What is the best way to implement nested dictionaries?

Question

I have a data structure which essentially amounts to a nested dictionary. Let\'s say it looks like this:

{\'new jersey\': {\'mercer county\': {\'plumbers\':          


        

Answer 1

I find setdefault quite useful; It checks if a key is present and adds it if not:

d = {}
d.setdefault('new jersey', {}).setdefault('mercer county', {})['plumbers'] = 3

setdefault always returns the relevant key, so you are actually updating the values of 'd' in place.

When it comes to iterating, I'm sure you could write a generator easily enough if one doesn't already exist in Python:

def iterateStates(d):
    # Let's count up the total number of "plumbers" / "dentists" / etc.
    # across all counties and states
    job_totals = {}

    # I guess this is the annoying nested stuff you were talking about?
    for (state, counties) in d.iteritems():
        for (county, jobs) in counties.iteritems():
            for (job, num) in jobs.iteritems():
                # If job isn't already in job_totals, default it to zero
                job_totals[job] = job_totals.get(job, 0) + num

    # Now return an iterator of (job, number) tuples
    return job_totals.iteritems()

# Display all jobs
for (job, num) in iterateStates(d):
    print "There are %d %s in total" % (job, num)