Lets say you have range(s, e), it will generate a sequence of numbers from s to e - 1, so if you do range(1, 100) it will be from 1 to 99. If you omit the first argument then python will use 0 as the default value, so in your example range is basically being called as range(0, 2) which will generate the sequence [0, 1] You can also pass a value to the step parameter to dictate how values of the range are incremented, the default value is 1. edit: You can also pass a negative value to step and it will generate a reverse sequence. So if you call range(10, 0, -2) it will return the sequence [10, 8, 6, 4, 2] More on reddit.com

It's embarrassing that python's range dosen't allow this, given how easy it is to implement a generator that does this even without accumulating rounding errors. Heck, even the seq tool in GNU coreutils allows one to do seq 0 0.1 1 without rounding errors! ... @josch: seq uses the C long double type internally, and is subject to rounding errors. For example on my machine, seq 0 0.1 1 gives 1 as its last output (as expected), but seq 1 0.1 2 ... More on stackoverflow.com

You can think of range as returning a sequence of numbers. Since we use the syntax for in : to loop or iterate over a sequence, naturally we can put a range as the sequence. Does that help? More on reddit.com

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. More on reddit.com