I'm coming from an R background and am confused about the following:
Suppose I had "hello" in list format saved in the variable txt. Thus
txt= ['h','e','l','l','o']
After some testing, the following works:
txt.reverse()
"."join(txt)
and produces as expected olleh.
However, the following does not work:
"".join(txt.reverse())
It gives an error. I'm curious why that is the case as I thought I can "nest" function calls within function calls?
Thank you for helping!
The problem is that the reverse() function does not return an useful value (it returns None), because it reverses the list in-place, as a way to avoid the need to create a new output list. So this:
"".join(txt.reverse())
... Will try to join None, which clearly will fail.
Being an in-place function, the reverse method of a list always returns None:
>>> a = [1, 2]
>>> print a.reverse()
None
>>> a
[2, 1]
>>>
You can get the behavior you want by using the reversed built-in:
>>> a = ['a', 'b']
>>> "".join(reversed(a))
'ba'
>>> a
['a', 'b']
>>>
a in the reversed example, as you did the modified value of a in the reverse one?
None). Conversely, functions that return a value usually do not have side-effects. It's a convention used pretty often. By the way: you can always obtain R behaviour using some ors: "".join(txt.reverse() or txt). txt.reverse() returns None, which is considered false, so python will evaluate txt and return it instead(return values of or and and need not be booleans). However this seems quite dirty.your problem is that txt.reverse() does NOT return the reversed list, it returns None,
ie, txt.reverse() reverses your list in place
You can use reversed instead:
".".join(reversed(txt))
As already said the list.reverse() replaces the list in-place, thus returns None
If you want a nestable version you can use the reversed function
In [7]: "".join(reversed(txt))
Out[7]: 'olleh'
reversedinstead:"".join(reversed(txt)).reversedis in-place. Tryreversed([1,2,3])on the interpreter, it returns alistreverseiteratorobject which means no copies are made. The difference is that.reverse()modifies the original list, whilereversed()iterates over it.reversedreturns an iterator that will return the items in reverse order. It does not modify the original argument, which thereversemethod does.