Python：我可以在 map function 中使用字典更新吗？

Question

I've got the following input inp = [{1:100}, {3:200}, {8:300}] and I would like to convert it to the dictionary. I can do

dct = dict()  
for a in inp:  
    dct.update(a)

or I can do

_ = [dct.update(a) for a in inp]

But I would like to do something like:

map(dct.update, inp)

Answer 1

Method map builds a iterator, so the computes are made only when need it. So you need to force the computing, using a list for example

list(map(dct.update, inp))
print(dct)

---

But you shouldn't use either the list comprehension or the map operation that are here to produce values

You'd better keep your for-loop or a dict-comprehension

dct = {key:val for item in inp for key,val in item.items()}

Answer 2

Why use map for this task?

If you want to solve it in one line you can just do:

dct={k:v for x in inp for k,v in x.items()}

Answer 3

With using Chain Map:

from collections import ChainMap

inp = [{1: 100}, {3: 200}, {8: 300}]
data = dict(ChainMap(*inp))

Answer 4

What you want is reduce , not map . map creates a sequence by applying a function to each element of the given sequence, whereas reduce accumulates an object over the sequence.

from functools import reduce
inp = [{1:100}, {3:200}, {8:300}]
dct = {}
reduce(lambda d1, d2: (d1.update(d2) or d1), inp, dct)

{1: 100, 3: 200, 8: 300}

This lambda part may look tricky. What this does is to create a function that updates a dict and returns the dictionary itself. This is because reduce expects the accumulating function returns the updated object while dict.update() returns None . Since None is interpreted as False, the or operator returns the second variable, which is the updated dictionary.

This or trick is credited to @kostya-goloveshko in this thread: Why doesn't a python dict.update() return the object?