26

I'm trying to use map in Python3. Here's some code I'm using:

import csv

data = [
    [1],
    [2],
    [3]
]

with open("output.csv", "w") as f:
    writer = csv.writer(f)
    map(writer.writerow, data)

However, since map in Python3 returns an iterator, this code doesn't work in Python3 (but works fine in Python2 since that version of map always return a list)

My current solution is to add a list function call over the iterator to force the evaluation. But it seems odd (I don't care about the return value, why should I convert the iterator into a list?)

Any better solutions?

yegle
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    Using `map` for side effects is what's odd. Python 2 `map` also collects the return values. The new behavior merely highlights it further. Just don't do that, [use a for loop](http://stackoverflow.com/q/5753597/395760). –  Aug 25 '13 at 20:45
  • @delnan Thank you for the link, indeed I shouldn't use `map` for side effect. – yegle Aug 25 '13 at 20:48
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    for Python 3, `list(map(lambda x:2*x, [1,2,3]))` – Zhe Hu Aug 05 '16 at 21:01

6 Answers6

29

Using map for its side-effects (eg function call) when you're not interested in returned values is undesirable even in Python2.x. If the function returns None, but repeats a million times - you'd be building a list of a million Nones just to discard it. The correct way is to either use a for-loop and call:

for row in data:
    writer.writerow(row)

or as the csv module allows, use:

writer.writerows(data)

If for some reason you really, really wanted to use map, then you can use the consume recipe from itertools and generate a zero length deque, eg:

from collections import deque
deque(map(writer.writerow, data), maxlen=0)
Jon Clements
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6

You can set up a zero length deque to do this:

with open("output.csv", "w") as f:
    writer = csv.writer(f)
    collections.deque(map(writer.writerow, data),0)

This is the same way that itertools.consume(iterator, None) recipe works. It functionally will exhaust the iterator without building the list.

You can also just use the consume recipe from itertools.

But a loop is more readable and Pythonic to me, but YMMV.

sds
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dawg
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3

If you don't care about the return value, then map is not the best tool for the job. A simple for would be better:

for d in data:
    writer.writerow(d)

That'll work fine in Python 2.x and 3.x. Notice that map is useful when you want to create a new list, if you're traversing over an iterable just for the effect, then use a for.

Óscar López
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2
list(map(lambda x: do(x),y))

will trigger an evaluation and stay in the nice, readable semantics that improve human runtime efficiency beyond the "for-loop (which is a map itself) plus new paragraph scope transition" semantics. ¯\(ツ)/¯

Note that there is no reason not to call it semantic sugar during an initial draft (in fact, for loops are usually easier since they are more modular: you may not know what your code needs to do during your first try at a problem), but when you are productionalizing or reverse engineering code that is in a working state, increasing semantic efficiency (or even merely rewriting in equivalently nice code) is a strong factor of success.

Anyhow, if you want to flush the map stack, trigger it with a list type conversion.

So:

import csv

data = [
    [1],
    [2],
    [3]
]

with open("output.csv", "w") as f:
    writer = csv.writer(f)
    list(map(writer.writerow, data))
Chris
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1

You can also use list comprehension, as suggested in the official FAQ:

with open("output.csv", "w") as f:
    writer = csv.writer(f)
    [writer.writerow(elem) for elem in data]

List comprehension will force the evaluation of each element even if you don't assign the newly created list to any variable.

Note, however, that the list may still be created behind the scenes, creating a potential performance pitfall, so while relatively succinct, it should not be used if the input sequence can get very long.

undercat
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0

I would use a function to extract data from the iterable using something like this:

def rake(what, where=None):
    for i in what: 
        if where: where.append(i)

rake(map(writer.writerow, data))

If you know up front that you won't ever be collecting the output of the mapped function then you could simplify that to just:

for i in what: pass

But neither approach keeps excess data around unless you supply a list to put it in. And this approach should work equally well with map, filter, reduce, generators, range, and anything else that you could pass in to the rake function that for can iterate over.

user1637146
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