Python Use User Defined String Class

Question

I know how to override string class with:

class UserString:
 def __str__(self):
 return 'Overridden String'
if __name__ == '__main__':
 print UserString()

But how can i use this class instead of built-in str class without defining implicitly UserString class?. To be clear I want this:

>>> print "Boo boo!"
Overridden String

• Steve Jessop: It's not possible. You have not "overridden string class", you have defined a class whose instances can be converted to strings. You can't change the type of string literals.
• Aziz Alfoudari: Do you want to use the class? Or are you simply interested on overriding print?
• guneysus: I am trying to found a new way -that would be integrate seamlessly- to uppercase special Turkish unicode characters. In other words, when a MYSTRING.upper() method triggerer, I want to trigger my own upper() method.

Answer 1

It is not possible. You have not overridden string class.

You cannot override classes. You can override methods. What you have done is defined a class and only overridden its str() method.

But you can do something like this...

def overriden_print(x):
 print "Overriden in the past!"
from __future__ import print_function # import after the definition of overriden_print
print = overriden_print
print("Hello")

Output:

Overriden in the past!

Answer 2

It's impossible to do what you want without hacking the python executable itself... after all, str is a built-in type, and the interpreter, when passed 'string' type immediates, will always create built-in strings.

However... it is possible, using delegation, to do something like this. This is slightly modified from another stackoverflow recipe (which sadly, I did not include a link to in my code...), so if this is your code, please feel free to claim it :)

def returnthisclassfrom(specials):
 specialnames = ['__%s__' % s for s in specials.split()]
 def wrapit(cls, method):
 return lambda *a: cls(method(*a))
 def dowrap(cls):
 for n in specialnames:
 method = getattr(cls, n)
 setattr(cls, n, wrapit(cls, method))
 return cls
 return dowrap

Then you use it like this:

@returnthisclassfrom('add mul mod')
class UserString(str):
 pass
In [11]: first = UserString('first')
In [12]: print first
first
In [13]: type(first)
Out[13]: __main__.UserString
In [14]: second = first + 'second'
In [15]: print second
firstsecond
In [16]: type(second)
Out[16]: __main__.UserString

One downside of this is that str has no radd support, so 'string1' + UserString('string2') will give a string, whereas UserString('string1') + 'string2' gives a UserString. Not sure if there is a way around that.

Maybe not helpful, but hopefully it puts you on the right track.

• guneysus: Thanks, this is not the exactly i am looking for but i liked it, because, it uses decorators. And your code looks like stackoverflow.com/a/1243045/1766716 :)
• Corley Brigman: that's the one... thanks for finding that! (annotating in code now...)