Python update a key in dict if it doesn't exist

You do not need to call d.keys(), so

if key not in d:
    d[key] = value

is enough. There is no clearer, more readable method.

You could update again with dict.get(), which would return an existing value if the key is already present:

d[key] = d.get(key, value)

but I strongly recommend against this; this is code golfing, hindering maintenance and readability.

Use dict.setdefault():

>>> d = {'key1': 'one'}
>>> d.setdefault('key1', 'some-unused-value')
'one'
>>> d    # d has not changed because the key already existed
{'key1': 'one'}
>>> d.setdefault('key2', 'two')
'two'
>>> d
{'key1': 'one', 'key2': 'two'}

Since Python 3.9 you can use the merge operator | to merge two dictionaries. The dict on the right takes precedence:

new_dict = old_dict | { key: val }

For example:

new_dict = { 'a': 1, 'b': 2 } | { 'b': 42 }

print(new_dict) # {'a': 1, 'b': 42}

Note: this creates a new dictionary with the updated values.

With the following you can insert multiple values and also have default values but you're creating a new dictionary.

d = {**{ key: value }, **default_values}

I've tested it with the most voted answer and on average this is faster as it can be seen in the following example, .

Speed test comparing a for loop based method with a dict comprehension with unpack operator method.

if no copy (d = default_vals.copy()) is made on the first case then the most voted answer would be faster once we reach orders of magnitude of 10**5 and greater. Memory footprint of both methods are the same.