CODEWITHSUNDEEP
pythonglobal-variableslocal-variables
vincentshiusun
vincentshiusun

Reputation: 7

python local variable- when do I have to assign a value?

I'm an amateur programmer and would like to seek advice while learning codes. Here I encounter some issues.

I found that when I remove the comment "#X=3" and make it into a code from the below then the code works. Without X=3, the code results into UnboundLocalError.

Browsed online, it's related to global and local variable but I can't see how it's related. And when does X has to be denoted before the while loop? and why "for y in primes" doesn't need to pre-define "y"?

Main purpose of the code: count the # of prime numbers up to (num)

def count_primes2(num):
    primes = [2]
    #x = 3
    if num < 2:
        return 0
    while x <= num:
        for y in primes:  # use the primes list!
            if x%y == 0:
                x += 2
                break
        else:
            primes.append(x)
            x += 2
    print(primes)
    return len(primes)

Upvotes: 0

Views: 68

Answers (3)

Alfe
Alfe

Reputation: 59466

You need to create (and assign a value to) a variable before you use it. If you try to use a variable's value before creating the variable, then you get the exception. You do exactly this in the while expression: You ask if its value is below or equal to num, but it does not even exist yet, it has no value, this raises the exception.

Now, why do you get the UnboundLocalError?

The compiler goes through your code before the code gets executed. In this compile step it notices that you somewhere in your function assign a value to X, namely in the line x += 2. (There are even two of them.) This marks the variable for the compiler as a local variable.

So if you try to access the variable before the assignment takes place, the variable doesn't exist yet, but the code already knows that is supposed to be a local variable, hence the UnboundLocalError.

Without any assignment statement to X in the function you would have gotten a NameError because during execution of the while statement the interpreter then searches for a global variable of this name.

Upvotes: 1

SuperStew
SuperStew

Reputation: 3054

To expand, since you are using x in the while loop criteria, yes, it has to be defined before. You don't need to define y before the for loop because the for y in primes line defines y as each item in the list.

A rough translation to plain English:

while x <= num: == As long as this number is less than or equal to this other number, do the following

for y in primes == do the following for each item, named y, in primes

hopefully that wasn't more confusing

Upvotes: 1

arjunan
arjunan

Reputation: 217

As per design pattern variable should be created just before to use. In the code you are using x without creating or initializing default value. "y" = you are iterating the list (primes). So in each iteration y will be initialized by the current value.So it will not give any error.

Upvotes: 2

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