Creating a new dictionary in Python

Question

I want to build a dictionary in Python. However, all the examples that I see are instantiating a dictionary from a list, etc . ..

How do I create a new empty dictionary in Python?

Tripoli

Answer 1

Call dict with no parameters

new_dict = dict()

or simply write

new_dict = {}

Insectivore

Is there any difference between dict() and {}? Or do people just prefer one over the other? – Foretell

@ Matt Apparently CPython 2.7 dict() is slower (6 times slower?), See: doughellmann.com/2012/11/… In any case I am starting to prefer the constructor syntax anyways since I find it easier to type and move code between dicts and function calls. – Lehmbruck

@DavidWheaton Thanks for pointing this out! Personally, I find new_dict = dict() slightly more readable, nonetheless if there is such a big difference in regards to performance... Would go for performance! ;) (For those who are confused about Python and Cphython see: here) – Lynnettelynnworth

I confirm it's 3 times faster to use { } than dict() in python 3.x – Guanase

Yeah, I get about 4 times faster in python 3.6 for {} over dict() and 5 times for [] over list(). – Oxbridge

In the vast majority of cases, it doesn't matter if it takes six times longer, since that's still an unnoticeably small amount of time. – Illassorted

{} is faster because it cant be overwritten but dict() can be so there are some extra checks and lookups – Tinware

Answer 2

You can do this

x = {}
x['a'] = 1

Grunion

Answer 3

Knowing how to write a preset dictionary is useful to know as well:

cmap =  {'US':'USA','GB':'Great Britain'}

# Explicitly:
# -----------
def cxlate(country):
    try:
        ret = cmap[country]
    except KeyError:
        ret = '?'
    return ret

present = 'US' # this one is in the dict
missing = 'RU' # this one is not

print cxlate(present) # == USA
print cxlate(missing) # == ?

# or, much more simply as suggested below:

print cmap.get(present,'?') # == USA
print cmap.get(missing,'?') # == ?

# with country codes, you might prefer to return the original on failure:

print cmap.get(present,present) # == USA
print cmap.get(missing,missing) # == RU

Jana

Good point! But I think the bit with cxlate makes your answer seem too complicates. I'd just keep the initialization part. (cxlate itself is too complicated. You could just return cmap.get(country, '?').) – Peregrinate

Consider using docs.python.org/2/library/… instead of writing a translate function or using .get() everywhere. – Vegetative

Perhaps I would, except that the documentation is absolutely opaque to me -- it's terrible. I have no idea what they're telling me to do, or why I should do it. And .get() seems to do exactly the right thing -- plus it's extremely flexible. I'm sure its a lack of understanding on my part. With that in mind, my questions are: why bother? What is saved here, easier here, faster here, etc.? Benefit is exactly what? – Jana

This appears useful, but a bit complicated for beginners – Libratory

Answer 4

>>> dict(a=2,b=4)
{'a': 2, 'b': 4}

Will add the value in the python dictionary.

Gat

Answer 5

d = dict()

or

d = {}

or

import types
d = types.DictType.__new__(types.DictType, (), {})

Toile

What is the difference between types.DictType.__new__(types.DictType, (), {}) and just {} – Thither

For anyone reading this: the last "solution" is a bit of a joke - you can use it (in python 2.x at least - won't work in py3k), but no one in it's own mind would ever want to do so ;-) – Felske

Answer 6

So there 2 ways to create a dict :

1. my_dict = dict()

2. my_dict = {}

But out of these two options {} is more efficient than dict() plus its readable. CHECK HERE

Directrix

Why are you referring to python2.7? (in a quite old article: 2012-11-12, "The Performance Impact of Using dict() Instead of {} in CPython 2.7") – Fredericksburg

(7 yrs later) performances did not changed even in python 3.x.x you can see it running a %timeit my_dict = {} vs %timeit my_dict = dict() – Enclitic

Answer 7

>>> dict.fromkeys(['a','b','c'],[1,2,3])


{'a': [1, 2, 3], 'b': [1, 2, 3], 'c': [1, 2, 3]}

Arenicolous

the question states that the dictionary should be a new empty instance – Less

Please check the details in question. The question is how do I create a new empty dictionary in Python? – Erode

Answer 8

I do not have enough reputation yet to be able comment, so I share this as an answer.

The link shared by @David Wheaton in his comment to the accepted answer is no longer valid as Doug Hellmann has migrated his site (source: https://doughellmann.com/posts/wordpress-to-hugo/).

Here is the updated link about "The Performance Impact of Using dict() Instead of {} in CPython 2.7": https://doughellmann.com/posts/the-performance-impact-of-using-dict-instead-of-in-cpython-2-7-2/

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