Syntax of Lists in Python

Question

I am learning python, now, i came across a code snippet which looks like this:

my_name={'sujit','amit','ajit','arijit'}
for i, names in enumerate(my_name):
    print "%s" %(names[i])

OUTPUT

s
m
i
t

But when I modify the code as:

my_name=['sujit','amit','ajit','arijit']
for i, names in enumerate(my_name):
    print "%s" %(names[i])

OUTPUT

s
m
i
j

What is the difference between {} and []? The [] is giving me the desired result for printing the ith character of the current name from the list. Bu the use of {} is not.

NPE · Accepted Answer · 2011-06-09 06:48:15Z

4

{} creates a set, whereas [] creates a list. The key differences are:

the list preserves the order, whereas the set does not;
the list preserves duplicates, whereas the set does not;
the list can be accessed through indexing (i.e. l[5]), whereas the set can not.

The first point holds the key to your puzzle. When you use a list, the loop iterates over the names in order. When you're using a set, the loop iterates over the elements in an unspecified order, which in my Python interpreter happens to be sujit, amit, arijit, ajit.

P.S. {} can also be used to create a dictionary: {'a':1, 'b':2, 'c':3}.

edited Jun 9, 2011 at 6:48

answered Jun 9, 2011 at 6:36

NPE

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2 Comments

Sujit Agarwal Over a year ago

But could you please explain the internal logic behind the last line of the first output set? i mean the t

mgiuca Over a year ago

It's the 't' from "ajit". Since sets don't preserve order, you are getting a random order out of enumerate. I would guess that the order coming out is ['sujit','amit','arijit','ajit']. That would make the first letter the 's' of 'sujit', the second letter the 'm' of 'amit', the third the 'i' of 'arijit' and the fourth the 't' of 'ajit'.

mgiuca · Accepted Answer · 2011-06-09 06:38:02Z

The {} notation is set notation rather than list notation. That is basically the same as a list, but the items are stored in a jumbled up order, and duplicate elements are removed. (To make things even more confusing, {} is also dictionary syntax, but only when you use colons to separate keys and values -- the way you are using it, is a set.)

Secondly, you aren't using enumerate properly. (Or maybe you are, but I'm not sure...)

enumerate gives you corresponding index and value pairs. So enumerate(['sujit','amit','ajit','arijit']) gives you:

[(0, 'sujit'), (1, 'amit'), (2, 'ajit'), (3, 'arijit')]

So this will get you the first letter of "sujit", the second letter of "amit", and so on. Is that what you wanted?

(And it will crash as soon as there is a word in the list whose index is greater than or equal to its length, which is why I said I don't think it's being used properly.)
Yes. I tried to modify the last element arijit to art and the code crashed.

Karl Knechtel · Accepted Answer · 2011-06-09 06:43:17Z

{} do not enclose a list. They do not enclose any kind of sequence; they enclose (when used this way) a set (in the mathematical sense). The elements of a set do not have a specified order, so you get them enumerated in whatever order Python put them in. (It does this so that it can efficiently ensure the other important constraint on sets: they cannot contain a duplicate value).

This is specific to Python 3. In 2.x, {} cannot be used to create a set, but only to create a dict. (This also works in Python 3.) To do this, you specify the key-value pairs separated by colons, thus: {'sujit': 'amit', 'ajit': 'arijit'}.

(Also, a general note: if you say "question" instead everywhere that you currently say "doubt", you will be wrong much less often, at least per the standards of English as spoken outside of India. I don't particularly understand how the overuse of 'doubt' has become so common in English as spoken by those from India, but I've seen it in many places across the Internet...)

Mark Tolonen · Accepted Answer · 2011-06-09 06:44:29Z

0

sets do not preserve order:

[] is a list:

>>> print ['sujit','amit','ajit','arijit']
['sujit', 'amit', 'ajit', 'arijit']

{} is a set:

>>> print {'sujit','amit','ajit','arijit'}
set(['sujit', 'amit', 'arijit', 'ajit'])

So you get s,m,i,j in the first case; s,m,i,t in the second.

answered Jun 9, 2011 at 6:44

Mark Tolonen

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