Python - Initializing List variable from List elements

Question

Ex. Lets consider, I have a list,

list_var = ['sales_qty' , 'returns_qty' , 'net_sales_qty' , 'sales_amt' , 'returns_amt' ,'product_discount_amt' , 'product_net_amt' ,'product_cost_amt' , 'gross_sales_amt' , 'supplier_disc_amt' , 'category_disc_amt' , 'topup_disc_amt' , 'value_at_cost']

from this list element, i want to init these elements as list in the memory, so i could use append, extend method for these elements.

As follow,

#Need these variable in memory..
sales_qty = [], returns_qty  = [], net_sales_qty = [] ... value_at_cost = []

#list operation on variable..
sales_qty.append(5)

Is there any simple way to do that, so i could remove element or add element in list easily?

What exactly is sales_qty in list_var = [sales_qty, ...]? A reference to some object? — NPE
– NPE, Commented Jan 29, 2013 at 12:16
@NPE: this is string element in the list, for which i need to define as list. — Niks Jain
– Niks Jain, Commented Jan 29, 2013 at 12:18

sloth · Accepted Answer · 2013-01-29 12:37:15Z

5

You could create a dictionary of lists based on list_var .

list_var = ['sales_qty' , 'returns_qty' , 'net_sales_qty' , 'sales_amt' , 'returns_amt' ,'product_discount_amt' , 'product_net_amt' ,'product_cost_amt' , 'gross_sales_amt' , 'supplier_disc_amt' , 'category_disc_amt' , 'topup_disc_amt' , 'value_at_cost']

list_dic = {k: [] for k in list_var}

Now can access your lists through the list_dic dict:

list_dic['sales_qty'].append(123)
list_dic['category_disc_amt'].append('abc')

If you're using a python version that does not support dict comprehensions (python 2.6 or lower), you can use a list comprehension instead (as DSM explained in his comment):

list_dic = dict((k, []) for k in list_var)

edited Jan 29, 2013 at 12:37

answered Jan 29, 2013 at 12:25

sloth

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2 Comments

Niks Jain Over a year ago

{k: [] for k in list_var} this dict format not supported for python2.5 :(

DSM Over a year ago

@Niks: then you can use dict((k, []) for k in list_var) instead. This is still the right approach.

ATOzTOA · Accepted Answer · 2013-01-29 14:35:18Z

2

Use vars if you are invoking from global scope. Otherwise, use globals.

list_var = ['sales_qty' , 'returns_qty' , 'net_sales_qty' , 'sales_amt' , 'returns_amt' ,'product_discount_amt' , 'product_net_amt' ,'product_cost_amt' , 'gross_sales_amt' , 'supplier_disc_amt' , 'category_disc_amt' , 'topup_disc_amt' , 'value_at_cost']

for item in list_var:
    vars()[item] = []

sales_qty.append(1)
sales_qty.append(3)
returns_qty.append(2)

print "Outside..."
print sales_qty
print returns_qty


def foo():
    global sales_qty

    sales_qty.append(4)

    print "Inside foo..."
    print sales_qty
    print returns_qty

    # This works if you want the whole thing inside a function
    print "Variable from inside foo..."
    globals()["new_var"] = []
    new_var.append(5)
    print new_var

foo()

print "Outside again..."
print sales_qty
print returns_qty

Output:

>>> 
Outside...
[1, 3]
[2]
Inside foo...
[1, 3, 4]
[2]
Variable from inside foo...
[5]
Outside again...
[1, 3, 4]
[2]
>>>

edited Jan 29, 2013 at 14:35

answered Jan 29, 2013 at 12:30

ATOzTOA

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8 Comments

Niks Jain Over a year ago

Are these init as local var or global var in the memory??

ATOzTOA Over a year ago

They will be local to the scope in which vars() is invoked.

DSM Over a year ago

Now try this from within a function. As your link says: "Note, the locals dictionary is only useful for reads since updates to the locals dictionary are ignored." It only works by fluke in your test because you're working at module level.

sloth Over a year ago

Even if this would work, I don't see the point in creating variables dynamically. If you want to access them in your script, you need to know the name in advance, in which case you could just create them like any other ordinary variable: sales_qty = []

DSM Over a year ago

Yes, globals() will work -- and for that matter, using "exec" would work in local scope if you absolutely, positively, wanted to create local variables dynamically. [Why, I've no idea.] But now you not only have the problem that @DominicKexel pointed out, you're polluting the global scope. That's deeply unpythonic.

|

isedev · Accepted Answer · 2013-01-29 16:58:53Z

0

You mean something like this:

list_var = [ 'abc', 'cde', 'fgh' ]

dict_var = dict()
for x in list_var:
    dict_var[x] = []

dict_var['abc'].append(5)

edited Jan 29, 2013 at 16:58

answered Jan 29, 2013 at 12:25

isedev

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1 Comment

Cristian Ciupitu Over a year ago

Take note that your vars variable hides the builtin function vars. Nevertheless it can still be called as __builtins__.vars.

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