Inheriting from “str” class

Question

I'm doing this little exercise... I want to reorder a string based on some weird dictionary. For example, according to my dictionary, the letters come in the order: "a", "b", "d", "c", "f", "e"

So I figured I should just overload the < operator for strings and call sorted()

Here goes:

class MyString(str):
     new_dict = dict((x,i) for i,x in enumerate(["a", "b", "d", "c", "f", "e"]))
     def __lt__(self,other):
         return self.new_dict[self] < self.new_dict[other]
     def __init__(self,x):
         str.__init__(self,x)

And then

In [59]: sorted((MyString(x) for x in "abcdef"))
Out[59]: ['a', 'b', 'd', 'c', 'f', 'e']

That's awesome. Or even:

In [64]: MyString("".join(sorted((MyString(x) for x in "abcdef"))))
Out[64]: 'abdcfe'

But why can't I just do sorted(MyString("abcdef")) ?

In [70]: sorted(MyString("abcdef"))
Out[70]: ['a', 'b', 'c', 'd', 'e', 'f']

Apparently the iterator of MyString is returning strings.

In [72]: for i in MyString("abcdef"):
             print type(i)
      ....:     
      <type 'str'>
      <type 'str'>
      <type 'str'>
      <type 'str'>
      <type 'str'>
      <type 'str'>

What happens if I call join on MyString:

In [63]: type(MyString("").join(sorted((MyString(x) for x in "abcdef"))))
Out[63]: str

Why does MyString have str iterators?

Answer 1

You need to override the __getitem__ method here:

class MyString(str):
    def __getitem__(self, i):
        return type(self)(super(MyString, self).__getitem__(i))

This returns a new instance of the current type:

>>> for i in MyString("abcdef"):
...     print type(i)
... 
<class '__main__.MyString'>
<class '__main__.MyString'>
<class '__main__.MyString'>
<class '__main__.MyString'>
<class '__main__.MyString'>
<class '__main__.MyString'>

str itself doesn't implement iteration (it has no __iter__ menthod , but does implement the sequence protocol (it has both a __len__ length method an a __getitem__ method); it is this that the for loop ultimately uses).

If using Python 3, the str object does have a __iter__ method and you need to override that instead:

class MyString(str):
    def __iter__(self):
        return (type(self)(i) for i in super().__iter__())

Note that str is an immutable type, overriding __init__ has little influence on the instance.

For ordering, you really need to implement all of the __gt__ , __ge__ , __eq__ , etc. methods too. Use the @functools.total_ordering() decorator to save yourself most of the work here:

from functools import total_ordering

@total_ordering
class MyString(str):
    sortmap = {x: i for i, x in enumerate("abdcfe")}

    def __lt__(self, other):
        return self.sortmap[self] < self.sortmap[other]

    # inherit __eq__ from str

    def __getitem__(self, i):
        return type(self)(super(MyString, self).__getitem__(i))

Last but not least, for sorting, just use the key argument to sorted() here:

>>> sortmap = {x: i for i, x in enumerate("abdcfe")}
>>> sorted('abcdef', key=sortmap.get)
['a', 'b', 'd', 'c', 'f', 'e']

Answer 2

You don need a subclass for customizing sort behavior - you can pass a key parameter to a sort method or sorted call, specifying a function that gives the relative weights of each element being compared.

Like in:

def mycomp(text): myseq = ("abdcfe") weigthed = [myseq.find(char) for char in text] return weigthed # this will place -1's for chars not found in your mapping string

Answer 3

You should indeed use the key parameter instead of your approach. The reason it is not working however is simply that you didn't overload the __iter__ function:

class MyString(str):
    # ...
    def __iter__(self):
        for x in super().__iter__():
            yield self.__class__(x)

In Python 2 you can use

class MyString(str):
    # ...
    def __iter__(self):
        for x in super(MyString, self).__str__():
            yield self.__class__(x)