[Tutor] Modify a dictionary using csv.DictWriter

[Judith Flores]

Hello,

   I have been struggling a while trying to figure out how to modify a csv file using the csv.DictWriter class. Let's suppose I have the following code:

import csv
outfile=open('template.csv','w')     # This is a pre-existing file that contains 3 variables (3 columns). The variables will (or will not) have a value each.

x='new_value'      

writing=csv.DictWriter(outfile, fieldnames=['variable1','variable2','variable2'])

data={'variable1' : x}   # What I intent to do here is to overwrite whatever value exists for variable 1. There will always exist only one value per variable.

writing.writerow(data)

I have several questions about how DictWriter works, and I am sorry if my questions have a very obvious answer, which I am 'obviously' missing.


1. When I run the code above for the first time, the contents of the pre-existing file disappear, if I run the script a second time, now I can see the value of x.
2. The fieldnames don't appear in the csv file, only the values. 
3. Is there a way to let the DictWriter know that the header of the csv file corresponds to all the fieldnames? To avoid typing all the names.


As usual, thank you very much for your help.


Judith