What's the best way to lookup the US county a US city resides in?

I'm looking for 开发者_运维问答the best/easiest way to programmatically grab the name of the US county a given US city resides in. It doesn't seem there's a straightforward API available for such a (seemingly simple) task?

You can download a freely-available database of county/city/zip code info such as this one: http://www.unitedstateszipcodes.org/zip-code-database/ (no need to register or pay)

Import it whole, or a subsection of it, into a local, persistent data store (such as a database) and query it whenever you need to look up a city's county

Note: County info has disappeared from the originally-linked .csv file since this answer was posted. This link no longer contains county information: http://federalgovernmentzipcodes.us/free-zipcode-database.csv

1) Cities span counties

2) Zips span both cities and counties, not even on the same lines

Any solution that uses zip as an intermediary is going to corrupt your data (and no, "zip+4" won't usually fix it). You will find that a city-to-zip-to-county data map (#2) has a larger number of city-to-county matches than the more accurate model (#1)--these are all bad matches.

What you're looking for is free census data. The Federal Information Processing Standards (FIPS) dataset you need is called "2010 ANSI Codes for Places": https://www.census.gov/geographies/reference-files/time-series/geo/name-lookup-tables.2010.html

Census "places" are the "cities" for our question. These files map "places" to one or more county.

It will not be easy to use geospace functions for this task because of the odd polygon shaped of counties and the point locations of cities.

Your best bet is to reference a database of cities and their respective counties, though I don't know where you could find one. Maybe Texas publishes one?
CommonDataHub doesn't contain this information.

Here is a bit of code to programmatically grab the name of a US county given a single US city/state using the Google Maps API. This code is slow/inefficient and does not have any error handling. However, it has worked reliably for me to match counties with a list of ~1,000 cities.

#Set up googlemaps API
import googlemaps
google_maps = googlemaps.Client(key='API_KEY_GOES_HERE')

#String of city/state
address_string = 'Atlanta, GA'

#Geocode
location = google_maps.geocode(address_string)

#Loop through the first dictionary within `location` and find the address component that contains the 'administrative_area_level_2' designator, which is the county level
target_string = 'administrative_area_level_2'
for item in location[0]['address_components']:     
    if target_string in item['types']: #Match target_string
        county_name = item['long_name'] #Or 'short_name'
        break #Break out once county is located
    else:
        #Some locations might not contain the expected information
        pass

This produces:

>>> county_name
Fulton County

Caveats:

1. code will break if google_maps.geocode() is not passed a valid address
2. certain addresses will not return data corresponding to 'administrative_area_level_2'
3. this does not solve the problem of US cities that span multiple counties. Instead, I think the API simply returns the county associated with the single latitude/longitude associated with address_string

The quickest and most non-evasive way might be to use a JSON/XML request from a free geolocation API (Easily found on Google). That way you don't need to create/host your own database.