Why does [:1] work in a for loop but not [0] in python?

Question

I just had a quick question about the difference between [:1] and [0]

The following program parses a text document and puts all instances of email addresses in said txt document in a dictionary, and then creates a list of tuples from this dictionary, then sorts the list and prints out the email with the most instances. Please read the last 2 lines of code on this program:

emails = dict()
fname = input('Enter file name: ')
try:
    fhand = open(fname)
except:
    print("sorry can't do that")
    quit()
for line in fhand:
    if not line.startswith("From "):
        continue
    line = line.split()
    emails[line[1]] = emails.get(line[1], 0) + 1
tmp = list()
for k, v in emails.items():
    newtup = (v, k)
    tmp.append(newtup)
tmp = sorted(tmp, reverse = True)
for v, k in tmp[:1]:
    print(k, v)

Specifically, the last 2 lines of this program:

for v, k in tmp[:1]:
        print(k, v)

Works perfectly. But when I tried to do the same thing with this syntax:

for v, k in tmp[0]:
        print(k, v)

I get the following traceback:

TypeError: cannot unpack non-iterable int object

I guess that [0] is in some way, not the same as [:1] ? Why does tmp[:1] work but tmp[0] doesn't in a for loop?

I have no complaints since my program is running perfectly with the tmp[:1] syntax, but why doesn't it work with the tmp[0] syntax? Are they not the same thing?

Thanks for your time and for reading this if you've come this far!

Answer 1

Emmm, here is an example.

>> tmp = [1, 2, 3]                                                                                                                                                                                    
>> tmp[:1]                                                                                                                                                                                            
[1]
>> tmp[0]                                                                                                                                                                                             
1
>> type([1])                                                                                                                                                                                          
list
>> type(1)                                                                                                                                                                                            
int

As you see, [:1] and [0] produces different stuff. [:1] makes list, [0] returns first element. Why you didn't try it just yourself?

Also made an example just for you:)

>> tmp = [(1,1),(2,2),(3,3)]

>> tmp[:1]
[(1, 1)]

>> tmp[0]
(1, 1)

>> for k,v in [(1, 1)]: pass

>> for k,v in (1, 1): pass
TypeError: cannot unpack non-iterable int object

Iteration like this does not make sence at all: for k,v in (1, 1): .

Answer 2

You need to read about Python list indexing and slicing, as that is the difference between the two.

tmp[:1]:
This is list slicing syntax. This general form of this syntax is like this,

list_object[start_idx: end_index]

This returns a sliced list with elements starting from(including) start_idx and ending with end_idx(not inclusive, so one less than this value).

So in your question tmp[:1] returns a sublist(which is considered an iterable ) with elements at 0. Since this is an iterable it is acceptable to iterate over in a for loop.

tmp[0]:
This is list indexing syntax. The general form is,

list_object[element_idex]

This returns an element at that index. The returned value may/may not be an iterable (it depends on what the list is composed of). In your case, tmp[0] returns the first element of the list.

Answer 3

Ok. Makes sense.

>>> tmp = [1, 2, 3]
>>> tmp[0]
1
>>> tmp[:1]
[1]

where tmp[0] could be any object, like something that can't be iterated through, like in my case, a tuple.