Python: Memory Conservation Tip: intern()

I'm working with a lot of data, and running out of memory is a problem. When I read a line of data, I've often seen the same data before. Rather than have two pointers that point to two separate copies of "foo", I'd prefer to have two pointers that point to the same copy of "foo". This makes a lot of sense in Python since strings are immutable anyway.

I knew that this was called the flyweight design pattern, but I didn't know if it was already implemented somewhere in Python. (Strictly speaking, I thought it was called the "flywheel" design pattern, and my buddy Drew Perttula corrected me.)

My first attempt was to write code like:

>>> s1 = "foo"
>>> s2 = ''.join(['f', 'o', 'o'])
>>> s1 == s2
True
>>> s1 is s2
False
>>> identity_cache = {}
>>> s1 = identity_cache.setdefault(s1, s1)
>>> s2 = identity_cache.setdefault(s2, s2)
>>> s1 == 'foo'
True
>>> s1 == s2
True
>>> s1 is s2
True

This code looks up the word "foo" by value and returns the same instance every time. Notice, it works.

However, Monte Davidoff pointed out that this is what the intern builtin is for. From the docs:

Enter string in the table of ``interned'' strings and return the interned string - which is string itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup - if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys. Changed in version 2.3: Interned strings are not immortal (like they used to be in Python 2.2 and before); you must keep a reference to the return value of intern() around to benefit from it.

Here it is in action:

>>> s1 = "foo"
>>> s2 = ''.join(['f', 'o', 'o'])
>>> s1 == s2
True
>>> s1 is s2
False
>>> s1 = intern(s1)
>>> s2 = intern(s2)
>>> s1 == 'foo'
True
>>> s1 == s2
True
>>> s1 is s2
True

Well did it work? My program still functions, but I didn't get a tremendous savings in memory. It turns out that I don't have enough dups, and that's not where I'm spending all my memory anyway. Oh well, at least I learned about the intern() function.