Question

Basically this started with my problem that I had when trying to find if the index exist in a dict:

if collection[ key ]: # if exist
    #do this
else: # if no exist
    #do this

But when the index really doesn’t exist it throws me a KeyError.
So, reading the Python documentation. If the missing() is defined it will not throw the KeyError.

collection = {}
def collection.__missing__():
    return false

The above code on the terminal gives me:

ghelo@ghelo-Ubuntu:~/Music$ python __arrange__.py
  File "__arrange__.py", line 16
    def allArts.__missing__():
               ^
SyntaxError: invalid syntax

So, how to do this correctly?
Btw, I’ll be needing to use Python 2.7 on this. And is there a difference when running on Python 3?

Answer 1

This is how you do it:

if key in collection:

or, as suggested by @sdolan, you can use the .get method, which returs a default (optional second parameter) if it does not exist.

if collection.get(key, None):

If you want to use __missing__ you would apply it to a class that extends dict (in this case):

class collection(dict):

    def __missing__(self, key):
        print "Too bad, {key} does not exist".format(key=key)
        return None


d = collection()
d[1] = 'one'

print d[1]

if d[2]:
    print "Found it"

OUTPUT

one
Too bad, 2 does not exist