Python Set Examples

Set. A set is a dictionary with no values.

It has only unique keys. Its syntax is similar to that for a dictionary. But we omit the values, and can use complex, powerful set logic.

In practice, sets are less often used. The initializer syntax is the same as a dictionary, but no pairs are specified—just keys. With union() and difference() we employ set logic.

First example. This program initializes a set. When we initialize a set, we do not include the values in the syntax as with a dictionary. We specify only the keys.

Then: We use len(), in and not-in to test the set. It has just 3 elements, despite specifying 5 strings.

Tip: When the set is initialized, duplicate values are removed. As with a dictionary, no two keys can have the same value.

In: The set also allows the in-keyword. This returns true or false depending on whether the item exists.

In

Based on:

Python 3

Python program that creates set

# Create a set.
items = {"arrow", "spear", "arrow", "arrow", "rock"}

# Print set.
print(items)
print(len(items))

# Use in-keyword.
if "rock" in items:
    print("Rock exists")

# Use not-in keywords.
if "clock" not in items:
    print("Cloak not found")

Output

{'spear', 'arrow', 'rock'}
3
Rock exists
Cloak not found

Add. Elements can be added to a set with the add method. Here we create an empty set with the set built-in method. We then add three elements to the set.

Tip: This is a good approach if you don't know beforehand what elements are to be added.

Empty: We use set() to create an empty set. We cannot use empty curly brackets as they indicate an empty dictionary, not set.

Python program that adds, discards, removes

# An empty set.
items = set()

# Add three strings.
items.add("cat")
items.add("dog")
items.add("gerbil")
print(items)

Output

{'gerbil', 'dog', 'cat'}

Create, list. We can create a set from a list (or other iterable collection). Consider this example. We pass a list with six different elements in it to set(). The duplicates are ignored.

Integers: A set can contain integers, strings, or any type of elements that can be hashed.

Python program that creates set from list

# Create a set from this list.
# ... Duplicates are ignored.
numbers = set([10, 20, 20, 30, 40, 50])
print(numbers)

Output

{40, 10, 20, 50, 30}

Subset, superset. In set theory, we determine relations between sets of elements. And with the Python set type, we can compute these with built-in methods.

Here: In this program, we introduce two sets. The method results depend on the numbers in the sets.

Is subset: This returns true in the program because numbers2 is a subset of numbers1.

Is superset: This method also returns true in this program. Numbers1 is a superset of numbers2.

Intersection: This method returns a new set that contains just the shared numbers. Other values are omitted.

Python program that uses set methods

numbers1 = {1, 3, 5, 7}
numbers2 = {1, 3}

# Is subset.
if numbers2.issubset(numbers1):
    print("Is a subset")

# Is superset.
if numbers1.issuperset(numbers2):
    print("Is a superset")

# Intersection of the two sets.
print(numbers1.intersection(numbers2))

Output

Is a subset
Is a superset
{1, 3}

Union. Another set operation that is available is union(). This combines two sets. Any element in either set is retained in the return value of union. But duplicates are eliminated.

Here: In the program, the sets each contained a 3, but the union method returns a set with just one 3.

Python program that unions two sets

# Two sets.
set1 = {1, 2, 3}
set2 = {6, 5, 4, 3}

# Union the sets.
set3 = set1.union(set2)
print(set3)

Output

{1, 2, 3, 4, 5, 6}

Subtract, difference. A set can be subtracted from another set. The difference() method is used in this case. The syntax that is clearer is the best choice.

Tip: Subtracting sets is not something I do every day. For this reason, I would prefer difference() to make the operation explicit.

Result: When we subtract set "b" from set "a", the string "connecticut" is removed. The other strings remain.

And: When we subtract set "a" from set "b", the string "connecticut" is also removed. The other two strings from "b" remain.

Python program that subtracts sets

a = {"new york", "connecticut", "new jersey"}
b = {"connecticut", "pennsylvania", "maine"}

# Subtract.
c = a - b
print(c)

# Difference.
c = a.difference(b)
print(c)

# Subtract in opposite order.
c = b - a
print(c)

# Difference in opposite order.
c = b.difference(a)
print(c)

Output

{'new jersey', 'new york'}
{'new jersey', 'new york'}
{'pennsylvania', 'maine'}
{'pennsylvania', 'maine'}

Discard. We pass discard() the value of an element we want to remove. If the element does not exist, discard will cause no error—it does nothing. Remove, however, will cause a KeyError.

So: To safely use remove(), you may need to use the in-operator beforehand. Discard meanwhile does not need this step.

Python that uses discard, remove

animals = {"cat", "dog", "parrot", "walrus"}
print(animals)

# Discard nonexistent element, nothing happens.
animals.discard("zebra")
print(animals)

# Discard element that exists.
animals.discard("cat")
print(animals)

# Remove element that exists.
animals.remove("parrot")
print(animals)

# Remove causes an error if the element is not found.
animals.remove("buffalo")

Output

{'walrus', 'dog', 'parrot', 'cat'}
{'walrus', 'dog', 'parrot', 'cat'}
{'walrus', 'dog', 'parrot'}
{'walrus', 'dog'}
Traceback (most recent call last):
  File "...", line 16, in <module>
    animals.remove("buffalo")
KeyError: 'buffalo'

Frozenset. This is an immutable set. We cannot add or remove elements. We can use it in every other way like a set. And because it cannot be changed, it can be used as a dictionary key.

Tip: In some program contexts, a frozenset can be used where a set cannot. It can be a dictionary key or a set element itself.

Here: We initialize a frozenset with the frozenset() built-in. We pass it a list of strings.

Python that uses frozenset

# Strings to put in frozenset.
keys = ["bird", "plant", "fish"]

# Create frozenset.
f = frozenset(keys)
print(f)

# Cannot add to frozenset.
try:
    f.add("cat")
except AttributeError:
    print("Cannot add")

# Can use frozenset as key to dictionary.
d = {}
d[f] = "awesome"
print(d)

Output

frozenset({'plant', 'bird', 'fish'})
Cannot add
{frozenset({'plant', 'bird', 'fish'}): 'awesome'}

Get keys, dictionary. A dictionary contains only unique keys. With the set() built-in, we can get these keys and convert them into a set.

Python that uses dictionary, set keys

# This dictionary contains key-value pairs.
dictionary = {"cat": 1, "dog": 2, "bird": 3}
print(dictionary)

# This set contains just the dictionary's keys.
keys = set(dictionary)
print(keys)

Output

{'bird': 3, 'dog': 2, 'cat': 1}
{'cat', 'bird', 'dog'}

Performance. How does a set compare in performance to a dictionary? Logically the performance should be similar. I test how an in-keyword test runs.

Result: The set was consistently faster. The set lookup was performed about 4% faster than the dictionary lookup in the simple benchmark.

Thus: When you need to test if an element exists in a collection, the set may offer improved performance over the dictionary.

Python that benchmarks set

import time

set1 = {"a", "b", "c", "z"}
dict1 = {"a": 1, "b": 2, "c": 3, "z": 4}

print(time.time())

# Use set.
i = 0
while i < 10000000:
    a = "z" in set1
    i += 1

print(time.time())

# Use dictionary.
i = 0
while i < 10000000:
    a = "z" in dict1
    i += 1

print(time.time())

Output

1346615677.741
1346615679.7   (Set        = 1.959 s)
1346615681.732 (Dictionary = 2.032 s)

1346615958.731
1346615960.692 (Set        = 1.961 s)
1346615962.736 (Dictionary = 2.044 s)

In my experience, sets are not widely used in computer programs. Instead collections like dictionaries are used more often. But sometimes a set is useful.

Overall: A dictionary is more powerful than a set. But in certain programs, a set is more graceful. It has methods such as intersection().

Map. Methods such as map can be used to transform collections. The result of map() is not a set. It is a "map object" which we can enumerate in a for-loop.

Map

Python that uses set and map

values = {10, 20, 30}

# Multiply all values in the set by 100.
result = map(lambda x: x * 100, values)

# Display our results.
for value in result:
    print(value)

Output

1000
2000
3000

Sometimes, the existence of keys is our main consideration. The keys have no specific value. Here a set is worthwhile. It avoids confusion with having unused values in a dictionary.

More advantages. A set shortens the syntax of programs. No values are specified. It provides mathematical methods like union that act on set logic.

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