On: Python's Mutable Default Problem

[eru]

Strings are also non-mutable, and thus have referential transparency. And so do numbers in Python.

[jerf → eru]

Referential transparency is a property of functions, not data (unless you're in a lambda mood and treat them as functions of zero arguments, but in that case you're not in Python so it's not relevant here). Even a function to "concatenate two strings" could be passed an object that overloads the addition operator to cause arbitrary modifications:

    >>> class Evil(object):
            def __init__(self):
                self.evil = 1
            def __add__(self, other):
                result = ("%s" % self.evil) + other
                self.evil += 1
                return result


    >>> def referentially_transparent_concat(a, b):
            return a + b

    >>> e = Evil()
    >>> print referentially_transparent_concat(e, "hi")
    1hi
    >>> print referentially_transparent_concat(e, "hi")
    2hi

You can program in a referentially-transparent style with Python, but you'll have to do it by adding your own restrictions to the code you write. Python will not help you with that.

[eru → jerf]

In your code the devil is in the data-type.