved with Python ?

Sorry, I cannot infer the pattern. How would you do that at all?

Peter

 

[Piet van Oostrum]

WN> Hi,
WN> loving Java (oo) as well as SML (fun) I use to practice both of them
WN> separately.
WN> Now, with Python I would like to combine 'oo.extend()' with 'functional
WN> map':

WN> Python 2.4.4 (#2, Oct 22 2008, 19:52:44)
WN> [GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)] on linux2
WN> Type "help", "copyright", "credits" or "license" for more information.WN> ... """list.reverse() returns None; reverse_ returns the reversed
WN> list"""
WN> ... list.reverse()
WN> ... return list
WN> ...

ll = [[11, 'a'], [33, 'b']]
l = ll[:] # make a copy !
l = map(reverse_, l[:]) # make a copy ?
ll.extend(l)
print("ll=", ll)

WN> ('ll=', [['a', 11], ['b', 33], ['a', 11], ['b', 33]])

WN> But I expected to get ...
WN> ('ll=', [[11, 22], [33, 44], [22, 11], [44, 33]])
WN> ... how would that elegantly be achieved with Python ?

I guess you later changed 22 to 'a' and 44 to 'b'.

You have made a copy of the outer list with ll[:] (actually twice) but
you have not made copies of the inner lists and these are the important
ones. So the reverse will reverse the *original* inner lists which are
both in ll and in l.

If you still want to do destructive reverses on the inner list you
should make a deep copy:

import copy
l = copy.deepcopy(ll)
l [[11, 'a'], [33, 'b']]
ll.extend(l)
ll

[[11, 'a'], [33, 'b'], ['a', 11], ['b', 33]]

But IMHO it would be better to use non-destructive reverses:

ll [[11, 'a'], [33, 'b']]
l = [list(reversed(x)) for x in ll]
l [['a', 11], ['b', 33]]
ll.append(l)
ll

[[11, 'a'], [33, 'b'], [['a', 11], ['b', 33]]]