Unexpected behavior from Python

Question

I'm new to Python and I am a bit confused with the way Python treats an empty object.

Consider this code piece;

a = {}

if a:
    print "a is alive!"
else:
    print "a is NOT alive!"

if not a:
    print "NOT a!"
else:
    print "a!"

if a is None:
    print "a is None!"
else:
    print "a is NOT None!"

I get the following output for this code piece.

a is NOT alive!
NOT a!
a is NOT None!

Edit::

I am under the assumption that an object initialized by {} is a Valid Object. Why doesn't Python treat it that way? and why do I get diff output for diff If conditions?

Edit 2::

In C++, when I say

 Object obj;
 if (obj){
 }

It will enter the IF block if obj is NOT NULL(regardless if it is garbage value or whatever)

But the same thing when I translate to python.

a = {} #This is a valid object
if a:
# Doesn't work!

Why? and I read Python evaluates {} as False. Why is that?

possible duplicate of [Python: Checking if a 'Dictionary' is empty doesn't seem to work](http://stackoverflow.com/questions/23177439/python-checking-if-a-dictionary-is-empty-doesnt-seem-to-work) — Anshul Goyal, Nov 14 '14 at 14:06
The output is correct and this is how it's done in Python. Even you said that you check with "If not". — ha9u63a7, Nov 14 '14 at 14:06
Downvoted on account that you don't include what you think the expected behaviour is. — wvdz, Nov 14 '14 at 14:08
Empty dict is the same as "False", but not the same as None. What did you expect? — Rusty, Nov 14 '14 at 14:09
@All... I know the output I'm getting, I am asking why diff If conditions are giving diff output. — Syed Mauze Rehan, Nov 14 '14 at 14:12
because they are different conditions and statements. do you expect the same thing will happen no matter what you type? — Karoly Horvath, Nov 14 '14 at 14:13
FYI: http://stackoverflow.com/questions/4600295/what-is-the-meaning-of-operator-bool-const-in-c — Karoly Horvath, Nov 14 '14 at 14:27
Everything is a valid python object. `False` evaluation has nothing to do with its validity. Show me an invalid python object, I don't think I've ever heard of these. — Rusty, Nov 14 '14 at 14:28

score 2 · Answer 1 · answered Nov 14 '14 at 14:12

2

Empy dict/sets/lists etc are evaluated as false. None is its own type, with only one possible value.

https://docs.python.org/2.4/lib/truth.html Tells you what is evaluated as true and false

answered Nov 14 '14 at 14:12

horns

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+1. I wanted to edit it in my answer, but you beat me to it, good job! – Rusty Nov 14 '14 at 14:13
2

saying that they are evaluated as false is misleading. only in *boolean* context. – Karoly Horvath Nov 14 '14 at 14:15
1

The key difference is that you are using `is` None. So not only is an empty dictionary not *equal* to None, it is also not exactly the same object. For the same reason `{} == {}` evaluates as True, but `{} is {}` evaluates False - they are both empty dicts, but they are not the *same* empty dictionary. – Tom Dalton Nov 14 '14 at 14:23

score 1 · Answer 2 · answered Nov 14 '14 at 14:11

1

It is a valid python object, but it is empty, so it is treated as a False, the same goes for lists [] or numbers 0. You do have a dict, but it is not a None object.

answered Nov 14 '14 at 14:11

Rusty

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Alberto Coletta · Accepted Answer · 2014-11-14T14:24:32.353

1

I see nothing weird in your output. Let's go step-by-step:

a is dictionary, more specifically a dictionary object;
a is a dictionary, but it's empty, so its truth value is False

Therefore:

The first if, since a is False, prints the else statement and that's right;
The second if, since not a evaluates to True because a is False, prints the if part and that's right too.
Last, but not least a is not a None object, but a dict object, so it's right too that the else part is taken and printed.

edited Nov 14 '14 at 14:24

answered Nov 14 '14 at 14:19

Alberto Coletta

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<<<>>> when I say a={}, does it means that it's not allocated any memory until I add an item to it? – Syed Mauze Rehan Nov 14 '14 at 14:24
If you do `a={}` memory is allocated (on my system 280 bytes are allocated); you can check doing `sys.getsizeof(object)`. The truth value of the dictionary is determined from the fact that is empty. It's a pythonic behaviour and it works also on empty strings, lists, sets and so on. – Alberto Coletta Nov 14 '14 at 14:35

score 0 · Answer 4 · answered Nov 14 '14 at 14:17

0

with

a = {}

you are creating an dictionary object which is not NoneType you can check the class of your object with

type(a)

which will give you:

type dict

if not a:

will return False if a has already any members and True if a is just and empty list

answered Nov 14 '14 at 14:17

arash javanmard

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Unexpected behavior from Python

Edit::

Edit 2::

4 Answers4