Currently I’m doing this:
try:
something = next(iterator)
# ...
except StopIteration:
# ...
But I would like an expression that I can place inside a simple if
statement. Is there anything built-in which would make this code look less clumsy? I only need to check for the first item.
4
if any(True for _ in iterator):
print('iterator had at least one element')
if all(False for _ in iterator):
print('iterator was empty')
Note that this will consume the first element of the iterable if it has at least one element.
7
Pass a sentinel value as the default value to next()
:
sentinel = object()
if next(iterator, sentinel) is sentinel:
print('iterator was empty')
You could also use any value which you “know” (based on application considerations) that the iterator can’t possibly yield as the sentinel value.
2
This isn’t really cleaner, but it shows a way to package it in a function losslessly:
def has_elements(iter):
from itertools import tee
iter, any_check = tee(iter)
try:
any_check.next()
return True, iter
except StopIteration:
return False, iter
has_el, iter = has_elements(iter)
if has_el:
# not empty
This isn’t really pythonic, and for particular cases, there are probably better (but less general) solutions, like the next default.
first = next(iter, None)
if first:
# Do something
This isn’t general because None can be a valid element in many iterables.
6
The best way to do that is with a peekable
from more_itertools
.
from more_itertools import peekable
iterator = peekable(iterator)
if iterator:
# Iterator is non-empty.
else:
# Iterator is empty.
Just beware if you kept refs to the old iterator, that iterator will get advanced. You have to use the new peekable iterator from then on. peekable
expects to be the only bit of code modifying that old iterator.
1
you can use:
if zip([None], iterator):
# ...
else:
# ...
but it’s a bit nonexplanatory for the code reader
1
What about:
In [1]: i=iter([])
In [2]: bool(next(i,False))
Out[2]: False
In [3]: i=iter([1])
In [4]: bool(next(i,False))
Out[4]: True
2
This is an overkill iterator wrapper that generally allows to check whether there’s a next item (via conversion to boolean). Of course pretty inefficient.
class LookaheadIterator ():
def __init__(self, iterator):
self.__iterator = iterator
try:
self.__next = next (iterator)
self.__have_next = True
except StopIteration:
self.__have_next = False
def __iter__(self):
return self
def next (self):
if self.__have_next:
result = self.__next
try:
self.__next = next (self.__iterator)
self.__have_next = True
except StopIteration:
self.__have_next = False
return result
else:
raise StopIteration
def __nonzero__(self):
return self.__have_next
x = LookaheadIterator (iter ([]))
print bool (x)
print list (x)
x = LookaheadIterator (iter ([1, 2, 3]))
print bool (x)
print list (x)
Output:
False
[]
True
[1, 2, 3]
1
A little late, but… You could turn the iterator into a list and then work with that list:
# Create a list of objects but runs out the iterator.
l = [_ for _ in iterator]
# If the list is not empty then the iterator had elements; else it was empty.
if l :
pass # Use the elements of the list (i.e. from the iterator)
else :
pass # Iterator was empty, thus list is empty.
3