Re: [Tutor] Adding to a dict through a for loop confusion.

[Steven D'Aprano]

On Tue, May 17, 2016 at 06:56:40PM -0400, Chris Kavanagh wrote:
> Thank you so much for the help, and the example!
> 
> So, by putting quotes around a dict key, like so dict["key"] or in my case
> cart["item"] this makes the dict have ONE key. The loop assigns the
> cart_items to this ONE key until the end of the loop, and I'm left with
> {'item': 5}. . .

Chris, you're still thinking about this the wrong way. Let's look at a 
slightly different example which hopefully will give you some better 
perspective. I'm going to use a list, not a dict.


I hope it is obvious what is going on here:

py> mylist = [100, 200, 300]
py> for i in (0, 1, 2):
... mylist[i] = 1000 + i
...
py> mylist
[1000, 1001, 1002]

If we "unroll" the for-loop and turn it into individual lines of code, 
we're effectively executing this:

i = 0
mylist[i] = 1000 + i  # here, i==0 so mylist[0] = 1000
i = 1
mylist[i] = 1000 + i  # here, i==1 so mylist[1] = 1001
i = 2
mylist[i] = 1000 + i  # here, i==2 so mylist[2] = 1002


Any questions?

Now, let's look at this one instead:

py> mylist = [100, 200, 300]
py> for i in (0, 1, 2):
... mylist[0] = 1000 + i
...
py> mylist
[1004, 200, 300]

Let's unroll the for-loop:

i = 0
mylist[0] = 1000 + i
i = 1
mylist[0] = 1000 + i
i = 2
mylist[0] = 1000 + i


So far, all of this should hopefully be obvious. If you keep assigning 
to mylist[0] each time, only the first item of the list will be changed. 
In the first case, you assigned to a different item each time through 
the loop, because you wrote mylist[i] and i changed each time. In the 
second case, you assigned to mylist[0] each time.

There's nothing fancy or tricky going on here. The difference is 100% 
and entirely because of the difference between writing the VARIABLE i 
and writing the CONSTANT 0.

And it is the same with dict keys. If you loop over the dict:

cart_items = ['one', 'two', 'three']
for item in cart_items:
cart[item] = item

you are assigning to a different key each time, because you have written 
the VARIABLE item, which changes each time through the loop. If we 
unroll the loop again, we see:

item = 'one'
cart[item] = item
item = 'two'
cart[item] = item
item = 'three'
cart[item] = item

But when you do this:

for item in cart_items:
cart['item'] = something

you have written the CONSTANT 'item', which is always exactly the same 
thing: a four letter word starting with 'i', ending with 'm', with 'te' 
in the middle. It is nothing but an unfortunately coincidence that the 
value of this constant, 'item', matches the name of the variable item. 
Let's get rid of the coincidence:

for item in cart_items:
cart['word'] = something

Unrolling the loop gives us:

item = 'one'
cart['word'] = item
item = 'two'
cart['word'] = item
item = 'three'
cart['word'] = item


And now hopefully it is clear what is going on! If you write this:

cart = {}
cart['word'] = 'one'
cart['word'] = 'two'
cart['word'] = 'three'

it should be clear why the end result is {'word': 'three'} rather than 
{'one': 'one', 'two': 'two', 'three': 'three'}.



-- 
Steve
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Re: [Tutor] Adding to a dict through a for loop confusion.

[Alan Gauld via Tutor]

On 17/05/16 23:56, Chris Kavanagh wrote:

> So, by putting quotes around a dict key, like so dict["key"] or in my case
> cart["item"] this makes the dict have ONE key. The loop assigns the
> cart_items to this ONE key until the end of the loop, and I'm left with
> {'item': 5}. . .
> 
> Where as if you do NOT put the key in quotes, dict[key] or cart[item], this
> basically means the dict has as many keys as you're iterating through. In
> other words it assigns the cart_item as a key and a value, and saves them
> all to the dict.
> 
> Is that correct?

Sort of, but not for the reasons you give.
Putting the key in quotes makes it a literal string.
That is, a fixed value, just like 2 or 5.5 or True.
These are literal integer, float and boolean values
respectively.

So you could just as well have done:

for item in cart_items:
cart[2] = item

And everything would be stored under a key of 2.

Or

for item in cart_items:
cart[True] = item

And everything would get stored on a key of True.

The quotes turn the sequence of characters 'i','t','e','m'
into the string value 'item'. The fact that the string is
the same as the variable name in your for loop is completely
coincidental. Python doesn't recognise them as in any way
related.

So in the first case the issue is that you stored your
values in a single unchanging key.

Whereas in the second loop you stored your values against
a key which was a variable that changed in each iteration.

The fact you used quotes is only significant in that quotes
are what you use to create a literal string value. But it
was the fact that it was a fixed value that made everything
appear in the same place (and hence be overwritten), not the
quotes per se.


-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] Adding to a dict through a for loop confusion.

[Chris Kavanagh]

Thank you so much for the help, and the example!

So, by putting quotes around a dict key, like so dict["key"] or in my case
cart["item"] this makes the dict have ONE key. The loop assigns the
cart_items to this ONE key until the end of the loop, and I'm left with
{'item': 5}. . .

Where as if you do NOT put the key in quotes, dict[key] or cart[item], this
basically means the dict has as many keys as you're iterating through. In
other words it assigns the cart_item as a key and a value, and saves them
all to the dict.

Is that correct?

On Tue, May 17, 2016 at 5:01 AM,  wrote:

> On 17May2016 04:28, Chris Kavanagh  wrote:
>
>> Could someone tell me why this different behavior occurs between these 2
>> code snippets, please. The 1st example has quotes around it ['item'] only
>> adds the last item to the dict (cart). In the 2nd example the item does
>> not
>> have quotes around it [item] and every entry is added to the dict.
>>
>> Why?
>>
> [...]
>
>> # Example #1
>> cart_items = ['1','2','3','4','5']
>> cart = {}
>> for item in cart_items:
>>cart['item'] = item
>>
>
> This for loop assigns the values '1','2','3','4','5' in succession to the
> variable named "item". Then the body of the loop assigns that value (via
> the variable "item") to the single dictionary slot with the fixed key with
> string value 'item' i.e. always the same slot.  And the last item is the
> one kept.  All the earlier assignments are overwritten by the later ones:
> they happen, but are replaced.
>
> print cart
>> #output
>> {'item': 5}
>>
>
> Which you see above.
>
> # Example #2
>> cart_items = ['1','2','3','4','5']
>> cart = {}
>> for item in cart_items:
>>cart[item] = item
>>
>
> Here, the variable named "item" takes on the values as before, but the
> diction slot chosen also comes form that variable. So each value ends up in
> its own slot as your output shows.
>
> print cart
>> # output
>> {'1': '1', '3': '3', '2': '2', '5': '5', '4': '4'}
>>
>
> The essential difference here is that in the first example the expression
> for the slot in the dictionary is the expression:
>
>  'item'
>
> which is simply the fixed string 'item'. In the second example the
> expression is:
>
>  item
>
> which produces the current value stored in the variable "item".
>
> Cheers,
> Cameron Simpson 
>
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Re: [Tutor] Adding to a dict through a for loop confusion.

[cs]

On 17May2016 04:28, Chris Kavanagh  wrote:

Could someone tell me why this different behavior occurs between these 2
code snippets, please. The 1st example has quotes around it ['item'] only
adds the last item to the dict (cart). In the 2nd example the item does not
have quotes around it [item] and every entry is added to the dict.

Why?

[...]

# Example #1
cart_items = ['1','2','3','4','5']
cart = {}
for item in cart_items:
   cart['item'] = item


This for loop assigns the values '1','2','3','4','5' in succession to the 
variable named "item". Then the body of the loop assigns that value (via the 
variable "item") to the single dictionary slot with the fixed key with string 
value 'item' i.e. always the same slot.  And the last item is the one kept.  
All the earlier assignments are overwritten by the later ones: they happen, but 
are replaced.



print cart
#output
{'item': 5}


Which you see above.


# Example #2
cart_items = ['1','2','3','4','5']
cart = {}
for item in cart_items:
   cart[item] = item


Here, the variable named "item" takes on the values as before, but the diction 
slot chosen also comes form that variable. So each value ends up in its own 
slot as your output shows.



print cart
# output
{'1': '1', '3': '3', '2': '2', '5': '5', '4': '4'}


The essential difference here is that in the first example the expression for 
the slot in the dictionary is the expression:


 'item'

which is simply the fixed string 'item'. In the second example the expression 
is:


 item

which produces the current value stored in the variable "item".

Cheers,
Cameron Simpson 
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Re: [Tutor] Adding to a dict through a for loop confusion.

[Alan Gauld via Tutor]

On 17/05/16 09:28, Chris Kavanagh wrote:

> # Example #1
> cart_items = ['1','2','3','4','5']
> 
> cart = {}
> 
> for item in cart_items:
> cart['item'] = item

'item' is a literal string. It never changes.
So you keep overwriting the dict entry so that
at the end of the loop the dict contains the
last value associated with your literal key 'item'

> #output
> {'item': 5}
> 
> 
> # Example #2
> cart_items = ['1','2','3','4','5']
> 
> cart = {}
> 
> for item in cart_items:
> cart[item] = item

here you use the actual item from your data as both
the key and the value. So you wind up with a dict
containing a key/value pair for each value in your
data list.

> # output
> {'1': '1', '3': '3', '2': '2', '5': '5', '4': '4'}

That's not a very common requirement though, usually
you would have different values from the keys.

Maybe something like:

for item in cart_items:
cart[item] = int(item)

which stores an integer value against the string
representation:

{'1': 1, '3': 3, '2': 2, '5': 5, '4': 4}

Notice also that dicts don't usually print out
in the same order as you created them. In fact
the 'order' can even change over the life of your
program, as you add/remove elements.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] Adding to a dict through a for loop confusion.

[Sibylle Koczian]

Hello,

Am 17.05.2016 um 10:28 schrieb Chris Kavanagh:

Could someone tell me why this different behavior occurs between these 2
code snippets, please. The 1st example has quotes around it ['item'] only
adds the last item to the dict (cart). In the 2nd example the item does not
have quotes around it [item] and every entry is added to the dict.

Why?


# Example #1
cart_items = ['1','2','3','4','5']

cart = {}

for item in cart_items:
cart['item'] = item

print cart

#output
{'item': 5}



Here you assign every item from your list to a dictionary entry with the 
same key 'item' - that's a string literal that hasn't got anything to do 
with the name item you give to each list element in turn.


Because a dictionary can only contain one entry for each key the value 
of that entry cart['item'] is overwritten in every step through the 
loop. Only the result of the last step is kept.




# Example #2
cart_items = ['1','2','3','4','5']

cart = {}

for item in cart_items:
cart[item] = item

print cart

# output
{'1': '1', '3': '3', '2': '2', '5': '5', '4': '4'}



Here you take the value from the list cart_items, called item, for the 
key and for the value of the dictionary entry. So every entry gets a 
different key and your dictionary has as many entries as the list has 
elements.


But did you really want to get a dictionary with the same values for key 
and value?


HTH
Sibylle


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[Tutor] Adding to a dict through a for loop confusion.

[Chris Kavanagh]

Could someone tell me why this different behavior occurs between these 2
code snippets, please. The 1st example has quotes around it ['item'] only
adds the last item to the dict (cart). In the 2nd example the item does not
have quotes around it [item] and every entry is added to the dict.

Why?


# Example #1
cart_items = ['1','2','3','4','5']

cart = {}

for item in cart_items:
cart['item'] = item

print cart

#output
{'item': 5}


# Example #2
cart_items = ['1','2','3','4','5']

cart = {}

for item in cart_items:
cart[item] = item

print cart

# output
{'1': '1', '3': '3', '2': '2', '5': '5', '4': '4'}


Thanks you for the help.
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