Python dictionary keys to csv file with column match

Question

I'm trying to push multiple dictionaries (keys and values) to a csv file by matching the key to a column in the csv header. Example:

import csv
d1 = {'a':1, 'b':2, 'c': 3}
d2 = {'d':4, 'e':5, 'f': 6}
with open('my_data.csv','wb') as f:
    w = csv.writer(f)
    w.writerow(['a', 'b', 'c', 'd', 'e', 'f'])

#iterate through all keys in d1,d2,dn
#if key matches column:
    #write value of key to the bottom of column
#else:
    #error key not found in header

expected result in mydata.csv

a,b,c,d,e,f
1,2,3,4,5,6

Not sure I get it. Are you merging all the dicts into a single line in the output file (as shown)? Or do you want each input dict to be a line in the output file? — RobertB
– RobertB, Commented Oct 28, 2015 at 21:54

little_birdie · Accepted Answer · 2015-10-28 22:13:09Z

The answer is.. don't just pass the column names to writerow().. put them in a variable columns and then use that to control the order in which the values are written out. Python dictionaries have no order.. you have to use a little bit of code to sort the values into the order you want.

The last line of code, which writes out the values into the CSV, uses a python feature called List Comprehension. It's a shortcut that saves 3-4 lines of code. Look it up, they are very handy.

import csv
d1 = {'a':1, 'b':2, 'c': 3}
d2 = {'d':4, 'e':5, 'f': 6}

columns = ['a', 'b', 'c', 'd', 'e', 'f']

# combine d1 and d2 into data.. there are other ways but this is easy to understand
data = dict(d1)
data.update(d2)

with open('my_data.csv','wb') as f:
    w = csv.writer(f)
    w.writerow(columns)
    # make a list of values in the order of columns and write them
    w.writerow([data.get(col, None) for col in columns])

Here is what it would look like without the list comprehension:

    row = []
    for col in columns:
        row.append(data.get(col, None)) 
    w.writerow(row)

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