How do I avoid KeyError when working with dictionaries?

Wigan Pier picture Wigan Pier · Sep 3, 2017 · Viewed 34.3k times · Source

Right now I'm trying to code an assembler but I keep getting this error:

Traceback (most recent call last):
  File "/Users/Douglas/Documents/NeWS.py", line 44, in 
    if item in registerTable[item]:
KeyError: 'LD'

I currently have this code:

functionTable = {"ADD":"00",
         "SUB":"01",
         "LD" :"10"}

registerTable = {"R0":"00",
         "R1":"00",
         "R2":"00",
         "R3":"00"}

accumulatorTable = {"A"  :"00",
            "B"  :"10",
            "A+B":"11"}

conditionTable = {"JH":"1"}

valueTable = {"0":"0000",
          "1":"0001",
          "2":"0010",
          "3":"0011",
          "4":"0100",
          "5":"0101",
          "6":"0110",
          "7":"0111",
          "8":"1000",
          "9":"1001",
          "10":"1010",
          "11":"1011",
          "12":"1100",
          "13":"1101",
          "14":"1110",
          "15":"1111"}

source = "LD R3 15"

newS = source.split(" ")

for item in newS:

        if item in functionTable[item]:
            functionField = functionTable[item]
        else:
            functionField = "00"

        if item in registerTable[item]:
            registerField = registerTable[item]
        else:
            registerField = "00"

print(functionField + registerField)

Help is appreciated.

Answer

MSeifert picture MSeifert · Sep 3, 2017

You generally use .get with a default

get(key[, default])

Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never raises a KeyError.

So when you use get the loop would look like this:

for item in newS:
    functionField = functionTable.get(item, "00")
    registerField = registerTable.get(item, "00")
    print(functionField + registerField)

which prints:

1000
0000
0000

If you want to do the explicit check if the key is in the dictionary you have to check if the key is in the dictionary (without indexing!).

For example:

if item in functionTable:   # checks if "item" is a *key* in the dict "functionTable"
    functionField = functionTable[item]  # store the *value* for the *key* "item"
else:
    functionField = "00"

But the get method makes the code shorter and faster, so I wouldn't actually use the latter approach. It was just to point out why your code failed.