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)
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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")
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setdefault
Suppose We have a dict which recording the result of a examination:
table = { "Nick" : "A", "Ada" : "B", "Mike": "B", "Leo": "C", "Sandy" : "D" }
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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"] }
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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)
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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)
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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)
print(type(result))
print(result)
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