How do I setting the API key using python googlemaps

the way I use it is the following, as skycrew said you need to pass your API key in the client class.

You'll notice here that being at work behind a proxy I had to pass some more arguments but it's not necessary if you're not in this situation.

import googlemaps
GOOGLE_MAP_KEY = 'yourkey'
PROXY = {'proxies': {"https": "https://user:password@proxyurl:proxyport/"}}
gmaps = googlemaps.Client(GOOGLE_MAP_KEY, requests_kwargs=PROXY)
results = gmaps.geocode(address=search)

now your error seems to come from the fact you didn't register your API properly. Go to https://console.developers.google.com and see the Credentials link on the left, click on it and add an API key, you should be set

You need to pass the key to Client() function as per docstring below:

Help on class Client in module googlemaps.client:

class Client(__builtin__.object)
 |  Performs requests to the Google Maps API web services.
 |  
 |  Methods defined here:
 |  
 |  __init__(self, key=None, client_id=None, client_secret=None, timeout=None, connect_timeout=None, read_timeout=None, retry_timeout=60, requests_kwargs=None, queries_per_second=10)
 |      :param key: Maps API key. Required, unless "client_id" and
 |          "client_secret" are set.
 |      :type key: string
 |      
 |      :param client_id: (for Maps API for Work customers) Your client ID.
 |      :type client_id: string
 |      
 |      :param client_secret: (for Maps API for Work customers) Your client
 |          secret (base64 encoded).
 |      :type client_secret: string
 |      
 |      :param timeout: Combined connect and read timeout for HTTP requests, in
 |          seconds. Specify "None" for no timeout.
 |      :type timeout: int
 |      
 |      :param connect_timeout: Connection timeout for HTTP requests, in
 |          seconds. You should specify read_timeout in addition to this option.
 |          Note that this requires requests >= 2.4.0.
 |      :type connect_timeout: int
 |      
 |      :param read_timeout: Read timeout for HTTP requests, in
 |          seconds. You should specify connect_timeout in addition to this
 |          option. Note that this requires requests >= 2.4.0.
 |      :type read_timeout: int
 |      
 |      :param retry_timeout: Timeout across multiple retriable requests, in
 |          seconds.
 |      :type retry_timeout: int
 |      
 |      :param queries_per_second: Number of queries per second permitted.
 |          If the rate limit is reached, the client will sleep for the
 |          appropriate amount of time before it runs the current query.
 |      :type queries_per_second: int
 |      
 |      :raises ValueError: when either credentials are missing, incomplete
 |          or invalid.
 |      :raises NotImplementedError: if connect_timeout and read_timeout are
 |          used with a version of requests prior to 2.4.0.
 |      
 |      :param requests_kwargs: Extra keyword arguments for the requests
 |          library, which among other things allow for proxy auth to be
 |          implemented. See the official requests docs for more info:
 |          http://docs.python-requests.org/en/latest/api/#main-interface
 |      :type requests_kwargs: dict

So you can do like this:

mapService = Client("you_key", "your_client_id", "your_client_secret")