What is the difference between implementation of list() and [] in py3.x?

Question

Here is the sample code1 and its result.

d ={
    "name":"looser",
    "roll":6666,
    "profile" : "developer"
}

print("list formatted dict is {}".format([d]))

for k,v in [d.items()]:
    if d[k] is "developer":
        d.pop(k)
    else:
        print(k)

result:

list formatted dict is [{'name': 'looser', 'roll': 6666, 'profile': 'developer'}]
Traceback (most recent call last):
  File "/Users/rough/try.py", line 18, in <module>
    for k,v in [d.items()]:
ValueError: too many values to unpack (expected 2)

Process finished with exit code 1

Here is the sample code2 with modified form.

d ={
    "name":"looser",
    "roll":6666,
    "profile" : "developer"
}

print("list formatted dict is {}".format([d]))

for k,v in list(d.items()):
    if d[k] is "developer":
        d.pop(k)
    else:
        print(k)

result:

list formatted dict is [{'name': 'looser', 'roll': 6666, 'profile': 'developer'}]
name
roll

Process finished with exit code 0

On many places people tell me there is no difference, is it true there is no difference.If there is no difference then why such result.

Because one is solving my problem of **RuntimeError: dictionary changed size during iteration **

forcing iteration to `list` isn't the same thing as putting one item (whatever it is) in a `list`. — Jean-François Fabre, Dec 21 '17 at 13:04
Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackoverflow.com/rooms/161729/discussion-on-question-by-theexorcist-what-is-the-difference-between-implementat). — deceze, Dec 21 '17 at 13:33

score 4 · Answer 1 · edited Jun 20 '20 at 09:12

there's almost no difference between:

a = list()

and

a = []

except for the time taken for the name lookup vs built-in [] (Why is [] faster than list()?)

but if you pass any item to list, list tries to iterate on it to build a new list.

From help(list) output:

class list(object)

| list() -> new empty list

| list(iterable) -> new list initialized from iterable's items

So there are 2 "modes" when creating a list object.

Try list(3) you'll get TypeError: 'int' object is not iterable error, try [3] you'll get a list with 1 element in it.

That's no different when passing your d.items() object.

list(d.items()) iterates/converts the items to a list of tuples.
[d.items()] creates a list with a single element inside: d.items() (which explains that unpacking to 2 elements fails)

Also note that for k,v in list(d.items()): is not necessary unless you want to remove items from your dictionary while iterating on it (which seems to be the problem you're having, seeing your recent edit)

Apart from that case, no need to turn to a list as d.items() is already iterable and wrapping that in a list does nothing useful (and it's even slower)

score 1 · Answer 2 · answered Dec 21 '17 at 13:07

1

[] is a literal which will create the list exactly as you define it. ['a'] is a list with one item. [d.items()] is also a list with one item. That one item is many items though which fail to unpack into just k, v.

list() is the constructor form which takes an iterable to create a new list. list('a') equals ['a']. list('ab') equals ['a', 'b']; it iterates over the iterable string 'ab'. list(d.items()) iterates over all items in d.items() and creates the list [('name', 'looser'), (...)]; that is a list with many two-item tuples, which individually unpack into k, v.

answered Dec 21 '17 at 13:07

deceze

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still my question doesn't get answered, so its not same in some way or I am in dilemma. – TheExorcist Dec 21 '17 at 13:13
If you understood what I wrote, then your question should be answered. `[d.items()]` is a list with one item; `list(d.items())` is a list with many tuples. So, yes, they're different. – deceze Dec 21 '17 at 13:16
so it looks same to noobies but on granular way of implementation it works differently.Can I agree on that. – TheExorcist Dec 21 '17 at 13:21
That's a weird way of characterising it. They're simply used differently. Again, `[]` creates the list *exactly as you write it.* It's a *list literal*. `list()` on the other hand is a function that takes exactly one argument; you can't write `list('a', 'b')` for example. That one argument must be an *iterable*. `list()` is a function to turn any *iterable* into a list. So, even superficially, they're not used the same way. – deceze Dec 21 '17 at 13:23

What is the difference between implementation of list() and [] in py3.x?

2 Answers2