json.dumps() give unexpected result when passing a variable of subclass of str in python 2.7

Question

I wrote a subclass of str like this:

class URL(str):
    def __init__(self, url):
        u = normalize_url(url)
        print u
        super(URL, self).__init__(string=u)

normalize_url() function is used for doing something like url encoding. Everything is ok, until I use json.dumps(). Here is my test code:

>>> u = URL('https://example.com/path contains space.html')
https://example.com/path%20contains%20space.html
>>> json.dumps(u)
'"https://example.com/path contains space.html"'

Why json.dumps() didn't give 'https://example.com/path%20contains%20space.html'

anthony sottile · Accepted Answer · 2019-05-01 19:59:19Z

1

str (like other immutable objects) does it's initialization in __new__

Construction of an object in python roughly looks like this:

inst = cls.__new__(cls, *args, **kwargs)
cls.__init__(inst, *args, **kwargs)

In your example, you call __init__, but it is too late, the object has already been set up in __new__

You can however fix this!:

class URL(str):
    __slots__ = ()

    def __new__(cls, val):
         val = val.replace(' ', '%20')
         return super(URL, cls).__new__(cls, val)

Now it works!

>>> x = URL('foo bar')
>>> x
'foo%20bar'
>>> json.dumps(x)
'"foo%20bar"'

Note that I've added __slots__ = () to restore the immutability that str had.

edited May 1, 2019 at 19:59

answered Dec 2, 2016 at 2:33

anthony sottile

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