Question

I am just learning python and I am going though the tutorials on https://developers.google.com/edu/python/strings

Under the String Slices section

s[:] is ‘Hello’ — omitting both always gives us a copy of the whole
thing (this is the pythonic way to copy a sequence like a string or
list)

Out of curiosity why wouldn’t you just use an = operator?

s = 'hello';
bar = s[:] 
foo = s 

As far as I can tell both bar and foo have the same value.

Answer 1

= makes a reference, by using [:] you create a copy. For strings, which are immutable, this doesn’t really matter, but for lists etc. it is crucial.

>>> s = 'hello'
>>> t1 = s
>>> t2 = s[:]
>>> print s, t1, t2
hello hello hello
>>> s = 'good bye'
>>> print s, t1, t2
good bye hello hello

but:

>>> li1 = [1,2]
>>> li = [1,2]
>>> li1 = li
>>> li2 = li[:]
>>> print li, li1, li2
[1, 2] [1, 2] [1, 2]
>>> li[0] = 0
>>> print li, li1, li2
[0, 2] [0, 2] [1, 2]

So why use it when dealing with strings? The built-in strings are immutable, but whenever you write a library function expecting a string, a user might give you something that “looks like a string” and “behaves like a string”, but is a custom type. This type might be mutable, so it’s better to take care of that.

Such a type might look like:

class MutableString(object):
    def __init__(self, s):
        self._characters = [c for c in s]

    def __str__(self):
        return "".join(self._characters)

    def __repr__(self):
        return "MutableString(\"%s\")" % str(self)

    def __getattr__(self, name):
        return str(self).__getattribute__(name)

    def __len__(self):
        return len(self._characters)

    def __getitem__(self, index):
        return self._characters[index]

    def __setitem__(self, index, value):
        self._characters[index] = value

    def __getslice__(self, start, end=-1, stride=1):
        return str(self)[start:end:stride]


if __name__ == "__main__":
    m = MutableString("Hello")
    print m
    print len(m)
    print m.find("o")
    print m.find("x")
    print m.replace("e", "a") #translate to german ;-)
    print m
    print m[3]
    m[1] = "a"
    print m
    print m[:]

    copy1 = m
    copy2 = m[:]
    print m, copy1, copy2
    m[1] = "X"
    print m, copy1, copy2

Disclaimer: This is just a sample to show how it could work and to motivate the use of [:]. It is untested, incomplete and probably horribly performant