I have a function whose input argument can either be an element or a list of elements. If this argument is a single element then I put it in a list so I can iterate over the
Typically, strings (plain and unicode) are the only iterables that you want to nevertheless consider as "single elements" -- the basestring
builtin exists SPECIFICALLY to let you test for either kind of strings with isinstance
, so it's very UN-grotty for that special case;-).
So my suggested approach for the most general case is:
if isinstance(input, basestring): input = [input]
else:
try: iter(input)
except TypeError: input = [input]
else: input = list(input)
This is THE way to treat EVERY iterable EXCEPT strings as a list directly, strings and numbers and other non-iterables as scalars (to be normalized into single-item lists).
I'm explicitly making a list out of every kind of iterable so you KNOW you can further on perform EVERY kind of list trick - sorting, iterating more than once, adding or removing items to facilitate iteration, etc, all without altering the ACTUAL input list (if list indeed it was;-). If all you need is a single plain for
loop then that last step is unnecessary (and indeed unhelpful if e.g. input is a huge open file) and I'd suggest an auxiliary generator instead:
def justLoopOn(input):
if isinstance(input, basestring):
yield input
else:
try:
for item in input:
yield item
except TypeError:
yield input
now in every single one of your functions needing such argument normalization, you just use:
for item in justLoopOn(input):
You can use an auxiliary normalizing-function even in the other case (where you need a real list for further nefarious purposes); actually, in such (rarer) cases, you can just do:
thelistforme = list(justLoopOn(input))
so that the (inevitably) somewhat-hairy normalization logic is just in ONE place, just as it should be!-)
That is an ok way to do it (don't forget to include tuples).
However, you may also want to consider if the argument has a __iter__ method or __getitem__ method. (note that strings have __getitem__ instead of __iter__.)
hasattr(arg, '__iter__') or hasattr(arg, '__getitem__')
This is probably the most general requirement for a list-like type than only checking the type.
You can put * before your argument, this way you'll always get a tuple:
def a(*p):
print type(p)
print p
a(4)
>>> <type 'tuple'>
>>> (4,)
a(4, 5)
>>> <type 'tuple'>
>>> (4,5,)
But that will force you to call your function with variable parameters, I don't know if that 's acceptable for you.
You can do direct type comparisons using type()
.
def my_func(input):
if not type(input) is list:
input = [input]
for e in input:
# do something
However, the way you have it will allow any type derived from the list
type to be passed through. Thus preventing the any derived types from accidentally being wrapped.
First, there is no general method that could tell a "single element" from "list of elements" since by definition list can be an element of another list.
I would say you need to define what kinds of data you might have, so that you might have:
list
against anything else
isinstance(input, list)
(so your example is correct)basestring
in Python 2.x, str
in Python 3.x)
isinstance(myvar, collections.Sequence) and not isinstance(myvar, str)
int
, str
, MyClass
isinstance(input, (int, str, MyClass))
.
try:
input = iter(input) if not isinstance(input, str) else [input]
except TypeError:
input = [input]
Your aproach seems right to me.
It's similar to how you use atom?
in Lisp when you iterate over lists and check the current item to see if it is a list or not, because if it is a list you want to process its items, too.
So, yeah, don't see anything wrong with that.