Understanding Python’s map, reduce, filter, and sorted Functions

The map() function takes a function and an iterable, applying the function to each element and returning a new iterator; for example:

def f(x):
    return x * x
L = map(f, [1,2,3,4,5])
list(L)  # [1, 4, 9, 16, 25]

The reduce() function from functools repeatedly applies a two‑argument function to the items of a sequence, accumulating a single result, e.g.:

from functools import reduce
def fn(x, y):
    return x * 10 + y
reduce(fn, [1,2,3,4,5])  # 12345

Combining map() and reduce() can convert a string of digits to an integer:

from functools import reduce
DIGITS = {'0':0, '1':1, '2':2, '3':3, '4':4, '5':5, '6':6, '7':7, '8':8, '9':9}
def str2int(s):
    def fn(x, y):
        return x * 10 + y
    def char2num(ch):
        return DIGITS[ch]
    return reduce(fn, map(char2num, s))

The filter() function also receives a function and a sequence, keeping elements for which the function returns True . For instance, removing empty strings:

def not_empty(s):
    return s and s.strip()
list(filter(not_empty, ['a', '', 'b', None, '  ']))  # ['a', 'b']

Using filter() together with a generator can produce prime numbers:

def _odd_iter():
    n = 1
    while True:
        n += 2
        yield n

def _not_divisible(n):
    return lambda x: x % n > 0

def primes():
    yield 2
    it = _odd_iter()
    while True:
        n = next(it)
        yield n
        it = filter(_not_divisible(n), it)

for n in primes():
    if n < 1000:
        print(n)
    else:
        break

The built‑in sorted() function returns a new sorted list and can accept a key function and a reverse flag. Examples:

sorted([36, 5, -12, 9, -21])  # [-21, -12, 5, 9, 36]
sorted([36, 5, -12, 9, -21], key=abs)  # [5, 9, -12, -21, 36]
sorted(['bob', 'about', 'Zoo', 'Credit'])  # ['Credit', 'Zoo', 'about', 'bob']
sorted(['bob', 'about', 'Zoo', 'Credit'], key=str.lower)  # ['about', 'bob', 'Credit', 'Zoo']
sorted(['bob', 'about', 'Zoo', 'Credit'], key=str.lower, reverse=True)  # ['Zoo', 'Credit', 'bob', 'about']