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

User · Accepted Answer

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

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

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'}}

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

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.

User · Answer

For arbitrary levels of nestedness   In  2   def nested dict                return collections defaultdict nested dict           In  3   a   nested dict    In  4   a Out 4   defaultdict  lt function   main   nested dict gt        In  5   a  a    b    c     1  In  6   a Out 6   defaultdict  lt function   main   nested dict gt                 a   defaultdict  lt function   main   nested dict gt                              b   defaultdict  lt function   main   nested dict gt                                           c   1

User · Answer

It is important to remember when using defaultdict and similar nested dict modules such as nested dict  that looking up a nonexistent 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 module   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

User · Answer

This thing is empty nested list from which ne will append data to empty dict  ls      a   a1   a2   a3     b   b1   b2   b3     c   c1   c2   c3       d   d1   d2   d3     this means to create four empty dict inside data dict data dict    f dict i      for i in range 4   for i in range 4       upd dict     val    ls i  0    val1    ls i  1   val2    ls i  2   val3    ls i  3        data dict f dict i    update upd dict   print data dict   The output   dict0     val    a    val1    a1    val2    a2    val3    a3     dict1     val    b    val1    b1    val2    b2    val3    b3    dict2     val    c    val1    c1    val2    c2    val3    c3     dict3     val    d    val1    d1    val2    d2    val3    d3

User · Answer

If you want to create a nested dictionary given a list  arbitrary length  for a path and perform a function on an item that may exist at the end of the path  this handy little recursive function is quite helpful  def ensure path data  path  default None  default func lambda x  x        quot  quot  quot      Function       - Ensures a path exists within a nested dictionary      Requires       -  data           - Type  dict         - What  A dictionary to check if the path exists     -  path           - Type  list of strs         - What  The path to check      Optional       -  default           - Type  any         - What  The default item to add to a path that does not yet exist         - Default  None      -  default func           - Type  function         - What  A single input function that takes in the current path item  or default  and adjusts it         - Default   lambda x  x    Returns the value in the dict or the default value if none was present      quot  quot  quot      if len path  gt 1          if path 0  not in data              data path 0              data path 0   ensure path data data path 0    path path 1    default default  default func default func      else          if path 0  not in data              data path 0   default         data path 0   default func data path 0        return data  Example  data   a    b  1   ensure path data data  path   a   c    default  1   print data     gt    a    b  1   c   1    ensure path data data  path   a   c    default  1   default func lambda x x  2   print data     gt    a     b   1   c    1  2

User · Answer

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

[python] How do you create nested dict in Python?

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