How do you create nested dict in Python

Question

I have 2 csv files. First one is data file and other one is mapping file. Mapping file has 4 columns Device_Name GDN Device_Type Device_OS These are also the columns which are present in data file and need to be worked upon.

Data file contains data with Device_Name column populated & rest 3 columns blank. Mapping file contains all columns populated. I want my Python code to open both files and for each device name in data file map its GDN, Device_Type & Device_OS value from mapping file.

I know how to use dict when only 2 columns are present (1 is needed to be mapped) but I don't know how to accomplish this when 3 columns need to be mapped.

Following is the code using which I tried to accomplish mapping of Device_Type:

x = dict([])
with open("Pricing Mapping_2013-04-22.csv", "rb") as in_file1:
    file_map = csv.reader(in_file1, delimiter=',')
    for row in file_map:
        typemap = [row[0],row[2]]
        x.append(typemap)
        with open("Pricing_Updated_Cleaned.csv", "rb") as in_file2, open("Data Scraper_GDN.csv", "wb")                                                                                         as out_file:
writer = csv.writer(out_file, delimiter=',')
for row in csv.reader(in_file2, delimiter=','):
    Try:
        row[27] = x[row[11]]
    except KeyError:
        row[27] = "" writer.writerow(row)
        #It returns the Atribute Error.

After some researching, I realized that I need to create a nested dict, but I don't have any idea on how to do this. Please help me in resolving this or nudge me in the right direction to resolve this.

Answer 1

It is important to remember when using defaultdict and similar nested dict modules such as nested_dict, that looking up a non existent key may inadvertently create a new key entry in the dict and cause a lot of havoc. Here is a Python3 example with nested_dict.

import nested_dict as nd 
nest = nd.nested_dict()
nest['outer1']['inner1'] = 'v11'
nest['outer1']['inner2'] = 'v12'
print('original nested dict: \n', nest)
try:
 nest['outer1']['wrong_key1']
except KeyError as e:
  print('exception missing key', e)
  print('nested dict after lookup with missing key. no exception raised:\n', nest)
# instead convert back to normal dict
 nest_d = nest.to_dict(nest)
try:
  print('converted to normal dict. Trying to lookup Wrong_key2')
  nest_d['outer1']['wrong_key2']
except KeyError as e:
  print('exception missing key', e)
  else:
  print(' no exception raised:\n')
  # or use dict.keys to check if key in nested dict.
  print('checking with dict.keys')
 print(list(nest['outer1'].keys()))
 if 'wrong_key3' in list(nest.keys()):
  print('found wrong_key3')
 else:
  print(' did not find wrong_key3')

Output is:

original nested dict: {"outer1": {"inner2": "v12", "inner1": "v11"}}
nested dict after lookup with missing key. no exception raised: {"outer1": {"wrong_key1": {}, "inner2": "v12", "inner1": "v11"}} 
converted to normal dict.
Trying to lookup Wrong_key2 exception missing key 'wrong_key2' checking with dict.keys ['wrong_key1', 'inner2', 'inner1'] did not find wrong_key3