printing a sequence...

[Florian Preknya]

Hi everybody,

I'm relatively new in using Python and I want to perform a simple job: I
have a list of objects of type A and want to print them to stdout.

class A:
def __init__(self, value):
self.value = value

def __str__(self):
return "value: %d" % self.value

a = [A(10), A(12), A(2)]
print str(a)

I obtain [<__main__.A instance at 0x00767D80>, <__main__.A instance at
0x00768698>, <__main__.A instance at 0x00768350>].
What I want to obtain is [value: 10, value: 12, value: 2].

I know that if I change the __str__ to __repr__ I obtain what I want, but
then I will have these informal strings in debuger also, and I do not want
that.

Can I write somehow the print statement to force calling the A.__str__
function?

Thanks,
Florian.

[Peter Otten]

Re: printing a sequence...

Florian Preknya wrote:
[color=blue]
> Hi everybody,
>
> I'm relatively new in using Python and I want to perform a simple job: I
> have a list of objects of type A and want to print them to stdout.
>
> class A:
> def __init__(self, value):
> self.value = value
>
> def __str__(self):
> return "value: %d" % self.value
>
> a = [A(10), A(12), A(2)]
> print str(a)
>
> I obtain [<__main__.A instance at 0x00767D80>, <__main__.A instance at
> 0x00768698>, <__main__.A instance at 0x00768350>].
> What I want to obtain is [value: 10, value: 12, value: 2].
>
> I know that if I change the __str__ to __repr__ I obtain what I want, but
> then I will have these informal strings in debuger also, and I do not want
> that.
>
> Can I write somehow the print statement to force calling the A.__str__
> function?[/color]

print map(str, a)

or

print [str(item) for item in a]

Peter

[Peter Otten]

Re: printing a sequence...

Peter Otten wrote:
[color=blue]
> Florian Preknya wrote:[/color]
[color=blue][color=green]
>> What I want to obtain is [value: 10, value: 12, value: 2].[/color][/color]
[color=blue][color=green]
>> Can I write somehow the print statement to force calling the A.__str__
>> function?[/color]
>
> print map(str, a)
>
> or
>
> print [str(item) for item in a][/color]

Oops, the above will still print the representation of the strings. When you
don't want quotes and escaped non-ascii characters, use

print "[%s]" % ", ".join(map( str, a))

Peter

[Christopher T King]

Re: printing a sequence...

On Fri, 16 Jul 2004, Peter Otten wrote:
[color=blue]
> Peter Otten wrote:
>[color=green]
> > Florian Preknya wrote:[/color]
>[color=green][color=darkred]
> >> What I want to obtain is [value: 10, value: 12, value: 2].[/color][/color]
>[color=green][color=darkred]
> >> Can I write somehow the print statement to force calling the A.__str__
> >> function?[/color]
> >
> > print [str(item) for item in a][/color]
>
> print "[%s]" % ", ".join(map( str, a))[/color]

I had the same problem with the same solution, and it makes me wonder, why
does a list's __str__() method call the __repr__() method of its members?
Personally, I think the __str__() call should be propagated, not turned
into a __repr__() call. I can't think of any instances in which such
behavior is what you want (if the __repr__() of the members is wanted,
then __repr__() should be used).

[Peter Otten]

Re: printing a sequence...

Christopher T King wrote:
[color=blue]
> On Fri, 16 Jul 2004, Peter Otten wrote:
>[color=green]
>> Peter Otten wrote:
>>[color=darkred]
>> > Florian Preknya wrote:[/color]
>>[color=darkred]
>> >> What I want to obtain is [value: 10, value: 12, value: 2].[/color]
>>[color=darkred]
>> >> Can I write somehow the print statement to force calling the A.__str__
>> >> function?
>> >
>> > print [str(item) for item in a][/color]
>>
>> print "[%s]" % ", ".join(map( str, a))[/color]
>
> I had the same problem with the same solution, and it makes me wonder, why
> does a list's __str__() method call the __repr__() method of its members?
> Personally, I think the __str__() call should be propagated, not turned
> into a __repr__() call. I can't think of any instances in which such
> behavior is what you want (if the __repr__() of the members is wanted,
> then __repr__() should be used).[/color]

I didn't find it in the FAQ; I think the argument goes that the results of
propagating str() are sometimes misleading, e. g:
[color=blue][color=green][color=darkred]
>>> class List(list):[/color][/color][/color]
.... def __str__(self):
.... return "[%s]" % ", ".join(map( str, self))
....[color=blue][color=green][color=darkred]
>>> List(["a", "b, c", "d"])[/color][/color][/color]
['a', 'b, c', 'd'][color=blue][color=green][color=darkred]
>>> print List(["a", "b, c", "d"])[/color][/color][/color]
[a, b, c, d][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

OK, there may be subtler examples. Personally, I would prefer the above
problem over the current inconsistency.

While I'm at it, here's another alternative for the OP that handles nested
lists nicely:
[color=blue][color=green][color=darkred]
>>> class D:[/color][/color][/color]
.... def __init__(self, value):
.... self.value = value
.... def __repr__(self):
.... if isinstance(self .value, list):
.... return str(map(D, self.value))
.... else:
.... return str(self.value)
....[color=blue][color=green][color=darkred]
>>> print D(["a", "b", "c"])[/color][/color][/color]
[a, b, c][color=blue][color=green][color=darkred]
>>> print D(["a", "b", ["c", "d"]])[/color][/color][/color]
[a, b, [c, d]][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Peter

[Lonnie Princehouse]

Re: printing a sequence...

To clarify what's already been said-

thing.__str__() is equivalent to str(thing)

thing.__repr__( ) is equivalent to repr(thing)


str(some_list) will call repr() for all of the list's items.

You could try:

print map(str, a)

or

print [str(x) for x in a]

for the desired effect, or you could overload __repr__ instead of __str__


"Florian Preknya" <bolo@coco.ro > wrote in message news:<cd8a1u$t6 d$1@nebula.dntt m.ro>...[color=blue]
> Hi everybody,
>
> I'm relatively new in using Python and I want to perform a simple job: I
> have a list of objects of type A and want to print them to stdout.
>
> class A:
> def __init__(self, value):
> self.value = value
>
> def __str__(self):
> return "value: %d" % self.value
>
> a = [A(10), A(12), A(2)]
> print str(a)
>
> I obtain [<__main__.A instance at 0x00767D80>, <__main__.A instance at
> 0x00768698>, <__main__.A instance at 0x00768350>].
> What I want to obtain is [value: 10, value: 12, value: 2].
>
> I know that if I change the __str__ to __repr__ I obtain what I want, but
> then I will have these informal strings in debuger also, and I do not want
> that.
>
> Can I write somehow the print statement to force calling the A.__str__
> function?
>
> Thanks,
> Florian.[/color]

[Peter Otten]

Re: printing a sequence...

Lonnie Princehouse wrote:
[color=blue]
> print [str(x) for x in a][/color]

will call repr(str(x)) for all x with undesirable effects for non-ascii
characters and extra quoting.

Looks like you made the same error that I did - after I corrected it :(

Peter

[Dan Bishop]

Re: printing a sequence...

Christopher T King <squirrel@WPI.E DU> wrote in message news:<Pine.LNX. 4.44.0407161004 150.10067-100000@ccc8.wpi .edu>...[color=blue]
> On Fri, 16 Jul 2004, Peter Otten wrote:
>[color=green]
> > Peter Otten wrote:
> >[color=darkred]
> > > Florian Preknya wrote:[/color][/color]
>[color=green][color=darkred]
> > >> What I want to obtain is [value: 10, value: 12, value: 2].[/color][/color]
>[color=green][color=darkred]
> > >> Can I write somehow the print statement to force calling the A.__str__
> > >> function?
> > >
> > > print [str(item) for item in a][/color]
> >
> > print "[%s]" % ", ".join(map( str, a))[/color]
>
> I had the same problem with the same solution, and it makes me wonder, why
> does a list's __str__() method call the __repr__() method of its members?
> Personally, I think the __str__() call should be propagated, not turned
> into a __repr__() call. I can't think of any instances in which such
> behavior is what you want (if the __repr__() of the members is wanted,
> then __repr__() should be used).[/color]

seq = ['A, B', 'C']
print seq