When you first start learning Python, chances are you write dictionary access as such:
dictionary['key']This is a valid way to retrieve the associated value in the dictionary, but it does come with a major drawback: what happens if the key doesn’t exist?
Take the following dictionary, for example:
cloud_information = {
"name" : "cumulonimbus",
"height" : "1000m",
"is_raining" : False
}Let’s say that I were to access the dictionary to get the name of the cloud:
dictionary["Height"]This will throw an error, as Height does not exist (but height does). The capitalization does matter and I’ll explain why below if you’d like a deeper dive into this topic.
Using the .get() method will help avoid this error and give us something to fall back on in case the key doesn’t exist in our dictionary. Generally, this is the preferred way of going about accessing dictionaries:
# Argument 1: the key value you're wanting to retrieve from the dictionary
# Argument 2: a "fallback" value if the key doesn't exist
cloud_information.get("Height", "Field does not exist")I personally never use dictionary access without .get() for this exact reason, even if it’s a key I’d expect to be in the dictionary; things happen and I’d want my logging to pick up on it.
Alternatively, you could wrap this statement in a try/except statement, but this does provide more code overhead and a presents a pattern I try to avoid unless it’s necessary:
try:
return dictionary['height']
except KeyError as e:
return "Field does not exist"How dictionaries work under the hood (and why they’re so quick!)
It might make sense to us humans that we want height regardless of capitalization, but computers don’t understand human text - it understands numbers.
Taking a peek under the hood, each key in our dictionary is stored in a “bucket”, which contains the key’s hash, a pointer to the key object, and a pointer to the value object:
Example bucket
When you index the dictionary (i.e. cloud_information[‘height’]), the requested key, height, hash is computed.
It uses this hash to jump directly to the bucket there the key should live, checks that the key matches, and then returns the value.
And if you’re someone who likes talking time complexity, this is why all dictionary lookups are constant time (O(1)) 😃
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