Posts Tagged ‘dict’

Python – “dict comprehension”

February 4, 2013 1 comment

I learned a new way to initialize dictionaries that I had never seen before, and I thought I’d share it with you. I previously blogged about three ways of creating dictionaries in Python. I showed that you can use an iterable of (key, value) tuples to initialize a dict:

professions_dict = dict(zip(names, professions))

Another way along these same lines is to perform the iteration within a list comprehension. Say that we have an iterable of Person objects, each of which has a name and profession. The professions_dict could be created as follows:

professions_dict = dict([(, x.profession) for x in people])

What I didn’t realize is you can skip the tuples and call to dict, and use the comprehension within the dictionary literal itself:

professions_dict = { x.profession for x in people}

In my opinion this is much cleaner. The syntax and official documentation can be found in 5.2.5. Displays for sets and dictionaries.


Three ways of creating dictionaries in Python

March 30, 2012 15 comments

Dictionaries are the fundamental data structure in Python, and a key tool in any Python programmer’s arsenal. They allow O(1) lookup speed, and have been heavily optimized for memory overhead and lookup speed efficiency.

Today I”m going to show you three ways of constructing a Python dictionary, as well as some additional tips and tricks.

Dictionary literals

Perhaps the most commonly used way of constructing a python dictionary is with curly bracket syntax:

d = {"age":25}

As dictionaries are mutable, you need not know all the entries in advance:

# Empty dict
d = {}
# Fill in the entries one by one
d["age"] = 25

From a list of tuples

You can also construct a dictionary from a list (or any iterable) of key, value pairs. For instance:

d = dict([("age", 25)])

This is perhaps most useful in the context of a list comprehension:

class Person(object):
    def __init__(self, name, profession): = name
        self.profession = profession

people = [Person("Nick", "Programmer"), Person("Alice","Engineer")]
professions = dict([ (, p.profession) for p in people ])
>>> print professions
{"Nick": "Programmer", "Alice": "Engineer"}

This is equivalent, though a bit shorter, to the following:

people = [Person("Nick", "Programmer"), Person("Alice","Engineer")]
professions = {}
for p in people:
    professions[] = p.profession

This form of creating a dictionary is good for when you have a dynamic rather than static list of elements.

From two parallel lists

This method of constructing a dictionary is intimately related to the prior example. Say you have two lists of elements, perhaps pulled from a database table:

# Static lists for purpose of illustration
names = ["Nick", "Alice", "Kitty"]
professions = ["Programmer", "Engineer", "Art Therapist"]

If you wished to create a dictionary from name to profession, you could do the following:

professions_dict = {}
for i in range(len(names)):
    professions_dict[names[i]] = professions[i]

This is not ideal, however, as it involves an explicit iterator, and is starting to look like Java. The more Pythonic way to handle this case would be to use the zip method, which combines two iterables:

print zip(range(5), ["a","b","c","d","e"])
[(0, "a"), (1, "b"), (2, "c"), (3, "d"), (4, "e")]

names_and_professions = zip(names, professions)
print names_and_professions
[("Nick", "Programmer"), ("Alice", "Engineer"), ("Kitty", "Art Therapist")]

for name, profession in names_and_professions:
    professions_dict[name] = profession

As you can see, this is extremely similar to the previous section. You can dispense the iteration, and instead use the dict method:

professions_dict = dict(names_and_professions)
# You can dispence the extra variable and create an anonymous
# zipped list:
professions_dict = dict(zip(names, professions))

Further reading



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