7: Functional Programming

Functional programming is a paradigm behind writing and organizing our code.

FP is all about separation of concerns.

Functions operate on well-defined structures such as lists and dictionaries.

Map

my_list = [1,2,3]

def multiply_by2(item):
    return item * 2

updated_list = list(map(multiply_by2, my_list)) # [2,4,6]
print(my_list) #[1,2,3]
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Note that you need to cast the map object to a list.

Filter

my_list = [1,2,3]

def filter_lt3(item):
    return item < 3

updated_list = list(filter(filter_lt3, my_list)) # [1,2]
print(my_list) # [1,2,3]
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Zip

my_list = [1,2,3]
people = ['a','b','c']

tuple_list = list(zip(my_list, people)) # [(1, 'a'), (2, 'b'), (3, 'c')]
print(my_list) # [1,2,3]
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It will zip lists together into a list of tuples.

Reduce

from functools import reduce

my_list = [1,2,3]

# This is operating as the accumulator
def add_items(item1, item2):
    return item1 + item2

tuple_list = list(reduce(add_items, my_list, 0)) # [1,3,6]
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It will zip lists together into a list of tuples.

Lambda Expressions

Anonymous functions that you only need once.

my_list = [1,2,3]

list(map(lambda item: item * 2, my_list)) # [2,4,6]
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If you wanted with more params, you need to pass them. For example, using a lambda with reduce:

from functools import reduce

my_list = [1,2,3]

tuple_list = list(reduce(lambda item1, item2: item1 + item2, my_list, 0)) # [1,3,6]
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List Comprehensions

Comprehensions are a quick way to create a list, set or dictionary without iterate over lists.

For example:

# Without comprehensions
my_list = []

for char in 'hello':
    my_list.append(char)

print(my_list) # ['h', 'e', 'l', 'l', 'o']

# With comprehensions: [param for param in iterable]
my_list = [char for char in 'hello']

my_list2 = [num for num in range(0, 100)] # list of [0, ..., 99]

# With conditions
my_list3 = [num * 2 for num in range(0, 100) if num % 2 == 0] # list of even numbers [0, ..., 198]
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Comprehensions can get confusing, so note the trade-off.

Set and Dictionary Comprehensions

For sets, we simple change list notation [] to a set {}.

my_list = {char for char in 'hello'}

my_list2 = {num for num in range(0, 100)} # set of {0, ..., 99}

# With conditions
my_list3 = {num * 2 for num in range(0, 100) if num % 2 == 0} # set of even numbers [0, ..., 198]
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For dictionaries:

simple_dict = {
    'a': 1,
    'b': 2,
}


my_dict = {key:value**2 for key, value in simple_dict.items()}

print(my_dict) # {'a': 1, 'b': 4}

my_dict2 = {num:num*2 for num in [1,2,3]}
print(my_dict2) # {1: 2, 2: 4, 3: 6}
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