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Python Functional Python The Lambda Lambada Reduce

Difference between A is not None, A not None and A != None ? Are they all valid in python? Do they all yield same result

A is not None, A not None and A != None ? Are they all valid in python? Do they all yield same result ?

4 Answers

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points

Using is and == or != are similar but different.

For the three cases you listed, the second one ("A not None") is illegal syntax. Here are three legal syntaxes:

>>> A is not None
True
>>> not A is None
True
>>> A != None
True

The keyword is relates to two objects being the exact same object, whereas == and != deal with two objects being equivalent but not necessarily the same object. All objects in Python have an ID related to their location in memory. Two objects are the exact same if they have the same ID:

>>> B = None
>>> C = None
>>> B is C
True
>>> id(B)
9999216
>>> id(C)
9999216
>>> id(None)
9999216

Two object can be equivalent but not the same object:

>>> B = [None]
>>> C = [None]
>>> B == C
True
>>> B is C
False
>>> id(B)
140030159982344
>>> id(C)
140030159808520
Camille Ferré
Camille Ferré
3,330 Points

Hi all,

I have an additional question related to this one.

Why would we write something like:

if books and if books is not None:

books would be True if it is indeed not None right ?

[MOD: added ```python formatting -cf]

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

In short, it's redundant.

There is a syntax error in your question. The second if should be removed:

if books and books is not None:

When two conditions are joined with and, the second condition is only evaluated if the first condition is True. The first condition is True if and only if books has a "truthy" value. That is, it can't be None. So if the first condition is True then the second condition must also be True. Therefore it is redundant and can be removed leaving:

if books:
Camille Ferré
Camille Ferré
3,330 Points

Thank you Chris, that's what I thought :)

So, does that mean each value has memory address. and if we assign any variable to that same value it is a pointer to that address ?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Sometimes, yes, Python may reuse memory addresses for some literal values since they're immutable (can not be altered). But it is extremely rare to write code to that takes advantage of this, so I would not focus on this.

Each object has a location at a specific address. In most cases, python will create a new object with a new address before assigning it to a variable.

$ python3
Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> # reference a string
... "some string"
'some string'
>>> # check id
... id("some string")
140351042828720
>>> # set variable to string
... s = "some string"
>>> # check s id
... id(s)
140351042828720
>>> # python reused existing string
... # reference string with same value
... "some string"
'some string'
>>> # check it's ID
... id("some string")
140351042828784
>>> # Since the first was assigned to s, a new one was created
... # assign same value to s
... s = "some string"
>>> # check s id
... id(s)
140351042828784
>>> # S was changed to point at new copy. Old copy destroyed
... id("some string")
140351042828720
>>> t = "some string"
>>> id(t)
140351042828720
>>> # can not tell if string was the same or if just address was reused.

Some small integers (0-256) are reused, but larger numbers are not:

>>> id(256)
10113984
>>> c = 256
>>> d = 256
>>> c is d is 256
True
>>> id(257)
140351043411856
>>> c = 257
>>> d = 257
>>> c is d is 257
False
>>> id(c)
140351043411856
>>> id(d)
140351042783152

Thank for taking the time to explain Chris !! Your time and knowledge is much appreciated :)