Why is this returning an empty list:
def word_lengths(phrase):
result = []
map(lambda x: result.append(x) , phrase.split())
return result
Where as this returns a list of the length of each word in the phrase:
def word_lengths(phrase):
return list(map(lambda x: len(x) , phrase.split()))
In Python 3, map results in a generator, which is lazily evaluated.
You need to iterate over it to take effect, for example by calling the list constructor on it:
result = []
list(map(lambda x: result.append(x) , phrase.split()))
return result
This mutates result as you had probably expected.
Note though, that the same can be achieved in a much simpler way:
return phrase.split()
phrase.split() directly is much more Pythonic. Abusing map for side-effects is profoundly misunderstanding the point of functional programming idioms. map (and list comprehensions and generator expressions) should always be side-effect free, to avoid lots of spooky action at a distance that makes it hard to reason about the code.map for side-effects makes sense (usually using the multiprocessing map-like functions to parallelize a task), the best way to force it to run to completion is to wrap it in a collections.deque constructor with a maxlen of 0 (which is optimized to run iterators to exhaustion without storing a thing), e.g. collection.deque(map(...), 0) which would avoid making a potentially large temporary list of Nones for no purpose.
lambda x: len(x)is just a more complicated way of sayinglen, and that the whole thing would be more pythonically expressed as a list comprehension:[len(word) for word in phrase.split()].dict.update, notlist.append, but everything else about it is the same)mapcall to anything in the former case. And even callinglist(map(...with theresult.append(x)as your lambda will just return a list ofNonefor as many elements as there are inphrase.split()