Difference between dict and set (python)


So, I know that this,

a = {}  # dict

constructs an empty dictionary. Now, I also picked up that this,

b = {1, 2, 3}  # set

creates a set. This can easily be verified, as,

<class 'dict'>

<class 'set'>

While I understand what it does, I fail to see why we use ‘set notation’ for empty dictionaries. I tried to find some more information about the logic behind this in the set and dict sections of the manual, but sadly, I got nothing out of it.

Could anyone explain to me why we do this in this way? Is it for historical reasons, or am I missing something blatantly obvious?

Asked By: Nelewout



The syntax is not the same. Dictionaries used curly braces the first and you specify key-value pairs, where the key and value are separated by a colon:

>>> {'foo': 'bar'}
{'foo': 'bar'}
>>> type(_)
<type 'dict'>

Sets were added to the language later on, and the {..} curly brace notation only names elements, not pairs:

>>> {'foo'}
>>> type(_)
<type 'set'>

Note that in Python 2, the interpreter echoes the object using the set() callable. That’s also how you specify an empty set:

>>> emptyset = set()

In Python 3, the newer {..} notation is used when echoing the object, unless it is empty:

>>> {'foo'}
>>> _ - {'foo'}  # difference, removing the one element

The set() type was added to the Python language in version 2.4 (see PEP 218), the curly brace syntax for set literals was added in Python 3 and back-ported to Python 2.7.

Answered By: Martijn Pieters

There were no set literals in Python 2, historically curly braces were only used for dictionaries. Sets could be produced from lists (or any iterables):

set([1, 2, 3])
set([i for i in range(1, 3)])

Python 3 introduced set literals and comprehensions (see PEP-3100) which allowed us to avoid intermediate lists:

{1, 2, 3}
{i for i in range(1, 3)}

The empty set form, however, was reserved for dictionaries due to backwards compatibility. References from [Python-3000] sets in P3K? states:

I’m sure we can work something out — I agree, {} for empty set and {:}
for empty dict would be ideal, were it not for backward compatibility. I
liked the “special empty object” idea when I first wrote the PEP (i.e.,
have {} be something that could turn into either a set or dict), but one
of the instructors here convinced me that it would just lead to confusion
in newcomers’ minds (as well as being a pain to implement).

The following message describes these rules better:

I think Guido had the best solution. Use set() for empty sets, use {}
for empty dicts, use {genexp} for set comprehensions/displays, use
{1,2,3} for explicit set literals, and use {k1:v1, k2:v2} for dict
literals. We can always add {/} later if demand exceeds distaste.

Answered By: myaut

The fact that {} is used for an empty dictionary and not for an empty set has largely historical reasons. The syntax {'a': 100, 'b': 200} for dictionaries has been around since the beginning of Python. The syntax {1, 2, 3} for sets was introduced with Python 2.7. Since {} has been used for such a long time it will stay as the way to define an empty dictionary. If Python would have had the new set syntax since the beginning, likely an empty set would be defined with {} and an empty dictionary with {:}.

Answered By: Mike Müller

A set is an unordered collection. A dictionary is an unordered collection of data that stores data in key-value pairs. Dictionaries and sets use hash tables in order to achieve their O(1) lookup and insertions.
Data-structures documentation to refer: https://docs.python.org/3/tutorial/datastructures.html

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Answered By: Sreehari
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