Could you say me, how to write a function, which takes as its (one an only) argument a list of numbers and returns a list of string representations of those numbers?
For example toNum([1, 2, 3, 4]) returns ["1", "2", "3", "4"].
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4 Answers
def to_num(a): return map(str, a) print to_num([1, 2, 3, 4]) prints
['1', '2', '3', '4'] 
3 Comments
Chris W.
Actually its more "Pythonic" to use list comprehension rather than map(), at least according to Guido. He actually wanted to completely get rid of map() and filter() in favor of list comprehension: artima.com/weblogs/viewpost.jsp?thread=98196
Sven Marnach
@Chris: There are two advantages of
map() though: (1) It's faster and (2) it's less verbose. I personally find it also more readable in simple cases like this one. And map() is still there even in Python 3, so it seems kind of pointless to claim that just any use of this built-in function is not "Pythonic" (a pretty meaningless word anyway).
ncoghlan
This kind of use case is precisely the reason why
map() and filter() stayed, so you can hardly call it unPythonic. However, using a lambda expression with map() instead of using a list comprehension crosses the line into code that wouldn't make it through any code review I was involved in.using list comprehension:
def stringify(input): return [str(num) for num in input] 
1 Comment
salezica
+1 python-style! (I personally favor list comprehension over
map())Adrien already gave you an elegant answer:
def stringify(input): return [str(num) for num in input] That works perfectly, but if you intend to only iterate through the representations (and don't need to keep the whole list in memory for any other reason), you should instead do:
(str(num) for num in the_list) The parenthesis instead of the brackets indicate a generator expression, just as iterable as a list, but won't fully expand on creation. This may be important if your list is large.
3 Comments
Adrien Plisson
using python 3, the two are equivalent: list comprehension always returns an iterator...
John Machin
@Adrien Plisson: Perhaps you are mistaken; Python 3.2 example:
>>> squares = [x * x for x in range(5)]; type(squares) produces <class 'list'>ncoghlan
While many things that returned lists in Python 2.x now return iterators (or other custom objects) in Python 3.x, list comprehensions aren't one of them. You may be thinking of other things like
range(), dict.keys(), dict.values() and dict.items().You just have to supply the parameters in the function call
def to_num(*numbers):# with the * you can enter as many parameters as you want return [str(x) for x in numbers] 