Python3: for-loop break and else (if statement)

Question

Background information:

hey, I want to do the following: I have a dictionary with IDs as keys and lists with various things as value. One of the items of the value is a string . I want to check, if a list contains this string . And I want to do it for all keys in my dictionary.

• If the list contains the string, I want to print "String is valid"
• If the list does not contain the string, I want to print "String is NOT valid"

So far, so good.

Furthermore, the lists I want to check depend on one console input of the user, which specifies, which list should be checked. The console input is "number" .

My idea was to iterate over my dictionary and my list with a nested for-loop and compare, if the string(the item of the value) is equal to any list item. If it is, I want to break out of the loop. If the String is not found in the list, I want to execute the else-statement to print my "String is not valid" message.

Code snippet:

def validationHelper(myDict, myList):
        for key in myDict:
            for value in myDict[key][0]:
                for item in myList:
                    if value==item:
                        validationHelper.true="String is valid"
                        break
                else:
                    validationHelper.true="Warning: String is NOT valid"

def validation(anyList,helperfunc):
    if anyList=="one":
        return helperfunc(finalDict,myList1)
    if anyList=="two":
         return helperfunc(finalDict,myList2)
    if anyList=="three":
         return helperfunc(finalDict,myList3)

validation(number, validationHelper)

print(validationHelper.true)

Problem:

I am running this, but no matter if the string is in the list or not, I always get my printout for the else-statement. So, I guess I have an error in reasoning in my for-loop? Or maybe, I did not understand for-loops at all?! I have tried out different indentions with the else-statement, but couldnt solve my problem.

Answer 1

I would suggest you to change your function the following way (without changing the logic):

def validationHelper(myDict, myList):
    for key in myDict:
        for value in myDict[key][0]:
            for item in myList:
                if value==item:
                    return "String is valid" # Add here to exit
    return "Warning: String is NOT valid" # will be returned inf nothing will be found in your 3 loops

def validation(anyList,helperfunc):
    if anyList=="one":
        return helperfunc(finalDict,myList1)
    if anyList=="two":
         return helperfunc(finalDict,myList2)
    if anyList=="three":
         return helperfunc(finalDict,myList3)

validation(number, validationHelper)

print(validationHelper)

This will help you to exit your 3 nested loops as it was mentioned in comments.
Because in the negative case on first wrong occurrence you don't need to check anything else.

Answer 2

Use return to break all of your loop. Having an else statement is not necessary if you don't have any if statement to begin with.

def validationHelper(myDict, myList):
    for item in myList:
        if item in myDict.values():
            return ("String is valid")
    return ("String is NOT valid")

def validation(anyList,helperfunc):
    if anyList=="one":
        return helperfunc(finalDict,myList1)
    elif anyList=="two":
        return helperfunc(finalDict,myList2)
    elif anyList=="three":
        return helperfunc(finalDict,myList3)

validation(number, validationHelper)

print(validationHelper.true)

Using elif instead of multiple if is a better practice. Be careful with indentions next time.

Also you might want to check .keys() and .values()

You can replace:

for key in myDict:
    for value in myDict[key][0]:

with:

for value in myDict.values():

Answer 3

The other answers give a good explanation of how to break out of multiple loops. But you could also simplify your code by using Python's built-in functions and list comprehensions, like this:

def validationHelper(myDict, myList):
    if any(v in myList for val in myDict.values() for v in val[0]):
        validationHelper.true="String is valid"
    else:
        validationHelper.true="Warning: String is NOT valid"

def validation(anyList,helperfunc):
    if anyList=="one":
        return helperfunc(finalDict,myList1)
    if anyList=="two":
         return helperfunc(finalDict,myList2)
    if anyList=="three":
         return helperfunc(finalDict,myList3)

validation(number, validationHelper)

print(validationHelper.true)

This should be as efficient as your code, since any short circuits at the first match. And it may be a little more readable. (Note that multi-level list comprehensions go in the same order as regular for loops.)