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pythondictionarytry-catch
PiccolMan
PiccolMan

Reputation: 5366

Python - How to use try/except statement in dictionary

I was trying to do this, but It didn't work. Just to clarify I want value to equal list[0] if it exists. Thanks.

    dictionary = {
    try:
        value : list[0],
    except IndexError:
        value = None
    }

Upvotes: 0

Views: 2407

Answers (2)

Martijn Pieters
Martijn Pieters

Reputation: 1121834

You'd have to put the try..exept around the assigment; you cannot put it inside an expression like you did:

try:
    dictionary = {value: list[0]}
except IndexError:
    dictionary = {value: None}

Alternatively, move the assignment to a separate set of statements:

dictionary = {value: None}
try:
    dictionary[value] = list[0]
except IndexError:
    pass

or explicitly test for the length of list so you can just select None with a conditional expression:

dictionary = {
    value: list[0] if list else None,
}

where the if list test is true if the list object is not empty.

You could also use the itertools.izip_longest() function (itertools.zip_longest() in Python 3) to pair up keys and values; it'll neatly cut off at the shortest sequence, and fill in None values for the missing elements:

from itertools import izip_longest
dictionary = dict(izip_longest(('key1', 'key2', 'key3'), list_of_values[:3]))

Here, if list_of_values does not have 3 values, then their matching keys are set to None automatically:

>>> from itertools import izip_longest
>>> list_of_values = []
>>> dict(izip_longest(('key1', 'key2', 'key3'), list_of_values[:3]))
{'key3': None, 'key2': None, 'key1': None}
>>> list_of_values = ['foo']
>>> dict(izip_longest(('key1', 'key2', 'key3'), list_of_values[:3]))
{'key3': None, 'key2': None, 'key1': 'foo'}
>>> list_of_values = ['foo', 'bar']
>>> dict(izip_longest(('key1', 'key2', 'key3'), list_of_values[:3]))
{'key3': None, 'key2': 'bar', 'key1': 'foo'}
>>> list_of_values = ['foo', 'bar', 'baz']
>>> dict(izip_longest(('key1', 'key2', 'key3'), list_of_values[:3]))
{'key3': 'baz', 'key2': 'bar', 'key1': 'foo'}

Upvotes: 6

blasko
blasko

Reputation: 690

You can actually use the 'in' keyword to see if something exists as a key in a dictionary

if list[0] in dictionary:
    value = list[0]
else:
    value = None

Just a note, avoid using 'list' as a variable name.

Here's what you're trying to do I'm assuming:

new_dictionary = dict()
if list[0] in dictionary:
    new_dictionary['value'] = list[0]
else:
    new_dictioanry['value'] = None

Upvotes: 0

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