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stackoverflow.com › questions › 4908666 › how-to-use-while-not-in

Using sets will be screaming fast if you have any volume of data

If you are willing to use sets, you have the isdisjoint() method which will check to see if the intersection between your operator list and your other list is empty.

MyOper = set(['AND', 'OR', 'NOT'])
MyList = set(['c1', 'c2', 'NOT', 'c3'])

while not MyList.isdisjoint(MyOper):
    print "No boolean Operator"

http://docs.python.org/library/stdtypes.html#set.isdisjoint

Answer from gahooa on Stack Overflow

reddit.com › r/learnpython › trouble understanding while not

r/learnpython on Reddit: Trouble understanding While Not

February 16, 2017 -

I'm doing "automate the boring stuff" and was on an early lesson about while loops. I can't wrap my head around "while not." The code is below

name = ''
while not name:
     print('Enter your name:')
     name = input()
print('How many guests will you have?')

While my limited knowledge, I would write it instead as

name = ''
while name == '':
    print('Enter your name:')
    name = input()
print('How many guests will you have?')

My question is how can they mean the same thing? Both start with name = False/empty. The way I interpret the top code is, while name is not False/empty. The way I interpret the bottom is, while name is equal to False/empty. In my mind, the top would keep looping with any True value. Thanks for your help!

EDIT: formatting

EDIT 2: Finally figured it out. I'm posting this edit for prosperity sake, for any other programming ogres like me. As long as the condition line is True, the loop will run. If the condition is False, the loop will not run. Even though name is False, the condition line is True and the loop will go until something is inputted. Once something is typed in, name becomes True, which would make the condition not name False, ending the loop.

Thanks everyone for helping me understand that! It's so simple now that I see it. I'm super new to Python and made a rookie mistake.

Top answer

1 of 4

An empty string coerces to False in a boolean context, while any other string coerces to True. The first version is the idiomatic way of writing that.

2 of 4

I almost committed suicide trying to get this straight in my head. I finally got it and maybe this will help someone else as well: In Python if variable is empty, i.e. = None or = "" - it is considered False (or Falsey to be more precise). While actually means While True and works as long as chosen condition is True. While not basically means While False and works as long as set condition is False. In this case we set name = False and say: as long as the name is False, i.e empty - do the loop. As soon as we add anything to the name through input - the name becomes True and the loop is ended. Jeeezoooos.

Stack Overflow

stackoverflow.com › questions › 65293083 › how-to-properly-use-while-not-loops-in-python

How to properly use "while not" loops in python? - Stack Overflow

This is to say it is basically "while guessed is false." So the while loop will run for as long as guessed is false and tries > 0. However, if guess is every made to be true, then the while loop will stop running.

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Stack Overflow

stackoverflow.com › questions › 4908666 › how-to-use-while-not-in

python - How to use: while not in - Stack Overflow

Top answer

1 of 8

Using sets will be screaming fast if you have any volume of data

If you are willing to use sets, you have the isdisjoint() method which will check to see if the intersection between your operator list and your other list is empty.

MyOper = set(['AND', 'OR', 'NOT'])
MyList = set(['c1', 'c2', 'NOT', 'c3'])

while not MyList.isdisjoint(MyOper):
    print "No boolean Operator"

http://docs.python.org/library/stdtypes.html#set.isdisjoint

2 of 8

The expression 'AND' and 'OR' and 'NOT' always evaluates to 'NOT', so you are effectively doing

while 'NOT' not in some_list:
    print 'No boolean operator'

You can either check separately for all of them

while ('AND' not in some_list and 
       'OR' not in some_list and 
       'NOT' not in some_list):
    # whatever

or use sets

s = set(["AND", "OR", "NOT"])
while not s.intersection(some_list):
    # whatever

Python.org

discuss.python.org › python help

If we assign false to a variable and then use a while not loop, why is the variable still false and not true? - Python Help - Discussions on Python.org

November 16, 2022 - if we assign false to a variable and then use a while not loop, why is the variable still false and not true?

W3Schools

w3schools.com › python › python_while_loops.asp

Python While Loops

With the break statement we can stop the loop even if the while condition is true: ... Note: The else block will NOT be executed if the loop is stopped by a break statement. ... If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail: sales@w3schools.com · If you want to report an error, or if you want to make a suggestion, send us an e-mail: help@w3schools.com · HTML Tutorial CSS Tutorial JavaScript Tutorial How To Tutorial SQL Tutorial Python Tutorial W3.CSS Tutorial Bootstrap Tutorial PHP Tutorial Java Tutorial C++ Tutorial jQuery Tutorial

reddit.com › r/learnpython › how do while not loops work

r/learnpython on Reddit: How do while not loops work

September 11, 2024 -

I was reading through this code and I'm just not getting how while loops work when not operator is also used.

https://pastebin.com/5mfBhQSb

I thought the not operator just inversed whatever the value was but I just can't see this code working if that's the case.

For example , the while not sequenceCorrect should turn the original value of False to True, so the loop should run while it's True. But why not just state while True then? And why declare sequenceCorrect = True again? Doesn't it just means while True, make it True? And so on.

The only way it makes sense to me is of the while not loop always means False (just like the while always means as long as it's True) even if the value is supposed be False and should be inverted to True.

So, is that the case? Can anyone explain why it works like that?

Top answer

1 of 8

In general, recall the while loop syntax. while : Conceptually, coding aside, consider filling a glass of water. Is there space for more water in the glass? Okay, then pour some. Without going in detail of how this funky glass object is defined, hopefully the following makes sense as a python analog of that process. while glass.has_space(): #we're still filling the glass glass.add_water() #glass is now full of water Without writing out how this glass object works, let's say it has another method for returning a boolean (True/False) for whether or not it is currently full, instead of the previous example of whether or not it had space for more water. We can fill the glass in the same way, but we have to negate that statement using the not operator while not glass.is_full(): #we're still filling the glass glass.add_water() #glass is now full of water Going back to the initial syntax, which is still the same, the only thing we've done is changing the condition statement of the loop. Instead of checking that glass.has_space() evaluates to True, we are now checking that the expression not glass.is_full() evaluates to True. That is the same statement as evaluating that glass.is_full() evaluates to False, because the only thing the not operator does to a boolean is negating it, i.e. True becomes False and vice versa. Now looking at your code linked, the condition for looping is that not sequenceCorrect evaluates to True, which is equivalent to the statement that sequenceCorrect is False. I won't paste the code here, but we see on line 3 that sequenceCorrect starts its life being False, so upon entering the while loop we do indeed step into that block of code because at that time not sequenceCorrect is in fact True. Then the first thing we do on line 5 is reassign it to True. If the remaining lines don't change it back to False, this will stop the while loop from repeating, since in this current state not sequenceCorrect evaluates to False. So you can say that line 5 is defaulting the loop to not going to be repeated. The only way for the loop to be repeated is for lines 6-8 with the nested for loop and if statement to find some character in dna that is not in "actgn", upon which sequenceCorrect will be assigned to False and thus the statement in the while loop not sequenceCorrect would evaluate to True and thus the loop would repeat itself once more. Personally, I think this is just a pretty unclear way to achieve the goal of checking the validity of that input string. I would have defined it another way, but I can't see anything wrong with it really. If the not operation in the while loop statement is bothering you, you could equally have rewritten the code with a variable for instance named sequenceIncorrect and just flipped all assignments i.e. True's become False's and vice versa.

2 of 8

The difference here is that by using not sequenceCorrect you are avoiding the use of the break keyword. It's cleaner and makes the loop terminate itself, rather than relying on a specific keyword and force terminate it. You could use while True, but that way is not descriptive enough and can potentially create and endless loop if you don't take care of the edge cases. Think about it: while not sequenceCorrect is telling you, without a single comment line, that the loop should run while sequenceCorrect is False, vs while True which the only thing that's telling you is that this loop will run endlessly for unknown reasons.

Appdividend

appdividend.com › how-to-use-while-not-in-python

While Not in Python

December 13, 2025 - The while not is a variation of the while loop that continues to execute a block of code as long as a specified condition is false in Python.

Stanford CS

cs.stanford.edu › people › nick › py › python-while.html

While Loop

Here is an example that uses while/True to go through the numbers 0..9. This not the best way to generate those numbers; it just shows how if/break can serve instead of a while/test at the top of the loop.

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freeCodeCamp

freecodecamp.org › news › while-loops-in-python-while-true-loop-statement-example

While Loops in Python – While True Loop Statement Example

July 19, 2022 - Next, it is time to construct the while loop. I am going to check if the variable user_input is not equal to the contents of the variable secret_keyword. Essentially, I am checking whether what the user has entered is not equal to the string 'Python'.

Real Python

realpython.com › lessons › not-in-while-loops

Using not in Boolean while Loops (Video) – Real Python

02:18

Just like with if, use not when there is a condition you want to be False for a loop to iterate, because writing a condition that should be True would be more complicated. 00:20 Suppose you’re writing a game or some other type of interactive ...

Published July 19, 2022

Real Python

realpython.com › python-while-loop

Python while Loops: Repeating Tasks Conditionally – Real Python

March 3, 2025 - The bad news is that Python doesn’t have one. The good news is that you can emulate it using a while loop with a break statement. Consider the following example, which takes user input in a loop: ... >>> while True: ... number = int(input("Enter a positive number: ")) ... print(number) ... if not number > 0: ...

Stack Overflow

stackoverflow.com › questions › 69599255 › confused-with-the-not-operator-in-while-loops-and-if-statements

python - Confused with the "not" operator in while loops and if statements - Stack Overflow

In Python if variable is empty, ... as long as chosen condition is True. While not basically means While False and works as long as set condition is False....

wiki.c2.com

While Not Done Loop

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TOOLSQA

toolsqa.com › python › python-while-loop

Python While Loop | While True and While Else in Python || ToolsQA

August 6, 2021 - Unlike the "for" loop in python, the while loop does not initialize or increment the variable value automatically. As a programmer, you have to write this explicitly, such as "i = i + 2".

Learn By Example

learnbyexample.org › python-while-loop

Python While Loop - Learn By Example

April 20, 2020 - If the condition is false at the start, the while loop will never be executed at all.

Luc

anh.cs.luc.edu › handsonPythonTutorial › whilestatements.html

3.3. While Statements — Hands-on Python Tutorial for Python 3

In Python, while is not used quite like in English. In English you could mean to stop as soon as the condition you want to test becomes false.

GeeksforGeeks

geeksforgeeks.org › python › loops-in-python

Loops in Python - GeeksforGeeks

08:47

If we want a block of code to execute ... while loop in Python to do so. Code given below uses a 'while' loop with the condition "True", which means that the loop will run infinitely until we break out of it using "break" keyword or some other logic. ... Note: It is suggested not ...

Published 1 month ago

Python documentation

docs.python.org › 3 › tutorial › controlflow.html

4. More Control Flow Tools — Python 3.14.3 documentation

The pass statement does nothing. It can be used when a statement is required syntactically but the program requires no action. For example: >>> while True: ...

reddit.com › r/learnpython › none object and while loop confusion.

r/learnpython on Reddit: None object and While loop confusion.

June 23, 2022 -

Player_1 = None

While (not Player_1):
    Player_1 = input('your name')

I'm not new to python, I understand classes and functions etc..but this is really baffling me and I need some help understanding 'None' and the While loop.

I understand that 'not' negates a Boolean value, I get that piece, but None is not a Boolean value, I've searched this and None is its own Object and None is also not equal to False or True.

The while loop runs if the condition evaluates to true, and doesn't run if evaluated to False.

What is making the loop run the first time and not the second time. Obviously, the first time the logic equates to True and the second time to False...But I don't get the logic.

I think it's the "not" which is confusing me, what is the not doing?

Can someone please enlighten me?

Top answer

1 of 3

I get that piece, but None is not a Boolean value, I've searched this and None is its own Object and None is also not equal to False or True. None isn't equal to False but None is "falsey", as are empty collections, the numbers 0 and 0.0, and the empty string. "Falsey" values are treated as boolean False in contexts where truth values are expected, so not None evaluates to True.

2 of 3

https://docs.python.org/3/library/stdtypes.html#truth-value-testing Here are most of the built-in objects considered false: constants defined to be false: None and False. zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1) empty sequences and collections: '', (), [], {}, set(), range(0) Anything else is treated as True. In your case the non-empty string from input() is considered True.

reddit.com › r/learnpython › while vs while is not none?

r/learnpython on Reddit: While vs While is not None?

October 22, 2022 -

Which is the recommended syntax? "while <list> is not None:" make more sense to me than "while <var>:" Same thing for "if <list> is not None". However, which way of writing is more proper or according to PEP8?

Top answer

1 of 5

If a list is involved in the loop terminating condition we usually use for not while. In the rare situation where a list is consumed (items are being deleted from the list inside the loop) it is perfectly correct to use while The same way, to verifying that an iterable (list, string, tuple and others) or a mapping (dict) is empty we just do if .

2 of 5

is not None is a tautology. A list is always not None because an empty list may be falsy (equal False) but it does not equal None.