python - How do I avoid KeyError when working with dictionaries?

Question

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 1

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.