Python Dictionary to CSV

Question

I have written code to read a CSV into a python dictionary, which works fine. I'm trying to get the dictionary back to a CSV. I have written the following:

import csv  itemDict={}  listReader = csv.reader(open('/Users/broberts/Desktop/Sum_CSP1.csv','rU'), delimiter = ',', quotechar='|')  for row in listReader:     fID = row[0]     fClassRange = row[1]     fArea = row[2]      if itemDict.has_key(fID):         itemDict[fID][fClassRange]=fArea     else:         itemDict[fID] = {'5.0 to 5.25':'','5.25 to 5.5':'','5.5 to 5.75':'','5.75 to 6.0':'','6.0 to 6.25':'','6.25 to 6.5':'','6.5 to 6.75':'','6.75 to 7.0':'','7.0 to 7.25':'','7.25 to 7.5':'','7.5 to 7.75':'','7.75 to 8.0':'','8.0 to 8.25':'',}         itemDict[fID][fClassRange]=fArea  listWriter = csv.writer(open('/Users/broberts/Desktop/Sum_CSP1_output.csv', 'wb'), delimiter=',', quotechar='|', quoting=csv.QUOTE_MINIMAL)  for a in itemDict:     print a     listWriter.writerow(a) 

In the last block, listWriter will not write anything to the CSV though it will print a. I believe this has something to do with a dictionary being unordered. I really need to write out the fID and each of the keys associated with each fID (fClassRange ex. "5.0 to 5.25") and then the value fArea associated with each fClassRange to the CSV, but I haven't even gotten that far in my code since I can't figure out how to write out even the fID.

I looked into using DictWriter, but I can't figure out how to tell it what the required fieldnames are.

Answer 1

The default writer expects a list, which is why it won't work for you. To use the dictwriter, just change your listwriter = line to this:

with open('/Users/broberts/Desktop/Sum_CSP1_output.csv', 'wb') as outfile:     listWriter = csv.DictWriter(        outfile,        fieldnames=itemDict[itemDict.keys()[0]].keys(),        delimiter=',',        quotechar='|',        quoting=csv.QUOTE_MINIMAL     ) 

Or, you can just set fieldnames to be fieldnames=['arbitrary','list','of','keys'] if you know what the fields are supposed to be.

Answer 2

Sample data:

mydict = [{"col1": 1000, "col2": 2000}, {"col1": 3000, "col2": 4000}] 

One-liner for converting a list of dicts to CSV, using pandas:

import pandas as pd  pd.DataFrame(mydict).to_csv('out.csv', index=False) 

Results:

col1,col2 1000,2000 3000,4000