map 在 python 中如何工作？

Question

I am new to Python programming. I encountered a way to take a list input using map.

code 1:

l = list(map(int, input().split()))
print(l)

code 2:

l = [map(int, input().split())]
print(l)

Both of them look identical to me,

But when I took the input 1 2 3 4 , code #1 gave the output [1,2,3,4] and code #2 for the same input gave the output [<map object at 0x0000024DFD85F0C8>] .

What is the difference between both methods? How are they different? Any help is appreciated.

Answer 1

What list() does is it takes any iterable like generator, tuple, string, map etc. and exhousts given object to create list. For example:

gen = (i**2 for i in range(10))  # -> <generator object <genexpr> at 0x7f5b7066e0f8>
list(gen)  # -> [0, 1, 4, 9]

tup = (1, 2, 3, 4)  # -> (1, 2, 3, 4)
list(tup)  # -> [1, 2, 3, 4]

str_ = '123'  # -> '123'
list(str_)  # -> ['1', '2', '3']

You can think of it as:

def list(it):
    ret_list = []
    for el in it:
        ret_list.append(el)
    return el

---

Where as surrounding instances with [...] simply creates list with whatever you've passed, so:

gen = (i**2 for i in range(10))  # -> <generator object <genexpr> at 0x7f5b7066e0f8>
[gen]  # -> [<generator object <genexpr> at 0x7f5b7066e0f8>]

tup = (1, 2, 3, 4)  # -> (1, 2, 3, 4)
[tup]  # -> [(1, 2, 3, 4)]

str_ = '123'  # -> '123'
[str_]  # -> ['123']

Naive implementation:

def brancets(*args):
    ret_list = []
    for arg in args:
        ret_list.append(arg)
    return ret_list

# [1, 2, 3] -> brackets(1, 2, 3)

Answer 2

The map() function executes a specified function for each item in a iterable. The item is sent to the function as a parameter. you have to specify the function to execute in each item. to display the final result you can use "list" for readability this is an example:

def myfunc(a, b):
   return a + b

x = map(myfunc, ('apple', 'banana', 'cherry'), ('orange', 'lemon', 'pineapple'))
print(x)
print(list(x)) #for readability 

in your case you can do this

def myfunc(a)
   return int(a)*2
x = map(myfunc,input().split())
print (list(x)) 

check this link: https://www.w3schools.com/python/ref_func_map.asp