python function that changes itself to list

Question

So I'm working on a chemistry project for fun, and I have a function that initializes a list from a text file. What I want to do s make it so the function replaces itself with a list. So here's my first attempt at it which randomly will or won't work and I don't know why:

def periodicTable():
    global periodicTable
    tableAtoms = open('/Users/username/Dropbox/Python/Chem Project/atoms.csv','r')
    listAtoms = tableAtoms.readlines()
    tableAtoms.close()
    del listAtoms[0]
    atoms = []
    for atom in listAtoms:
        atom = atom.split(',')
        atoms.append(Atom(*atom))
    periodicTable = atoms

It gets called in in this way:

def findAtomBySymbol(symbol):
    try:
        periodicTable()
    except:
        pass
    for atom in periodicTable:
        if atom.symbol == symbol:
            return atom
    return None

Is there a way to make this work?

Don't make the function replace itself with a list. Using a list and a function is different - how do you call a list or slice a function? - and trying to deal with this cases differently is messy as the using code shows. Also, "randomly" isn't a good error description - not swallowing all exceptions might give a better failure indicator. — user2864740
– user2864740, Commented Oct 13, 2013 at 18:59
You may be interested in "memoization functions", although I would just do: periodicTable = loadPeriodicTable() (say once, at the start of the program) myself as there is no obvious need for caching or memoization here. — user2864740
– user2864740, Commented Oct 13, 2013 at 19:01
@user2864740 Yeah, I thought about doing that, but I really enjoy exploring intricacies of python, much to the annoyance of some users here. — Hovestar
– Hovestar, Commented Oct 13, 2013 at 19:06
Then see one of the various memoization/cache answers shown. With memoization/caching the same name is always bound to a function, however. Thus it would always used like a function - for atom in periodicTable() - (with a function call and) with the difference being that subsequent invocations return the cached data without needing to reload from the file. — user2864740
– user2864740, Commented Oct 13, 2013 at 19:07

l4mpi · Accepted Answer · 2013-10-13 19:05:22Z

4

Don't do that. The correct thing to do would be using a decorator that ensures the function is only executed once and caches the return value:

def cachedfunction(f):
    cache = []
    def deco(*args, **kwargs):
        if cache:
            return cache[0]
        result = f(*args, **kwargs)
        cache.append(result)
        return result
    return deco

@cachedfunction
def periodicTable():
    #etc

That said, there's nothing stopping you from replacing the function itself after it has been called, so your approach should generally work. I think the reason it doesn't is because an exception is thrown before you assign the result to periodicTable and thus it never gets replaced. Try removing the try/except block or replacing the blanket except with except TypeError to see what exactly happens.

answered Oct 13, 2013 at 19:05

l4mpi

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4 Comments

Ethan Furman Over a year ago

Nice. A little more upfront work, but less confusion with mutable defaults. :)

l4mpi Over a year ago

@EthanFurman Yeah, this has the advantage of being reuseable and keeping the original function signature. If it's just for a small throwaway script your solution could be preferable...

Ethan Furman Over a year ago

Nitpick: It's only keeping the original signature because there isn't one; if there was it would be much more complicated to keep it. Check out Michele Simianoto's Decorator module if you want to see the gory details.

l4mpi Over a year ago

@EthanFurman Yeah, when you consider introspection that's a different matter. I meant it in the way that you'll still get a TypeError when you call it with a wrong signature, which I'd usually consider good enough.

Ethan Furman · Accepted Answer · 2013-10-13 22:05:05Z

3

This is very bad practice.

What would be better is to have your function remember if it has already loaded the table:

def periodicTable(_table=[]):
    if _table:
        return _table
    tableAtoms = open('/Users/username/Dropbox/Python/Chem Project/atoms.csv','r')
    listAtoms = tableAtoms.readlines()
    tableAtoms.close()
    del listAtoms[0]
    atoms = []
    for atom in listAtoms:
        atom = atom.split(',')
        atoms.append(Atom(*atom))
    _table[:] = atoms

The first two lines check to see if the table has already been loaded, and if it has it simply returns it.

edited Oct 13, 2013 at 22:05

answered Oct 13, 2013 at 19:05

Ethan Furman

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2 Comments

user2864740 Over a year ago

This won't work. The assignment to _table at the end won't persist beyond the function scope. Instead, the list (originally named by _table) needs to be modified.

l4mpi Over a year ago

@user2864740 is right, this should be _table.extend(atoms) or _table[:] = atoms - you can't change the reference of the default argument.

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