confused about __str__ vs. __repr__

Diez B. Roggisch deets at nospam.web.de
Thu Dec 18 16:13:18 CET 2008


Neal Becker wrote:

> Tino Wildenhain wrote:
> 
>> Neal Becker wrote:
>> ...
>>>>> So if __str__ is "meant for human eyes", then why isn't print using
>>>>> it!
>>>> it is:
>>>>
>>>>  > print x
>>>> str
>>>>
>>>> but dict just uses repr() for all its childs to print.
>>>>
>>>> T.
>>> That makes no sense to me.  If I call 'print' on a container, why
>>> wouldn't it recursively  print on the contained objects?  Since print
>>> means call str, printing a container should recursively call str on the
>>> objects.
>> 
>> Every class is free on how to best implement __str__, you will find
>> the same behavior on tuple and list as well.
>> 
>> Maybe its discussable to change the implementation sensibly, best if you
>> would come with a proposal? Perhaps pprint.pprint is a starting point?
>> 
>> Regards
>> Tino
> 
> First, I'd like to know if there is a rationale for the current design. 
> Am I correct in thinking this is a defect?

I don't think so. First of all, there is no "generic" way of printing a
collection. And the current implementation tries to give an overview what
is contained in the collection, without trying to make it "fancy" - any
such thing needed to be hand-coded anyway.

Using repr for that is better suited, as for example string keys are printed
with quotes around them - making clear what they are, and not irritating
the user through potentially contained spaces or even things that look as
if they are python objects.

For example, if repr *wasn't* used,

{ "{foo=bar}" : "baz"}

would be printed

{{foo=bar} : baz}

Which is *not* what the dictionary actually contains!

The same goes for unicode-objects. They appear with their "funny" characters
as \xXX-codes - instead of bailing out on you with unicode-errors.

So, IMHO the current behavior is desired. 

Diez



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