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Behaviour of list comprehensions

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  Behaviour of list comprehensions arsh.py@gmail.com - 2016-02-02 05:30 -0800
    Re: Behaviour of list comprehensions Steven D'Aprano <steve@pearwood.info> - 2016-02-03 00:58 +1100
    Re: Behaviour of list comprehensions Peter Otten <__peter__@web.de> - 2016-02-02 15:08 +0100

#102404 — Behaviour of list comprehensions

I am having some understandable behaviour from one of my function name week_graph_data() which call two functions those return a big tuple of tuples, But in function week_graph_data() line no. 30 does not work(returns no result in graph or error). 

I have check both functions are called in week_graph_data() individually and those run perfectly. (returning tuple of tuples)

here is my code: pastebin.com/ck1uNu0U

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#102405

Hi Arsh, and welcome!

On Wed, 3 Feb 2016 12:30 am, arsh.py@gmail.com wrote:

> I am having some understandable behaviour from one of my function name
> week_graph_data() which call two functions those return a big tuple of
> tuples, But in function week_graph_data() line no. 30 does not
> work(returns no result in graph or error).
> 
> I have check both functions are called in week_graph_data() individually
> and those run perfectly. (returning tuple of tuples)
> 
> here is my code: pastebin.com/ck1uNu0U

There's no need to waste your time, and other people's time, by making a
pastebin. Since it's less than 30 lines, just include it in your email.

(Be careful with Gmail that it doesn't kill the indentation. Just make sure
you send plain text, with no "rich text" -- actually HTML -- formatting.)



def week_list_func():
    """ A function returning list of last 6 days in 3-tuple"""
    date_list = [] 
    for i in xrange(7):
        d=date.today() - timedelta(i)
        t = d.year, d.month, d.day
        date_list.append(t)
    return reversed(date_list)

def week_graph_data():
    day_list_inbox = inbox_week()  # returns big Tuple of tuples   
    day_list_outbox = outbox_week() # Same as inbox_week()
    dates_inbox = tuple(a[:-7] for a in day_list_inbox)
    dates_outbox = tuple(a[:-7] for a in day_list_outbox)
    week_list = week_list_func()
    bar_chart = pygal.Bar()
    bar_chart.title = 'Weekly Email Analysis'
    bar_chart.x_labels = (list(week_list_func()))
    bar_chart.add('Received', 
            [sum(t==i for t in dates_outbox) for i in list(week_list)]
            )
    bar_chart.add('Sent', 
            [sum(m==n for n in dates_inbox) for m in list(week_list)]
            )
    week_chart = bar_chart.render(is_unicode = True)
    return bar_chart.render_to_file('ok_.svg')

week_graph_data()



Your problem is that week_list_func() returns an iterator, so once you
convert it to a list the first time, it becomes empty. For example:


py> values = [1, 2, 3, 4, 5, 6, 7]
py> days = reversed(values)
py> print days
<listreverseiterator object at 0xb7e578ac>
py> print list(days)
[7, 6, 5, 4, 3, 2, 1]
py> print list(days)
[]


Here, days is an iterator. Once you consume the iterator completely the
first time, there's nothing left, so the second time you get an empty list.

You can fix this by having week_list_func() return a list or tuple:


# Version 1
def week_list_func():
    """ A function returning list of last 6 days in 3-tuple"""
    date_list = [] 
    for i in xrange(7):
        d=date.today() - timedelta(i)
        t = d.year, d.month, d.day
        date_list.append(t)
    return date_list[::-1]  # reverse the list with a slice


# Version 2
def week_list_func2():
    """ A function returning list of last 6 days in 3-tuple"""
    date_list = [] 
    for i in xrange(6, -1, -1):
        d=date.today() - timedelta(i)
        t = d.year, d.month, d.day
        date_list.append(t)
    return date_list


Because this function returns an actual list, there is no need to call
list() on the result.


-- 
Steven

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#102406

arsh.py@gmail.com wrote:

> I am having some understandable behaviour from one of my function name
> week_graph_data() which call two functions those return a big tuple of
> tuples, But in function week_graph_data() line no. 30 does not
> work(returns no result in graph or error).
> 
> I have check both functions are called in week_graph_data() individually
> and those run perfectly. (returning tuple of tuples)
> 
> here is my code: pastebin.com/ck1uNu0U

> def week_list_func():
>     """ A function returning list of last 6 days in 3-tuple"""
>    
>     date_list = []
>     for i in xrange(7):
>         d=date.today() - timedelta(i)
>         t = d.year, d.month, d.day
>         date_list.append(t)
>     return reversed(date_list)

reversed(date_list) returns an "iterator":

>>> items = reversed(["a", "b", "c"])
>>> items
<list_reverseiterator object at 0x7f43622e9a20>

This means that iteration will "consume" the items

>>> list(items)
['c', 'b', 'a']
>>> list(items)
[]

and thus work only once. To allow for multiple iterations you can return a 
list instead:

>>> items = ["a", "b", "c"]
>>> items.reverse()
>>> items
['c', 'b', 'a']
>>> list(items)
['c', 'b', 'a']
>>> list(items)
['c', 'b', 'a']

Note that list.reverse() is a mutating method and by convention returns 
None. Therefore

   return date_list.reverse() # wrong!

will not work, you need two lines

   date_list.reverse()
   return date_list

in your code.

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