Variation of "running a function only once" in Python 3

Question

I'm using rpy2 to wrap R libraries (modules in Python speak) within python, through the importr function provided by this module.

The issue is that importr can be very expensive at runtime (it does a number of things when invoked) and I'd like to have it called just once (for each importr call I have several functions using its result, and at the same time I can't just put everything on top of the module, or it would slow import time significantly).

Currently, for each module where I use importr I do:

myrlib = None

def do_stuff_with_r(param):
    global myrlib
    if myrlib is None:
        myrlib = importr(myrlib)

I'd like to generalize it since I do this kind of operation in many different modules and thus these lines are duplicated all over.

However I'm not sure how to do this: this solution returns None after the first invocation, which is not really what I'd like to do. Assuming this is doable, how do I ensure importr() for a specific argument is called just once?

note: Python import statement uses sys.modules i.e., if myrlib is a Python module (implemented however you like (pure Python, C, Fortran, etc)) then multiple imports of the same module are fast. — jfs
– jfs, Commented Oct 6, 2014 at 13:40

Ben · Accepted Answer · 2014-10-06 08:13:12Z

2

You could write your own wrapper function that caches the result of the import:

def import_r(lib, cache={}):
    if lib not in cache:
        cache[lib] = importr(lib)
    return cache[lib]

And use that every time you want want to use importr instead?

answered Oct 6, 2014 at 8:13

Ben

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2 Comments

Einar Over a year ago

Wanted to, but I had to wait until the timer ran out. ;)

jfs Over a year ago

@Einar: to avoid unnecessary lookups ret = cache.get(lib) \n if ret is None: ret = cache[lib] = importr(lib); \n return ret. The general (perhaps slower) version is functools.lru_cache() decorator.

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