Explaining differences between dict.get(key) and dict[key] in Python

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The aim of this explainer💡 is to define the difference between

<dict>.get(<key>) VS <dict>[<key>]

in Python.

1. Get() is used when the missing key should not throw an exception, [] syntax when it should

• It allows you to provide a default value/expression if the key is missing
• …a sort of if/then mechanism without an if statement

>>> d = {"name": "mr_paul"}
>>> d.get("name")
'mr_paul'
>>> d.get("job", print("seems to be unemployed"))
seems to be unemployed

• If omitted in the .get() , default_value is None, such that

>>> d.get("job", None)

• is the same as

>>> d.get("job")  # <-- No default specified -- defaults to None

• however, checking for a non-existent key with [] syntax throws an ERROR

>>> d["name"]
'mr_paul'
>>> d["job"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'job'

'job'

• raises a KeyError which you can properly catch if needed

2. Let’s get to real life: I am get() this for say checking a value of an environmental variable

• I’m in a Docker
• I am doing case analysis based on environmental variables and I don’t want the script to crash if a key is not found

if os.environ.get("IMAGE_NAME") == "my_foo_docker image":
        return input('Please enter client tag (e.g. com_acme): ')
    else:
        return input("Please enter org id: ")

• A simple reason is that I can look_up org_id in one container based on a nicer client tags

3. In addition, you can return values for non-found keys by using get()

• the following counts words in a string

def count_words(doc):
    normalised_doc = "".join(c.lower() if c.isalpha() else " " for c in doc)
    frequencies = {}
    for word in normalised_doc.split():
        frequencies[word] = frequencies.get(word, a0) + 1
    return frequencies

• frequencies.get(word, 0) means that
• KeyError is never raised because of the use of get method
• the 0 is another optional parameter to be returned as a value if the key does not exist
• the default is None

LINKS

• Understanding .get() method in Python — Stack Overflow