KeyError python

Question

I wrote a program that shifts letters in a string and returns a dictionary. It works, unless the letter 'w' is within the string. Does anyone know why this happens? This is my code:

def buildCoder(shift):
    import string
    impStr = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
    resultDict = {}

    lowerValues = dict()
    for index, letter in enumerate(string.ascii_lowercase):
        lowerValues[letter] = index + 1

    upperValues = dict()
    for index, letter in enumerate(string.ascii_uppercase):
        upperValues[letter] = index + 1

    inv_lowerValues = {v:k for k, v in lowerValues.items()}

    inv_upperValues = {v:k for k, v in upperValues.items()}

    for char in impStr:
        if char in string.ascii_lowercase:
            value = lowerValues[char]
            value += shift
            if value <= 25:
                resultDict[char] = inv_lowerValues[value]
            else:
                value = abs(26-value)
                resultDict[char] = inv_lowerValues[value]
        elif char in string.ascii_uppercase:
            value = upperValues[char]
            value += shift
            if value <= 25:
                resultDict[char] = inv_upperValues[value]
            else:
                value = abs(26-value)
                resultDict[char] = inv_upperValues[value]
    return resultDict

This is the error I get (if 'w' is included in impStr):

KeyError                                  Traceback (most recent call last)
<ipython-input-117-0f4ac72e3c63> in <module>()
----> 1 buildCoder(3)

/var/folders/CV/CVkEwo3TGuu4+2cStWCRyE+++TQ/-Tmp-/tmpMr_sV1.py in buildCoder(shift)

KeyError: 0

I also saw this error:

Traceback (most recent call last):
  File "submission.py", line 24, in buildCoder
    resultDict[char] = inv_lowerValues[value]
KeyError: 0

*** ERROR: Expected to find a dictionary in the line:
Traceback (most recent call last):
  File "submission.py", line 24, in buildCoder
    resultDict[char] = inv_lowerValues[value]
KeyError: 0

 ***

It's trying to look up 0, not w. My guess is value = abs(26-value) gets triggered if value == 26, which means it's checking inv_lowerValues[0] which doesn't exist. — Adam Smith
– Adam Smith, Commented Jul 30, 2014 at 18:25

Adam Smith · Accepted Answer · 2014-07-30 18:46:21Z

2

Let's use a bit of introspection on your code:

lowerValues['w'] == 23
inv_lowerValues[23] == 'w'

# presumably shift == 3

for char in impStr: # when char==w
    if char in string.ascii_lowercase: # it is
        value += shift # 23 + 3 == 26
        if values <= 25 # it's not
        else:
            value = abs(26-value)
            # abs( 26 - 26 ) == 0
            resultDict[char] = inv_lowerValues[value]
            # resultDict['w'] = inv_lowerValues[0]; KeyError

It sounds like you just want to be sure that your shift wraps around the alphabet. That's not abs(26-value), that's (value % 25)+1.

edited Jul 30, 2014 at 18:46

answered Jul 30, 2014 at 18:40

Adam Smith

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Comments

Daniel · Accepted Answer · 2014-07-30 18:33:53Z

1

I think, you want to write

value = abs(value-25)

instead of

value = abs(26-value)

otherwise you would get 0 as value.

edited Jul 30, 2014 at 18:33

answered Jul 30, 2014 at 18:28

Daniel

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3 Comments

Erik Over a year ago

Absolute values are always positive...except when it's zero.

Adam Smith Over a year ago

Aren't we wrapping, here? Why abs(26-value) instead of (value % 25)+1 (and, better question, why are we not using 0 as a key anyway?

Padraic Cunningham Over a year ago

a better point is that the code does not return what it should even with this fix

Padraic Cunningham · Accepted Answer · 2014-07-30 20:16:14Z

0

A more concise version of what you are doing:

import string

def buildCoder(shift):
    alpha = string.ascii_lowercase
    ALPHA = string.ascii_uppercase
    if 0 <= shift < 26:
        unshifted_letters = ALPHA + alpha
        shifted_letters = ALPHA[shift:] + ALPHA[:shift] + alpha[shift:] + alpha[:shift]
        return dict(zip(unshifted_letters, shifted_letters))

answered Jul 30, 2014 at 20:16

Padraic Cunningham

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