Python: Dict setdefault and getdefault

In this post, we will discuss dict’s setdefault and getdefault in Python.

These are two handy programming idioms in Python.

getdefault

When we get the value from a dict, if the key does not exist in dict, a None will be returned.

dict.get(key, default = None)

The second parameter for get is the default value. Here is an example:

dict = { 'Name': 'Nick', 'Age': 23 }
print "Value : %s" % dict.get('Age')
print "Value : %s" % dict.get('Education', "Master")

# => Value : 23
# => Value : Master

setdefault

Suppose We have a dict which recording the result of a examination:

table = { "Nick" : "A", "Ada" : "B", "Mike": "B", "Leo": "C", "Sandy" : "D" }

We want to convert it to a dict with grade -> list of names so that we can check how many students in each grade. The result will be:

{ "A" : ["Nick"], "B" : ["Ada", "Mike"], "C" : ["Leo"], "D" : ["Sandy"] }

A beginner maybe will write the code in this way:

table = { "Nick" : "A", "Ada" : "B", "Mike": "B", "Leo": "C", "Sandy" : "D" }
result = {}
for name, score in table.items():
    if score not in result:
        result[score] = [name]
    else:
        result[score].append(name)

print(result)

# => {'A': ['Nick'], 'C': ['Leo'], 'B': ['Mike', 'Ada'], 'D': ['Sandy']}

In each iteration, we need to check whether the new key exists in the result.

A more pythonic way is using setdefault :

table = { "Nick" : "A", "Ada" : "B", "Mike": "B", "Leo": "C", "Sandy" : "D" }
result = {}
for name, score in table.items():
    g = result.setdefault(score, [])
    g.append(name)

print(result)

# => {'A': ['Nick'], 'C': ['Leo'], 'B': ['Mike', 'Ada'], 'D': ['Sandy']}

The code can be more simpler if we use defaultdict, but the result is an object of defaultdict:

table = { "Nick" : "A", "Ada" : "B", "Mike": "B", "Leo": "C", "Sandy" : "D" }
from collections import defaultdict

result = defaultdict(list)
for name, score in table.items():
    result[score].append(name) # all keys have a default already

print(type(result))
print(result)

# => <type 'collections.defaultdict'>
# => defaultdict(<type 'list'>, {'A': ['Nick'], 'C': ['Leo'], 'B': ['Mike', 'Ada'], 'D': ['Sandy']})

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