Python insert operation on list

Question

I am newbie to Python and I have a doubt regarding insert operation on the list.

Example 1:

mylist = ['a','b','c','d','e']
mylist.insert(len(mylist),'f')
print(mylist)

Output:

['a', 'b', 'c', 'd', 'e', 'f']

Example 2:

mylist = ['a','b','c','d','e']
mylist.insert(10,'f')
print(mylist)

Output:

['a', 'b', 'c', 'd', 'e', 'f']

In second example why it still inserts 'f' element in the list even if I am giving index 10 to insert method?

What do you expect the other indexes to contain if it did put 'f' in the 10th index? — Volatility
– Volatility, Commented Aug 3, 2013 at 13:26
@Volatility a not unreasonable assumption would be an error of some sort — Jon Clements
– Jon Clements, Commented Aug 3, 2013 at 13:32
I was expecting error. Why it's working that way and not giving any error. — thatisvivek
– thatisvivek, Commented Aug 3, 2013 at 13:33
@vivekratnaparkhi If you really thing it is wrong, submit a Python Enhancement Proposal. Note: There are no more changes, ever, to existing interfaces in the Python 2.x family. — Eric Urban
– Eric Urban, Commented Aug 3, 2013 at 17:53

Jon Clements · Accepted Answer · 2013-08-03 13:30:23Z

4

The list.insert function will insert before the specified index. Since the list is not that long anyways in your example, it goes on the end. Why not just use list.append if you want to put stuff on the end anyways.

x = ['a']
x.append('b')
print x

Output is

['a', 'b']

edited Aug 3, 2013 at 13:30

Jon Clements

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answered Aug 3, 2013 at 13:25

Eric Urban

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3 Comments

Sukrit Kalra Over a year ago

@vivekratnaparkhi : This behaviour is clearly stated in the Python Docs. Quoting - a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

Eric Urban Over a year ago

@vivekratnaparkhi If you want the philisophical answer, read up on the principle of least astonishment.

SethMMorton Over a year ago

The OP is not talking about using insert to add to the end of a list per se... he expected that trying to insert at a position way past the end of the list might raise an error (probably IndexError). i.e, why should a.insert(len(a), x) be equivalent to a.insert(len(a)+1000000000, x). That, I'm guessing, is where the doubt still lies.

Pavel Anossov · Accepted Answer · 2013-08-03 13:42:29Z

2

The concept here is "insert before the element with this index". To be able to insert at the end, you have to allow the invalid off-the-end index. Since there are no well-defined "off-the-end iterators" or anything in Python, it makes more sense to just allow all indices than to allow one invalid one, but no others.

answered Aug 3, 2013 at 13:42

Pavel Anossov

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