palindrome function

[kdt]

Hi all,

Can someone please explain to me why the following evaluates as false?

>>>list=['a','n','n','a']
list==list.reverse()
False

I'm stumped :s

[Denis Kasak]

On Sat, Jul 12, 2008 at 12:22 AM, kdt <do********@googlemail.comwrote:

Hi all,

Can someone please explain to me why the following evaluates as false?

>>>>list=['a','n','n','a']
list==list.reverse()
False

I'm stumped :s

Read the documentation on list.reverse().

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome"

Also, 'list' is a really bad name for a list, since this is the name of
the builtin type object for the list type.

--
Denis Kasak

[kdt]

On Jul 11, 11:34*pm, Denis Kasak <[e-mail]>
wrote:

On Sat, Jul 12, 2008 at 12:22 AM, kdt <[e-mail]:

*Hi all,
*>
*Can someone please explain to me why the following evaluates as false?
*>
*>>>>list=['a','n','n','a']
*>>>>list==list.reverse()
*>>>>False
*>
*I'm stumped :s

Read the documentation on list.reverse().

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
* * print "Palindrome"

Also, 'list' is a really bad name for a list, since this is the name of
the builtin type object for the list type.

--
Denis Kasak

thanks for the explanation :D

[bearophileHUGS]

Denis Kasak:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome"

An alternative version:

>>txt = "anna"
txt == txt[::-1]

True

>>txt = "annabella"
txt == txt[::-1]

False

Bye,
bearophile

[Mensanator]

On Jul 11, 5:34*pm, Denis Kasak <[e-mail]>
wrote:

On Sat, Jul 12, 2008 at 12:22 AM, kdt <[e-mail]:

*Hi all,
*>
*Can someone please explain to me why the following evaluates as false?
*>
*>>>>list=['a','n','n','a']
*>>>>list==list.reverse()
*>>>>False
*>
*I'm stumped :s

Read the documentation on list.reverse().

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
* * print "Palindrome"

You could also do

>>spam = ['a','n','n','a']
if spam == [i for i in reversed(spam)]:

print "Palindrome"

>
Also, 'list' is a really bad name for a list, since this is the name of
the builtin type object for the list type.

--
Denis Kasak

[Paul McGuire]

On Jul 11, 6:20*pm, Mensanator <[e-mail]:

Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
* * print "Palindrome"

You could also do

>spam = ['a','n','n','a']
if spam == [i for i in reversed(spam)]:

* * * * print "Palindrome"

Or instead of this:

[ i for i in generator_expression]

just use the list constructor:

list(generator_expression)

-- Paul

[Peter Otten]

Denis Kasak wrote:

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome"

Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome", either.

Peter

[Denis Kasak]

Peter Otten wrote:

Denis Kasak wrote:

>Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome"

Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome", either.

Of course. Thank you for the correction. I guess you know your caffeine
has started to wear off when you start making the same mistakes you were
trying to fix. :-)

--
Denis Kasak

[Terry Reedy]

Peter Otten wrote:

Denis Kasak wrote:

>Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome"

Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome", either.

I don't know if this was posted yet, but 'seq.reversed() == seq' is the
simple way to test for 'palindomeness'.

[Mensanator]

On Jul 12, 2:18�pm, Terry Reedy <[e-mail]:

Peter Otten wrote:

Denis Kasak wrote:

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
� � print "Palindrome"

Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome", either.

I don't know if this was posted yet,

It hasn't. and here's why:

IDLE 2.6b1

>>seq=['a','n','n','a']
seq.reversed()

Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
seq.reversed()
AttributeError: 'list' object has no attribute 'reversed'

but 'seq.reversed() == seq' is the
simple way to test for 'palindomeness'.

Not in 2.x.

[Terry Reedy]

Mensanator wrote:

It hasn't. and here's why:

IDLE 2.6b1

>>>seq=['a','n','n','a']
seq.reversed()

Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
seq.reversed()
AttributeError: 'list' object has no attribute 'reversed'

My apologies. reversed() is a builtin func, not a method, and it
produces an iterator, not a seq. SO, corrected,

>>for s in ((1,2,3,2,1), [1,2,3,2,1]):

.... type(s)(reversed(s)) == s
....
True
True

>>s = '12121'
''.join(reversed(s)) == s

True