如何检查用户输入是否在字典中

Question

how do I check if a value that the user has given is in a dictionary of randomly generated numbers and that the key and value match?

card_numbers = {}

def create_card():
    first_half = 400000
    second_half = random_digits_gen(10)
    card_num = int(str(first_half) + str(second_half))
    pin_num = random_digits_gen(4)
    store_nums(card_num, pin_num)
    print('Your card has been created\n'
          'Your card number:\n'
          f'{card_num}\n'
          'Your card PIN:\n'
          f'{pin_num}')


def store_nums(number, pin):
    card_numbers[number] = pin


def check_nums():
    card_num = input('Enter your card number:\n')
    card_pin = input('Enter your pin:\n')
    if card_num in card_numbers.keys() and card_pin in card_numbers.values():
        print('Success!')
    else:
        print('nope')

create_card()
check_nums()

What is the right way to check for this?

Answer 1

You have to first check that the card_num input by the user exists in the card_numbers . In case of existence, check that the values does match. Your function should look like this:

def check_nums():
    card_num = int(input('Enter your card number:\n'))
    card_pin = int(input('Enter your pin:\n'))

    stored_card_pin = card_numbers.get(card_num)  # does return None if key is not found
    if stored_card_pin is not None:
        print('Success!' if stored_card_pin == card_pin else 'nope')

Answer 2

Looks like you are checking if the card number and the card pin are part of the dictionary. Also as @goalie1998 mentioned, you need to convert the input into integers.

The way your code is currently setup, if you have the following examples:

card_numbers{100:20,200:40}

and your inputs are 100 and 40 , your code will be True.

Let's examine it.

The first part of the if statement

if card_num in card_numbers.keys() will be True as 100 is a key.

It will then check the second part:

and card_pin in card_numbers.values()

This will also be True as 40 is one of the values in the card_numbers dictionary. However, 40 is not part of 100 .

The correct way to check this will be as shown below.

if card_num in card_numbers and card_pin in card_numbers[card_num]:

The way python checks is from left to right. So it goes through the first if statement ( if card_num in card_numbers ). If this is True , then it checks if card_pin is in card_numbers[card_num] .

If card_num is not in card_numbers , it exits the if statement. So it never goes to check if card_pin is in card_numbers .

By keeping the correct order of the if statement, you can avoid error handling situations.

The updated code will be as follows:

def check_nums():
    while True:
        try:
            card_num = int(input('Enter your card number:\n'))
            card_pin = int(input('Enter your pin:\n'))
            break
        except:
            print('input not a number. retry')
    if card_num in card_numbers and card_pin in card_numbers[card_num]:
        print('Success!')
    else:
        print('nope')

Answer 3

You need to first check that card_num exists in the dictionary and then compare the card_pin against that card number:

def check_nums():
    card_num = input('Enter your card number:\n')
    card_pin = input('Enter your pin:\n')
    if card_num in card_numbers and card_pin == card_numbers[card_num]:
        print('Success!')
    else:
        print('nope')

Answer 4

1/ you have to chose how you store your card numbers and PIN. As leading zero can be important I recommend to store them as strings. Otherwise your pin number or card number first and second half could not stard with 0, without being shortened.

2/ second your card_numbers variable should be made global in each function, so the same variable is used in the whole piece of code.

3/ you have to change the pin comparison part like this: card_numbers[card_num] == card_pin

Here is the whole code, I just replaced random_digits_gen calls by hardcoded numbers to make things easier for testing. Here the card number is 4020 and pin is 1111.

card_numbers = {}

def create_card():
    first_half = "40"
    second_half = "20" ## random_digits_gen(10)
    card_num = str(first_half) + str(second_half)
    pin_num = "1111" ## random_digits_gen(4)
    store_nums(card_num, pin_num)
    print('Your card has been created\n'
          'Your card number:\n'
          f'{card_num}\n'
          'Your card PIN:\n'
          f'{pin_num}')


def store_nums(number, pin):
    global card_numbers
    card_numbers[number] = pin


def check_nums():
    global card_numbers
    card_num = input('Enter your card number:\n')
    card_pin = input('Enter your pin:\n')
    if card_num in card_numbers.keys() and card_numbers[card_num] == card_pin:
        print('Success!')
    else:
        print('nope')

create_card()
check_nums()

Output is:

Enter your card number:
4020

Enter your pin:
1111
Success!