Ignore IndexError in python (Something like null coalescing operator on list subscription)

Question

Is there a pythonic way to make python ignore IndexError and just return a default value or None when I access a list/array with an out-of-range index?

import sys
input = sys.argv[1]
output = sys.argv[2]

This may cause IndexError when the program is run with no more than 2 parameters. However if I want to say that there are default values for argv[1] and argv[2], then I should write like:

import sys
input = len(sys.argv) > 1 ? sys.argv[1] : DefaultValue1
output = len(sys.argv) > 2 ? sys.argv[2] : DefaultValue2

Is there any pythonic way to shorten this statements exept try?

(like name = input or "Not set" for pythonic null-colaescing operator)

Related post? (using or operator)

using [`argparse`](https://docs.python.org/3/library/argparse.html) module? — Azat Ibrakov, May 23 '19 at 07:20
or how about making a `dict` from `sys.argv` list and using `dict.get` method? — Azat Ibrakov, May 23 '19 at 07:22
A more high-level alternative to argparse would be [click](https://click.palletsprojects.com/en/7.x/quickstart/). — Klaus D., May 23 '19 at 07:24
@AzatIbrakov Thank you for introducing me that wonderful module! But what should I do if I want to solve this problem without argparse? Should I create a dictionary for the lists? — KYHSGeekCode, May 23 '19 at 07:24
@KlausD. I am surprised that there are something like annotations in python too. — KYHSGeekCode, May 23 '19 at 07:27
There are annotations, but they are something else. These are decorators, which can modify a class, method or function. — Klaus D., May 23 '19 at 07:33

score 1 · Accepted Answer · answered May 23 '19 at 08:36

1

Unsure whether this is really pythonic, but you could extend sys.argv with an array containing None and slice it. Then you just have to test the values against None:

input, output = (sys.argv + [None] * 2)[:2]
if input is None: input = defaultValue1
if output is None: output = defaultValue2

Or in a one liner:

defaultValues = [defaultValue1, defaultValue2]
input, output = (val if val is not None else defaultValues[i]
                 for i, val in enumerate((sys.argv + [None] * 2)[:2]))

Close to a matter of taste...

answered May 23 '19 at 08:36

Serge Ballesta

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And that may be used in other applications? :) – KYHSGeekCode May 23 '19 at 12:11
1

For a serious application, `argparse` or any other true command line parsing tool will be a must. You asked for a possible short implementation, I just proposed one :-) – Serge Ballesta May 23 '19 at 12:20

Gokhan Gerdan · Answer 2 · 2019-05-23T08:09:47.630

0

Set your deafult values in default_val_a and default_val_b and set your index in index a is your sys.arg[1] and b is your sys.arg[2]. This is the most pythonic and single line way that i can come up:

import sys

a = [1]
b = []

default_val_a = [1, 2, 3]
default_val_b = [4, 5, 6]

index = 1

a = default_val_a[index] if len(a) <= index or not a else a[index]
b = default_val_b[index] if len(b) <= index or not b else b[index]

print(a, b)

And the structure is like that:

VAR = DEFAULT_VALUE[INDEX] if len(VAR) <= INDEX or not VAR else VAR[INDEX]

edited May 23 '19 at 08:09

answered May 23 '19 at 07:33

Gokhan Gerdan

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6
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I am not worried about `TypeError`, but I' worried about `IndexOutOfBoundsException` or `IndexError`. – KYHSGeekCode May 23 '19 at 07:57

Ignore IndexError in python (Something like null coalescing operator on list subscription)

2 Answers2