Handy little decorator to cache a list of dictionaries returned from some long running operation like web queries.

save_dict_list.py

from functools import wraps
import csv
import os.path


def save_dict_list(filename, **kwargs):
    """Decorator to take the results of a function call (assumed to be a 
    ``list`` of ``dicts``) and cache them in a local file via 
    csv.DictWriter and serialize them with csv.DictReader"""
    def decorator(f):
        @wraps(f)
        def wrapper(*args, **kwargs):
            # check to see if we already have a cached version, if so, return
            # that
            if os.path.isfile(filename):
                with open(filename, 'r') as fp:
                    reader = csv.DictReader(fp, **kwargs)
                    return list(reader)

            # fetch the list of dictionaries
            res = f(*args, **kwargs)
            if len(res) == 0: return res  # don't bother saving for just one

            # save results to file
            with open(filename, 'w') as fp:
                # instead of specifying fieldnames, just assume alpha order
                fieldnames = sorted(res[0].keys())
                writer = csv.DictWriter(fp, fieldnames, **kwargs)
                writer.writerow({k: k for k in fieldnames})  # header
                for row in res:
                    writer.writerow(row)

            return res
        return wrapper
    return decorator


@save_dict_list('mylist.csv')
def foo():
    # first time this runs, it'll call foo for the results, second 
    # time it'll return results in mylist.csv
    print 'Foo called!'
    return [
        {'first_name': 'Albert', 'last_name': 'Einstein'},
        {'first_name': 'Neils', 'last_name': 'Bohr'},
    ]


@save_dict_list('addresses.csv')
def bar():
    # do some long running / expensive query to a webservice
    pass