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I have a somewhat stupid question which drives me nuts for a couple of days now. Heavy web search did not help either.

Within some larger SQLAlchemy application I want to check whether some value exists inside a list or not (before the SQL is executed). Therefore I have defined two functions: one which checks the existence of a value inside a list, and gives back True or False. Plus a second one which loops as long as the value is not in the list:

def check_value(itemlist, value_check):
    print(itemlist) #only for debugging reasons
    if value_check in itemlist:
        print('Item is already in list') #only for debugging reasons
        return False
    else:
        print('Item is not in list') #only for debugging reasons
        return True

def check_input(itemlist, value_check):
    while check_value(itemlist ,value_check)==False:
        value_check = input('Please input valid value')
    return value_check

now if I run this code lets say:

def check_value(itemlist, value_check):
    print(itemlist) #only for debugging reasons
    if value_check in itemlist:
        print('Item is already in list') #only for debugging reasons
        return False
    else:
        print('Item is not in list') #only for debugging reasons
        return True

def check_input(itemlist, value_check):
    while check_value(itemlist ,value_check)==False:
        value_check = input('Please input valid value')
    return value_check

if __name__ == "__main__":

        items = [1, 18272, 18279, 12, 298]
        value_check = 18272
        check_input(items, value_check)

IDLE gives me the correct answer at first:

[1, 18272, 18279, 12, 298]
Item is already in list
Please input valid value

Then I give it a number that is also in the list let's say "1" but it still tells me the value is not in the list:

[1, 18272, 18279, 12, 298]
Item is not in list
>>>

Its clear that this has been asked several times and I have found some help here already:How to check if a specific integer is in a list I tried this method x is in (see code above).

Also I found this thread: Finding the index of an item given a list containing it in Python which would be my workaround to check for index of a given value and write some try/except around it. Nevertheless I do not understand why x is y is not working here.

I have the feeling I am missing something very basic here but I am not able to grasp what.

BayerischerSchweitzer
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  • Use `value_check = int(input('Please input valid value'))` instead of `value_check = input('Please input valid value')` – tkhurana96 Jan 09 '18 at 22:28
  • You have a `list` of `int`egers and your `input` gives you a `str`ing. You are checking if the `str`ing `1` is in the `list` , and it is not, only an `int` of `1` is in - that is not the same thing. – Patrick Artner Jan 09 '18 at 22:34

1 Answers1

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Use value_check = int(input('Please input valid value')) instead of value_check = input('Please input valid value') input function returns a string and your list contains only numbers thereby making the while condition True i.e. item not in list

tkhurana96
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