Why is the error handling not working for None input?

Question

def copy_list(t):
try:
    if type(t) is list:
        t_copy=[]
        n=len(t)
        i=0
        while i<n:
            t_copy.append(t[i])
            i+=1
        return t_copy
except TypeError:
        return "Not a list"

The problem says that I should write a function which takes a list of integers as an input and returns a copy of it. It should raise an exception if the input is not a list. I am unable to understand that why my code is unable to raise an exception if the value is not of the list type or when the input is None?

Because your `if` statement will only trigger on `t` being a list, none of the other statements will try to run on non lists, so you never hit any code that will raise an exemption. — Morgan Thrapp, Oct 27 '15 at 14:56
You're catching `TypeError`; which line in the `try` block do you expect to throw that exception? (Furthermore, since you're *catching* the exception, you're violating the stipulation that your function should *throw* an exception. Since the function is supposed to throw, you shouldn't catch errors there.) — Rob Kennedy, Oct 27 '15 at 14:57
So how can I modify my code in order to reach the except statement? — Varun Upadhyay, Oct 27 '15 at 14:57
This is not a good use of `try..except` . Just `if..else` should be sufficient. — karthikr, Oct 27 '15 at 14:57
@karthikr I used a if-else also but it is not able to check for None input — Varun Upadhyay, Oct 27 '15 at 15:06
[How to clone or copy a list in Python?](http://stackoverflow.com/q/2612802/33732) — Rob Kennedy, Oct 27 '15 at 15:07

score 0 · Answer 1 · answered Oct 27 '15 at 15:07

0

Your code doesn't raise an exception because you check whether the type of t is a list using if type(t) is list. When you provide None as an input, it doesn't pass the if statement and falls through so nothing is returned and no exception is raised.

You can remove the if statement to raise your exception. n=len(t) will trigger the exception, as you cannot get the length of None (TypeError: object of type 'NoneType' has no len() ), and "Not a list" will be returned.

try:
    t_copy=[]
    n=len(t)
    i=0
    while i<n:
        t_copy.append(t[i])
        i+=1
    return t_copy
except TypeError:
    return "Not a list"

answered Oct 27 '15 at 15:07

Robert Lacher

656
1
6
16

This code may still fail the test when passed a non-list, non-None value. – Rob Kennedy Oct 27 '15 at 15:10
Its still failing for None input – Varun Upadhyay Oct 27 '15 at 16:02
This does NOT fail the None test, although it will fail if passed a dict – Rolf of Saxony Oct 29 '15 at 06:20

Rolf of Saxony · Answer 2 · 2015-10-29T06:17:08.867

Just throw it into a for loop, the if type should catch anything else.

def copy_list(t):
    if type(t) is list:
        t_copy=[]
        for i in t:
            t_copy.append(i)
        return t_copy
    else:
        return "Not a list"
y = None
x = copy_list(y)
print x
y = "abc"
x = copy_list(y)
print x
y = [1,2,3,4,5,6,7,8,9]
x = copy_list(y)
print x

Or more succinctly:

def copy_list(t):
    if type(t) is list:
        t_copy = list(t)
        return t_copy
    else:
        return "Not a list"
y = ""
x = copy_list(y)
print x,"\t", type(y)
y = []
x = copy_list(y)
print x,"\t\t", type(y)
y = None
x = copy_list(y)
print x,"   ", type(y)
y = 10
x = copy_list(y)
print x,"   ", type(y)
y = "abc"
x = copy_list(y)
print x,"   ", type(y)
y = [1,2,3,4]
x = copy_list(y)
print x,"   ", type(y)
y = ["a",2,"b"]
x = copy_list(y)
print x,"   ", type(y)
y = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
x = copy_list(y)
print x,"   ", type(y)

Results in:

Not a list  <type 'str'>
[]      <type 'list'>
Not a list  <type 'NoneType'>
Not a list  <type 'int'>
Not a list  <type 'str'>
[1, 2, 3, 4]    <type 'list'>
['a', 2, 'b']   <type 'list'>
Not a list  <type 'dict'>

@Varun I strongly dispute that comment. See the printed results. — Rolf of Saxony, Oct 29 '15 at 06:19

score 0 · Accepted Answer · edited May 23 '17 at 12:23

0

The try/except block is used for gracefully handling exceptions that are thrown by the interpreter when an unexpected or illegal value is encountered, not for raising exceptions intentionally. For that you want the raise keyword. See this question: How to use "raise" keyword in Python

As a suggestion, your code could look something like this:

def copy_list(t):
    if isinstance(t, list):
        t_copy=[]
        n=len(t)
        i=0
        while i<n:
            t_copy.append(t[i])
            i+=1
        return t_copy
    else:
        raise Exception('Not a list')

Edit: I think you're also going to want the isinstance function, and I have edited the code accordingly. Info on that can be found here.

edited May 23 '17 at 12:23

Community

1
1

answered Oct 27 '15 at 16:06

McGlothlin

2,059
1
15
28

You're welcome! Consider accepting answers when they work for you, and welcome to the site! http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work – McGlothlin Oct 27 '15 at 16:25
In the interests of fairness, having commented on the other answers, this answer passes the tests with flying colours. – Rolf of Saxony Oct 29 '15 at 06:27

Why is the error handling not working for None input?

3 Answers3