Using 'not' in a 'for-loop statement'

Question

1. Why is not possible to use a not in a for statement? Assuming that both object and list are iterable

2. If you can't do that is there another way around?

Here is an example, but "obviously" a syntax error:

tree = ["Wood", "Plank", "Apples", "Monkey"]

plant = ["Flower", "Plank", "Rose"]

for plant not in tree:
    # Do something
    pass 
else:
    # Do other stuff
    pass

Answer 1

Here's one way, using sets and assuming that both objects and list are iterable:

for x in set(objects).difference(lst):
    # do something

First of all, you should not call a variable list , that'll clash with a built-in name. Now the explanation: the expression set(objects).difference(lst) performs a set difference , for example:

lst = [1, 2, 3, 4]
objects = [1, 2, 5, 6]
set(objects).difference(lst)
=> set([5, 6])

As you can see, we found the elements in objects that are not in the list.

Answer 2

If objects and list are two lists, and you want to iterate over every element of objects that isn't in list , you want the following:

for object in objects:
    if object not in list:
        do_whatever_with(object)

This loops over everything in objects and only processes the ones that aren't in list . Note that this won't be very efficient; you could make a set out of list for efficient in checking:

s = set(list)
for object in objects:
    if object not in s:
        do_whatever_with(object)

Answer 3

It looks like you are confusing a couple of things. The for loop is used to iterate over sequences (lists, tuples, characters of a string, sets, etc). The not operator reverses boolean values. Some examples:

>>> items = ['s1', 's2', 's3']
>>> for item in items:
...   print item
...
s1
s2
s3
>>> # Checking whether an item is in a list.
... print 's1' in items
True
>>> print 's4' in items
False
>>>
>>> # Negating
... print 's1' not in items
False
>>> print 's4' not in items
True

Answer 4

If you mean to iterate over a list except few:

original = ["a","b","c","d","e"]
to_exclude = ["b","e"]
for item [item for item in orginal if not item in to_exclude]: print item

Produces:

a
c
d

Answer 5

Here is a simple way to achieve what you want:

 list_i_have = [1, 2, 4]  
 list_to_compare = [2, 4, 6, 7]

 for l in list_i_have:
     if l not in list_to_compare:
         do_something()
     else:
         do_another_thing()

Foreach item in the list you have, you can have a exclude list to check it is inside of list_to_compare.

You can also achieve this with list comprehension:

["it is inside the list" if x in (3, 4, 5) else "it is not" for x in (1, 2, 3)]

Answer 6

You may use list comprehension combined with inline if:

>>> lst = [1, 2, 3, 4]
>>> objects = [1, 2, 5, 6]
>>> [i for i in objects if i not in lst]
[5, 6]

Answer 7

And another way:

from itertools import ifilterfalse
for obj in ifilterfalse(set(to_exclude).__contains__, objects):
    # do something