Sorting Lists of Instances

Sorting_Lists_of_Instances.py

L = ["Cherry", "Apple", "Blueberry"]

print(sorted(L, key=len))
#alternative form using lambda, if you find that easier to understand
print(sorted(L, key= lambda x: len(x)))

# Output:
# ['Apple', 'Cherry', 'Blueberry']
# ['Apple', 'Cherry', 'Blueberry']


# When each of the items in a list is an instance of a class, you need to provide a function that takes one 
# instance as an input, and returns a number. The instances will be sorted by their numbers.

class Fruit():
    def __init__(self, name, price):
        self.name = name
        self.price = price

L = [Fruit("Cherry", 10), Fruit("Apple", 5), Fruit("Blueberry", 20)]
for f in sorted(L, key=lambda x: x.price):
    print(f.name)

# Output:
# Apple
# Cherry
# Blueberry



# Sometimes you will find it convenient to define a method for the class that does some computation on the data in an 
# instance. In this case, our class is too simple to really illustrate that. But to simulate it, I’ve defined a method 
# sort_priority that just returns the price that’s stored in the instance. Now, that method, sort_priority takes one 
# instance as input and returns a number. So it is exactly the kind of function we need to provide as the key parameter 
# for sorted. Here it can get a little confusing: to refer to that method, without actually invoking it, you can refer 
# to Fruit.sort_priority. This is analogous to the code above that referred to len rather than invoking len().

class Fruit():
    def __init__(self, name, price):
        self.name = name
        self.price = price

    def sort_priority(self):
        return self.price

L = [Fruit("Cherry", 10), Fruit("Apple", 5), Fruit("Blueberry", 20)]
print("-----sorted by price, referencing a class method-----")
for f in sorted(L, key=Fruit.sort_priority):
    print(f.name)

print("---- one more way to do the same thing-----")
for f in sorted(L, key=lambda x: x.sort_priority()):
    print(f.name)


# Output:
# -----sorted by price, referencing a class method-----
# Apple
# Cherry
# Blueberry
# ---- one more way to do the same thing-----
# Apple
# Cherry
# Blueberry