Pass list of lists of list to function

Question

I have values in a list of lists.

I would like to send the whole block to a conversion function which then returns all the converted values in the same structure.

my_list = [sensor1...sensor4] = [hum1...hum3] = [value1, value2, value3, value4]

So several nested lists

def conversion(my_list): dictionaries
    for sensor in my_list:
        for hum in sensor:
            for value in hum:
                map(function, value)

Is there a way to do a list comprehension as a one liner? I'm not sure how to use the map function in comprehensions especially when you have several nested iterations.

It's not even pseudocode; why not provide a [mcve] that *actually shows what you're doing?* — jonrsharpe, Jul 21 '15 at 14:20
possible duplicate of [Flattening a shallow list in Python](http://stackoverflow.com/questions/406121/flattening-a-shallow-list-in-python) — piglei, Jul 21 '15 at 14:21
Isin't your list more like `[ [ [1, 2, 3], [4, 5, 6], [7, 8, 9] ], [...] ]` — Gab, Jul 21 '15 at 14:23
Wait. Function returns dictionary. Where is the dictionary ? — FunkySayu, Jul 21 '15 at 14:24

poke · Accepted Answer · 2015-07-21T14:34:20.430

map(function, value)

Since you are just mapping a function on each value, without collecting the return value in a list, using a list comprehension is not a good idea. You could do it, but you would be collecting list items that have no value, for the sole purpose of throwing them away later—just so you can save a few lines that actually serve a much better purpose: Clearly telling what’s going on, without being in a single, long, and complicated line.

So my advice would be to keep it as it is. It makes more sense like that and clearly shows what’s going on.

I am however collecting the values. They all need to be converted and saved in the same structure as they were.

In that case, you still don’t want a list comprehension as that would mean that you created a new list (for no real reason). Instead, just update the most-inner list. To do that, you need to change the way you’re iterating though:

for sensor in my_list:
    for hum in sensor:
        for i, value in enumerate(hum):
            hum[i] = map(function, value)

This will update the inner list.

Alternatively, since value is actually a list of values, you can also replace the value list’s contents using the slicing syntax:

for sensor in my_list:
    for hum in sensor:
        for value in hum:
            value[:] = map(function, value)

Also one final note: If you are using Python 3, remember that map returns a generator, so you need to convert it to a list first using list(map(function, value)); or use a list comprehension for that part with [function(v) for v in value].

I am however collecting the values. They all need to be converted and saved in the same structure as they were. Which means in those nested for loops I need to pass everything back down the chain :S Efficient it ain't — cc6g11, Jul 21 '15 at 14:31
Value is not a list, they are the actual floats needing converted with the function — cc6g11, Jul 21 '15 at 14:42
@cc6g11 You should *really* get your question straight. It’s conflicting with what you are saying here. In your question, you seem to have a value list, and that also makes sense since you are calling `map` on it; you can’t call `map` on a single float. — poke, Jul 21 '15 at 14:43

FunkySayu · Answer 2 · 2015-07-22T07:06:05.967

0

This is the right way to do it. You can use list comprehension to do that, but you shouldn't for code readability and because it's probably not faster.

edited Jul 22 '15 at 07:06

answered Jul 21 '15 at 14:23

FunkySayu

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@jonrsharpe ho... I learned something in english today. I mean readability – FunkySayu Jul 22 '15 at 07:05

Pass list of lists of list to function

2 Answers2