Handling a keyerror with a python dictionary

Question

How can I fix a KeyError in a python dictionary?

I was asked to create a dictionary and create a function number_dictionary(text, n). I'm able to return True or False if the text matches the n, but there's a KeyError when I put something that's not in the dictionary when it is supposed to return False.

I've put the two conditions together (when it does not match and not in the dictionary), but then it returns everything as False.

def number_dictionary(text,n):
    numbers={'two': '2', 'three': '3', 'four': '4'}
    if n != numbers[text] or n or text not in numbers:
        return(False)
    else:
        return(True)

See how to ask particularly mcve.

Open AI - Opting Out
– Open AI - Opting Out

2015-09-14 09:03:36 +00:00
Commented Sep 14, 2015 at 9:03 — Open AI - Opting Out
– Open AI - Opting Out, Commented Sep 14, 2015 at 9:03

Martijn Pieters · Accepted Answer · 2015-09-14 09:09:50Z

8

Three options:

Catch the exception:

try:
     foo = somedictionary[bar]
except KeyError:
     return False

Use the dict.get() method to return a default value instead, that default value defaults to None:
```
foo = somedictionary.get(bar)  # return None if bar is not a key
```

Test for the key seperately:

if bar not in somedictionary:
    return False

In your case you tested for the key after trying to access it. You could swap the test (dropping the or n):

def number_dictionary(text, n):
    numbers={'two': '2', 'three': '3', 'four': '4'}
    return text not in numbers or n != numbers[text]

Or just use dict.get(), the default None will never match a string n:

def number_dictionary(text, n):
    numbers = {'two': '2', 'three': '3', 'four': '4'}
    return numbers.get(text) == n

Both the not in and == comparison tests already produce either True or False, so there is no need to use if and separate return statements.

edited Sep 14, 2015 at 9:09

answered Sep 14, 2015 at 8:58

Martijn Pieters

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4 Comments

Ihor Pomaranskyy Over a year ago

I would put .get() the first and try .. except the last.

Martijn Pieters Over a year ago

@IgorPomaranskiy: Why would that be? Both are valid options, neither has a clear advantage in general. It depends on how frequently the key is missing which option is faster, for example. In other words: it depends on the specific code and the typical frequencies of the keys being probed.

ameliagunawan Over a year ago

Thank you so much!! It fixed the problem :) there's actually a test for a hidden input do you know what that is?

Martijn Pieters Over a year ago

@ameliagunawan: no idea what that might be; perhaps you could ask that in a new question? Don't forget to include a minimal reproducible example!

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