Getting a default value on index out of range in Python [duplicate]

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Question :

Getting a default value on index out of range in Python [duplicate]
a=['123','2',4]
b=a[4] or 'sss'
print b

I want to get a default value when the list index is out of range (here: 'sss').

How can I do this?

Asked By: zjm1126

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Answer #1:

In the Python spirit of “ask for forgiveness, not permission”, here’s one way:

try:
    b = a[4]
except IndexError:
    b = 'sss'
Answered By: Thomas

Answer #2:

In the non-Python spirit of “ask for permission, not forgiveness”, here’s another way:

b = a[4] if len(a) > 4 else 'sss'
Answered By: Thomas

Answer #3:

In the Python spirit of beautiful is better than ugly

Code golf method, using slice and unpacking (not sure if this was valid 4 years ago, but it is in python 2.7 + 3.3)

b,=a[4:5] or ['sss']

Nicer than a wrapper function or try-catch IMHO, but intimidating for beginners. Personally I find tuple unpacking to be way sexier than list[#]

using slicing without unpacking:

b = a[4] if a[4:] else 'sss'

or, if you have to do this often, and don’t mind making a dictionary

d = dict(enumerate(a))
b=d.get(4,'sss')
Answered By: 00500005

Answer #4:

another way:

b = (a[4:]+['sss'])[0]
Answered By: maep

Answer #5:

You could create your own list-class:

class MyList(list):
    def get(self, index, default=None):
        return self[index] if len(self) > index else default

You can use it like this:

>>> l = MyList(['a', 'b', 'c'])
>>> l.get(1)
'b'
>>> l.get(9, 'no')
'no'
Answered By: gecco

Answer #6:

For a common case where you want the first element, you can do

next(iter([1, 2, 3]), None)

I use this to “unwrap” a list, possibly after filtering it.

next((x for x in [1, 3, 5] if x % 2 == 0), None)

or

cur.execute("SELECT field FROM table")
next(cur.fetchone(), None)
Answered By: Pepijn

Answer #7:

You could also define a little helper function for these cases:

def default(x, e, y):
    try:
        return x()
    except e:
        return y

It returns the return value of the function x, unless it raised an exception of type e; in that case, it returns the value y. Usage:

b = default(lambda: a[4], IndexError, 'sss')

Edit: Made it catch only one specified type of exception.

Suggestions for improvement are still welcome!

Answered By: Thomas

Answer #8:

try:
    b = a[4]
except IndexError:
    b = 'sss'

A cleaner way (only works if you’re using a dict):

b = a.get(4,"sss") # exact same thing as above

Here’s another way you might like (again, only for dicts):

b = a.setdefault(4,"sss") # if a[4] exists, returns that, otherwise sets a[4] to "sss" and returns "sss"
Answered By: Vlad the Impala

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