Why is there a logical error with using the keyword 'and' in an if statement with dict?

Question

I have a empty dictionary which I assign keys from a list of letters and values from user input

valuesDict = {}
letters = ['S', 'u', 'v', 'a', 't']
i = 0
while i < 5:
    newValue = input('Enter ' + letters[i] + ' ')
    if newValue != '':
        valuesDict.update({letters[i]: newValue})
    i = i + 1

and a cutdown if statement to show my problem which prints a number corresponding to the items in the dictionary

if 'S' and 'v' not in valuesDict.keys():
    print('1')
elif 'u' and 'v' not in valuesDict.keys():
    print('2')

If I enter values for u, a, and t it outputs '1' correctly

Enter S
Enter u 2
Enter v
Enter a 5
Enter t 7
1

However, when I enter values for S, a, and t for the elif part of the statement '1' is outputted when it is meant to be '2'

Enter S 2
Enter u 
Enter v 
Enter a 5
Enter t 7
1

Why is this happening, and how can I fix this and avoid it in the future?

score 1 · Answer 1 · answered Dec 24 '17 at 00:27

1

The syntax if 'S' and 'v' not in valuesDict.keys() is not logically equivalent to if 'S' not in valuesDict.keys() and 'v' not in valuesDict.keys().

answered Dec 24 '17 at 00:27

Omni

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Correct. The logical expression was not valid. – pstatix Dec 24 '17 at 00:28

score 0 · Accepted Answer · edited Dec 24 '17 at 03:45

0

You get:

>>> 'S' and 'v'
'v'

and only check for v not S.

You need to check for both:

if ('S' not in valuesDict.keys()) and ('v' not in valuesDict.keys()):

Example:

>>> 'S' and 'v' not in 'xS'  # equivalent to: 'v' not in 'xS' 
True
>>> ('S' not in 'xS') and ('v' not in 'xS')
False

edited Dec 24 '17 at 03:45

Klaus D.

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answered Dec 24 '17 at 00:30

Mike Müller

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Why is there a logical error with using the keyword 'and' in an if statement with dict?

2 Answers2