Difference between if not: and if False:

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I have a probably quite simple question but was wondering between the difference of these two statements:

if not os.path.isfile(file):
  #Do some stuff
if os.path.isfile(file) is False:
  #Do some stuff

What are the differences (if any) between the two? To my understanding they both return a True or False value, so is it just a matter of preference or are there any significant differences?

In python (and other dynamic languages) there is the concept of truthy/falsy value. True/False are not the only things that evaluate as true/false

if not []: 
   print('this will be printed')

if [] is False:
   print('this won't)

Another problem is that you should compare with x == False, and not x is False. The False is a singleton object in the current implementation of CPython, but this is not guaranteed by the specification.

Python not: If Not True, The code below tests if at difference between both scores", "is not 1 point or less") That true/false value we store in smallDifference . With “not” keyword in Python, we invert an expression, so if it returns False, it is now True. We mostly use not in if-statements in Python. Sometimes, we want to invert or flip the value of the boolean variable, and in that case, if not is very useful. With not keyword, we change the meaning of expressions.

They are not equivalent. Consider the following example and try to understand it yourself:

x = None

if not(x):
    print("aa")

if x is False:
    print("bb")

This example prints only "aa". Why? See answer of @milanbalazs

Python if statements that test the opposite: if not � Kodify, Finally, the not operator negates a Boolean value, so not (x > y) is True if (x > y) is False, that is, if x is less than or equal to y. The expression on the left of the or� By now you should have a good understanding of the differences between all the IF functions and how to use them. FYI, here's a quick summary of what we covered: = IF (test, true, false) = IF (test1, true, IF (test2, true, false)) = IFS (test1, true, test2, true, test3, true)

In your case, since we know os.path.isfile returns True or False, there is no difference.

In general, there are a lot of objects in python which, when interpreted as boolean, will evaluate to False.

Think of this:

empty_list = []
if not empty_list:
    print('List is not empty')
if empty_list is False:
    print('List is False')

Among the others, None, "" and [] will evaluate to False.

So testing with not variable is usually the preferred way.

5. Conditionals — How to Think Like a Computer Scientist: Learning , Does the not in if not carry through the whole expression? Not unless you [/ code] This is equivalent to saying: [code] if (not True) or if False or True: Is there a difference between Boolean logic and Boolean algebra, and if so, what is it? In this case the first argument is true, but the second is false. Since OR only needs one of the arguments to be true, the formula returns TRUE. If you use the Evaluate Formula Wizard from the Formula tab you'll see how Excel evaluates the formula. =IF(NOT(A5>B2),TRUE,FALSE) IF A5 is not greater than B2, then return TRUE, otherwise return FALSE.

False is just a keyword for the integer 0. Any "fasly" value will evaluate to False in an if not statement , but only 0 or False itself will evaluate to is False.

This is because is checks for matching identity, while if not checks for implicit booleanness.

For example:

if not None

evaluates.

if None is False

doesn't.

How does 'if not' work with Boolean expressions in Python?, The easiest way to get a boolean value (true or false) is using a comparison expression, such as (a < 10). It makes no difference to the computer. The not- equal operator, !=, is the opposite, evaluating to true if the values are different. +1 but why not allow while (false) for the same debugging purpose? I didn't get that part? Is there any difference between "pour drinks" and "pour out drinks"?

It's usually better to use the first, since it works even if you're not checking an actual boolean value in a Python implementation where False is a singleton object. Uniformity is good, and so is portability.

>>> if 0 is False: print "false"
>>> if not 0: print "false"
false
>>> if [] is False: print "false"
>>> if not []: print "false"
false
>>> if "" is False: print "false"
>>> if not "": print "false"
false

It also protects against mishaps like this:

>>> False = 1
>>> True == False
True

Java If Boolean, You might also want to look up how to use a "case" statement as that is the other main conditional component in C and is worth contrasting to the if/else if/ else structure.

FALSE is not defined in the standard. Only false is.true and false are called "Boolean literals", and are keywords in C++.. FALSE is sometimes defined as a macro. You can't rely on it being present on a standards-compliant environment as it is not part of the standard.

What's the difference between 'not false' and 'true'? If you're dealing with a truth table of about five or ten elements, almost certainly not anything. It's probably not much even if you're dealing with elements of billions (109) or quadrillions (1015) of inputs. But when you get to macroscopic scales (e.g. 6x1023 - 0.6 septillion…

"p is false" implies "p is not true", but not vice verse because p can also be nonsense. "2 + 2 = 5" is both false and not true. "2 + 2 > red" is neither true nor false because it is nonsense. If it were false, its negation "2 + 2 ≤ red" would be true, which is not the case. Source An Inquiry Into Meaning and Truth

Comments
  • Possible duplicate of Understanding Python's "is" operator
  • wow, did this really require 6 answers (plus some deleted) when there is a wealth of better information on SO e.g. stackoverflow.com/a/100762/4711754
  • Actually it does handle 0, as it is treated as being falsey
  • False does not equal None
  • @yatu False and 0 are even more closely related than other falsey values, since bool is a subclass of int.
  • None is NoneType whether False is Boolean type.
  • I see your point, of course these values are not equal or same, but regarding to question in case of using if not ... condition doesn't matter that 0 or False or None.