Python defaultdict(default) vs dict.get(key, default)

Question

Suppose I want to create a dict (or dict -like object) that returns a default value if I attempt to access a key that's not in the dict .

I can do this either by using a defaultdict :

from collections import defaultdict

foo = defaultdict(lambda: "bar")
print(foo["hello"]) # "bar"

or by using a regular dict and always using dict.get(key, default) to retrieve values:

foo = dict()
print(foo.get("hello", "bar")) # "bar"
print(foo["hello"]) # KeyError (as expected)

Other than the obvious ergonomic overhead of having to remember to use .get() with a default value instead of the expected bracket syntax, what's the difference between these 2 approaches?

Answer 1

Asides from the ergonomics of having .get everyone, one important difference is if you lookup a missing key in defaultdict it will insert a new element into itself rather than just returning the default. The most important implications of this are:

• Later iterations will retrieve all keys looked up in a defaultdict
• As more ends up stored in the dictionary, more memory is typically used
• Mutation of the default will store that mutation in a defaultdict , with .get the default is lost unless stored explicty

from collections import defaultdict 
 
default_foo = defaultdict(list) 
dict_foo = dict()                                                                                                                                                                                                                                                                                           

for i in range(1024): 
    default_foo[i] 
    dict_foo.get(i, []) 
                                                                                                                                                                                                                                                                                                 
print(len(default_foo.items())) # 1024
print(len(dict_foo.items())) # 0

# Defaults in defaultdict's can be mutated where as with .get mutations are lost
default_foo[1025].append("123")
dict_foo.get(1025, []).append("123")

print(default_foo[1025]) # ["123"]
print(dict_foo.get(1025, [])) # []

Answer 2

The difference here really comes down to how you want your program to handle a KeyError.

foo = dict()

def do_stuff_with_foo():
    print(foo["hello"])
    # Do something here
   
if __name__ == "__main__":
    try:
        foo["hello"] # The key exists and has a value
    except KeyError:
        # The first code snippet does this
        foo["hello"] = "bar"
        do_stuff_with_foo()

        # The second code snippet does this
        exit(-1)

It's a matter of do we want to stop the program entirely? Do we want the user to fill in a value for foo["hello"] or do we want to use a default value?

The first approach is a more compact way to do foo.get("hello", "bar") But the kicker is the matter of is this what we really want to happen?