Why can’t I subclass str (truly)

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December 4th, 2012, 05:23 AM

SuperOscar

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Why can’t I subclass str (truly)

For a very specific and hidden purpose, I tried to subclass strings. That can be done. What cannot be done, however, is modifying the __init__() function so that the new subclass could take additional parameters different from those of the superclass (which is exactly what I needed).

You can subclass lists this way:

Code:

>>> class Spam(list):
    def __init__(self, data=None, eggs=0):
        if data is None:
            data = []
        list.__init__(self, data)
        self.eggs = eggs


>>> test = Spam()
>>> test.eggs
0
>>> another_test = Spam(['a', 'b', 'c'], 10)
>>> another_test.eggs
10

Both tests work: I can create a Spam() object, a subclass of list, either with default or explicit values.

With string subclasses, only default value seem to work after the first parameter:

Code:

>>> class MoreSpam(str):
    def __init__(self, data='', eggs=0):
        str.__init__(self, data)
        self.eggs = eggs


>>> test = MoreSpam('')
>>> test.eggs
0
>>> wtf = MoreSpam('', 10)
Traceback (most recent call last):
  File "<pyshell#35>", line 1, in <module>
    wtf = MoreSpam('', 10)
TypeError: str() argument 2 must be str, not int

It seems that for whatever reason, the __init__() function of the str subclasses is not re-defining the parameter list. Instead, Python looks straight at whatever is in the base class’s parameter list, namely two string parameters: the value of the string object itself and its encoding.

Am I getting this right? Shouldn’t this rightly be called a bug, or am I doing something completely different than I think I’m doing?

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December 4th, 2012, 12:54 PM

b49P23TIvg

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looks like a bug to me!

Code:

>>> class c(tuple):
...  def __init__(s,a,b):
...   tuple.__init__(s,a)
...   self.b = b
...
>>> c(tuple(),666)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: tuple() takes at most 1 argument (2 given)

I guess you'll need to write some extra code to work around this. Affects my installations of python 2 and 3.
I'll report it to bugs.python.org
reported

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December 4th, 2012, 01:47 PM

b49P23TIvg

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I'm rebuffed at bugs.python.org.

You can't call __init__ on an immutable.

You'll need to use type or str.__new__ or object__new__ but I completely forget how to make this work.

December 4th, 2012, 02:08 PM

SuperOscar

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Quote:

Originally Posted by b49P23TIvg

I'm rebuffed at bugs.python.org.

You can't call __init__ on an immutable.

You'll need to use type or str.__new__ or object__new__ but I completely forget how to make this work.

Okay, looks like subclassing immutable objects isn’t so easy

I guess I could revert to subclassing “object” and use a silly class variable like self.data to hold the string. Darn. The methods would like much nicer when I could be slicing just “self” instead of “self.data”.

December 4th, 2012, 02:37 PM

Nyktos

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Quote:

Originally Posted by b49P23TIvg

You'll need to use type or str.__new__ or object__new__ but I completely forget how to make this work.

Code:

class MoreSpam(str):
    def __new__(cls, data='', eggs=0):
        return str.__new__(cls, data)

    def __init__(self, data='', eggs=0):
        self.eggs = eggs

Works just fine:

Code:

>>> test = MoreSpam('')
>>> test.eggs
0
>>> wtf = MoreSpam('', 10)
>>> wtf.eggs
10

Edit: You could also put everything into the __new__ method:

Code:

class MoreSpam(str):
    def __new__(cls, data='', eggs=0):
        spam =  str.__new__(cls, data)
        spam.eggs = eggs
        return spam

Comments on this post

b49P23TIvg agrees: Please post the object creation sequence. Thanks!

December 5th, 2012, 02:48 AM

SuperOscar

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Quote:

Originally Posted by Nyktos

Edit: You could also put everything into the __new__ method:

Code:

class MoreSpam(str):
    def __new__(cls, data='', eggs=0):
        spam =  str.__new__(cls, data)
        spam.eggs = eggs
        return spam

Nice! Although the necessity to “return” anything from __new__() seems strange to me but who cares if it works and doesn’t require much extra code lines.

Why can’t I subclass str (truly)

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