Unpacking parameters dict inside function

1

Hi everyone,

So I’m using different parameter dictionaries for different calibrations of my model by passing each time a different parameters dict to the model functions as argument. So I was wondering what is the most elegant way to unpack these parameters inside a function i.e. populate the local namespace of the function with these parameters without having to manually assign them? I hope I’m clear enough :grin:

Thanks for your input.

Cheers,

2

Hi @m.aitlahcen, thanks for posting here. Can you tell us what programming language you are using?

Thanks

3

Ah yes sorry. I’m using Python 3.6.

4

Sometimes what I do is define a simple function that does the unpacking for me.

Something like this

def _unpack(d):
    return d["one"], d["two"], d["three"]

And then I use it like this:

one, two, three = _unpack(d)

That isn’t a fantastic solution, but unfortunately I don’t know of a better one in Python

5

Thanks a lot for the answer. That’s more or less what I do. The trouble with this is that I have to write all the parameters everytime I unpack the dict. Is there any way to loop over the dict keys and values and do the assignment within the loop?
That would be great.

Thanks again Spencer.

Cheers,

6

Hi @m.aitlahcen, if your parameters are arguments to your function you can use the dictionary unpack operator ** like so:

def f(alpha=0, beta=1):
    return alpha + 2 * beta

d = {'alpha': 10, 'beta': 20}

f(**d)
7

Thanks a lot @john.stachurski for the suggestion. What I’m trying to do is to pass the whole parameters dict to the function and then unpack inside it. By doing this I’m trying to avoid having the parameters as global variables and also handling the use of different sets of parameters for the purpose of model calibration without resorting to OOP.

I found some people suggesting the use of locals().update(dict) but I haven’t tried it yet.

8

@m.aitlahcen I understand. It’s important to minimize global vars.

Personally I usually use a simple class that wraps the parameters and then unpack it manually, as in a, b = obj.a, obj.b.

9

I see. I guess I will just stick with what you suggested.

Thanks a lot.

10

Sorry to get to this discussion late. I have one last thought on this – If you don’t want to build up a full class or if you wanted to pass these parameters into a numba function then you could use what is called a namedtuple (https://docs.python.org/3.6/library/collections.html#collections.namedtuple). It isn’t that different than OOP, but is a little lighter and is compatible with numba.

Here is a very contrived example of what that might look like

from collections import namedtuple
from numba import jit

# This is a simple model where agent always will
# consume one  unit of consumption good.
# Utility is CRRA with risk aversion gamma and 
# discounts future with beta
MyModel = namedtuple("MyModel", ["beta", "gamma"])

@jit(nopython=True)
def utility(m, c):
    return (c**(1-m.gamma) / (1-m.gamma))

@jit(nopython=True)
def solve(m):
    """
    Simple model where agent always gets 1 consumption.
    This function solves for corresponding VF...

    Insert more fancy and instructive documentation here
    """
    V = 0.0
    dist, tol = 10.0, 1e-8
    while dist>tol:
        Vnew = utility(m, 1.0) + m.beta*V
        dist = Vnew - V
        V = Vnew
    return V
11

Great! Thanks a lot @cc7768. These named tuples seem to be exactly what I needed! Looks similar to Matlab structures I guess. I will have a deeper look at them.

Cheers,