In earlier posts we covered Dictionary Comprehensions and List Comprehensions. As you probably have guessed, the biggest difference with a Set Comprehension is we're creating a Set instead. Since the List and Set being so similar, using that post as a starting point this should be quick. 😃
Creating a new set using a for loop looks very much like how we'd create a List.
numbers_list = [1, 2, 3, 4, 5, 6, 7, 8]
new_set = set()
for num in numbers_list:
new_set.add(num)
We don't need to use an old set to create a new set. As seen here we can use a list and add each to a new set. Of course a shorter method would be new_set = set(numbers_list).
Here is the Set Comprehension of the for loop above.
numbers_list = [1, 2, 3, 4, 5, 6, 7, 8]
new_set = { num for num in numbers_list }
We are using curly braces { } just like when making a Dictionary Comprehension. How does Python know we want a Set instead of a Dictionary? The expression part of the Comprehension returns a single value instead of a key:value pair.
Modifying the Value
With a Set Comprehension we can modify the expression being added to the new set.
Here we'll make a new set that has the values double of the original list.
new_set = { num * 2
for num in numbers_list }
Filtering the Set
Let's filter the list so we include only even numbers.
new_set = { num
for num in numbers_list
if num%2 == 0 }
Examples
The difference between a list and set is mainly that a set won't contain duplicate values. We can use that to remove duplicates without adding extra checks to out code.
Price Point
Let's say we have a list of prices and we want to know the dollar price points of the prices. We don't want duplicates.
prices = [1.49, 1.99, 3.49, 4.25, 3.99, 1.25]
What we want returned is {1, 3, 4}.
price_points = { int(price)
for price in prices }
The int() will give us the dollar amount but also would give use duplicates. Using a set removes those duplicates without checks written by us.
Product Colors
We've been given a list that contains a dictionary of products. Our task is to provide a set of colors.
products = [
{ 'model': 'AVE-101', 'color': 'blue' },
{ 'model': 'QRE-333', 'color': 'gold' },
{ 'model': 'AVE-101', 'color': 'green' },
{ 'model': 'QRE-333', 'color': 'blue' }
]
Looping over the dictionary we'll insert the color value into the set and let the set handle removing the duplicates.
product colors = { product['color']
for product in products }
Top Salespeople
We have a list of dictionaries and want to know which salespeople that have had sales above $1,000.
sales = [
{'Employee': 'Aang', 'Amount': 985},
{'Employee': 'Sokka', 'Amount': 1444},
{'Employee': 'Katara', 'Amount': 135},
{'Employee': 'Sokka', 'Amount': 135},
{'Employee': 'Aang', 'Amount': 1339},
{'Employee': 'Sokka', 'Amount': 135},
]
top_sales = {
sale['Employee']
for sale in sales
if sale['Amount'] > 1000
}