How do you create nested dict in Python?

Question

I have 2 CSV files: 'Data' and 'Mapping':

• 'Mapping' file has 4 columns: Device_Name, GDN, Device_Type, and Device_OS. All four columns are populated.
• 'Data' file has these same columns, with Device_Name column populated and the other three columns blank.
• I want my Python code to open both files and for each Device_Name in the Data file, map its GDN, Device_Type, and Device_OS value from the 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 Attribute Error.

After some researching, I think I need to create a nested dict, but I don't have any idea how to do this.

Answer 1

A nested dict is a dictionary within a dictionary. A very simple thing.

>>> d = {} >>> d['dict1'] = {} >>> d['dict1']['innerkey'] = 'value' >>> d['dict1']['innerkey2'] = 'value2' >>> d {'dict1': {'innerkey': 'value', 'innerkey2': 'value2'}} 

You can also use a defaultdict from the collections package to facilitate creating nested dictionaries.

>>> import collections >>> d = collections.defaultdict(dict) >>> d['dict1']['innerkey'] = 'value' >>> d  # currently a defaultdict type defaultdict(<type 'dict'>, {'dict1': {'innerkey': 'value'}}) >>> dict(d)  # but is exactly like a normal dictionary. {'dict1': {'innerkey': 'value'}} 

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You can populate that however you want.

I would recommend in your code something like the following:

d = {}  # can use defaultdict(dict) instead  for row in file_map:     # derive row key from something      # when using defaultdict, we can skip the next step creating a dictionary on row_key     d[row_key] = {}      for idx, col in enumerate(row):         d[row_key][idx] = col 

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According to your comment:

may be above code is confusing the question. My problem in nutshell: I have 2 files a.csv b.csv, a.csv has 4 columns i j k l, b.csv also has these columns. i is kind of key columns for these csvs'. j k l column is empty in a.csv but populated in b.csv. I want to map values of j k l columns using 'i` as key column from b.csv to a.csv file

My suggestion would be something like this (without using defaultdict):

a_file = "path/to/a.csv" b_file = "path/to/b.csv"  # read from file a.csv with open(a_file) as f:     # skip headers     f.next()     # get first colum as keys     keys = (line.split(',')[0] for line in f)   # create empty dictionary: d = {}  # read from file b.csv with open(b_file) as f:     # gather headers except first key header     headers = f.next().split(',')[1:]     # iterate lines     for line in f:         # gather the colums         cols = line.strip().split(',')         # check to make sure this key should be mapped.         if cols[0] not in keys:             continue         # add key to dict         d[cols[0]] = dict(             # inner keys are the header names, values are columns             (headers[idx], v) for idx, v in enumerate(cols[1:])) 

Please note though, that for parsing csv files there is a csv module.

Answer 2

UPDATE: For an arbitrary length of a nested dictionary, go to this answer.

Use the defaultdict function from the collections.

High performance: "if key not in dict" is very expensive when the data set is large.

Low maintenance: make the code more readable and can be easily extended.

from collections import defaultdict  target_dict = defaultdict(dict) target_dict[key1][key2] = val