Try/except if an element of a list exists

Question

The problem

I have the following list in Python 3.6

Piko = {}
Piko['Name']='Luke'

I am trying to write a function that give the value of the element if it exist and is set and give None otherwise. For example:

INPUT: isset(Piko['Name']) OUTPUT: Luke
INPUT: isset(Piko['Surname']) OUTPUT: None

What I have tried

1st try; based on my know how:

def isset1(x):
    try:
        x
    except KeyError:
        print(None)
    else:
        print(x)

2nd try; based on this answer:

def isset2(x):
    try:
        t=x
    except IndexError:
        print(None)

3rd try; based on this answer:

def isset3(x):
    try:
        x
    except Exception:
        print(None)
    else:
        print(x)

Any one of the previous gives me KeyError: 'Surname' error and does not output None as I wanted. Can anybody help me explaining how could I manage correctly the KeyError?

the arguments are evaluated _before_ entering the function so it's just not possible. Using `get` method seems to do exactly what you want. — Jean-François Fabre, Mar 15 '18 at 21:15
For reference: python dictionary [get](https://www.tutorialspoint.com/python/dictionary_get.htm) method — Chad Lewis, Mar 15 '18 at 21:16

score 4 · Accepted Answer · edited Jun 20 '20 at 09:12

4

Piko.get('Surname')

Piko.get('Surname', None)

are identical and return None since "Surname" is not in your dictionary.

For future reference you can quickly discover this from the Python shell (eg ipython) by typing:

In[4]: help(Piku.get)

Which produces:

Help on built-in function get:

get(...)

D.get(k[,d]) -> D[k] if k in D, else d. d defaults to None.

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 15 '18 at 21:17

John

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score 3 · Answer 2 · answered Mar 15 '18 at 21:19

The exception is happening before it even gets into your isset function. When you do this:

isset(Piko['Name'])

… it's basically the same as doing this:

_tmp = Piko['Name']
isset(_tmp)

No matter what code you put inside isset, it's not going to help, because that function never gets called. The only place you can put the exception handling is one level up, in the function that calls isset.

Or, alternatively, you can not try to lookup dict[key] to pass into isset, and pass the dict and the key as separate parameters:

def isset(d, key):
    try:
        print(d[key])
    except KeyError:
        print(None)

But at this point, you're just duplicating dict.get in a clumsier way. You can do this:

def isset(d, key):
    print(d.get(key, None))

… or just scrap isset and do this:

print(Piko.get('Name', None))

Or, since None is the default if you don't specify anything:

print(Piko.get('Name'))

Try/except if an element of a list exists

2 Answers2