Adding to a value in Python Dictionary

Question

I am having trouble with the following code. I need to add a key to a python dictionary if it does not exist, and if it does then I need to add to the value. My output should look like this.

{'STA': {'value':-62**.***, 'count': 4}}.....

But I'm getting this instead.

{'STA': {'value': -1194.14562548, 'count': 0}}, 
{'STA': {'value': -5122.985396600001, 'count': 0}}, 
{'STA': {'value': 25.2293, 'count': 0}}, 
{'STA': {'value': 34.0099, 'count': 0}},

What am I doing wrong?

new_dict = []
for item in sales_orders['progress_output']:
    ex = (list(filter(lambda ex:ex['_branch_shortname'].strip() == item['_branch_shortname'].strip(), expenses)))
    value = float(item['gross_profit'].strip().replace(',', '')) - (float(item['load_factor_extended'].strip().replace(',', '')) * float(ex[0]['value']))
    branch = item['_branch_shortname'].strip()

    if branch in new_dict:
        new_dict[branch]['value'] += value
        new_dict[branch]['count'] += 1
    else:
        new_dict.append({branch: {'value': value, 'count': 0}})

    #print(item['_branch_shortname'], value)
print(new_dict)

In Python normal dict is not ordered sequence, there is no append. — Grijesh Chauhan
– Grijesh Chauhan, Commented Mar 31, 2015 at 16:02
new_dict is a list. Change [] to {} and it will be declared as an empty dictionary. Better yet, use dict(). — Spice
– Spice, Commented Mar 31, 2015 at 16:02
Ken, I also took your suggestion and added new_dict = dict() — thomasw_lrd
– thomasw_lrd, Commented Mar 31, 2015 at 16:16

AChampion · Accepted Answer · 2015-03-31 16:13:01Z

1

You can use setdefault to ensure there is a default value, setdefault returns the value if it already exists for key (first parameter) or adds the default (second parameter) and returns it:

 new_dict = {}
 for ...:
     b  = new_dict.setdefault(branch, {'value': 0, 'count': 0})
     b['value'] += value
     b['count'] += 1

You can also use a defaultdict:

from collections import defaultdict
new_dict = defaultdict(lambda: {'value': 0, 'count': 0})
for ...:
    new_dict[branch]['value'] += value
    new_dict[branch]['count'] += 1

edited Mar 31, 2015 at 16:13

answered Mar 31, 2015 at 16:06

AChampion

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