issue with Looping through lists to write new rows in a csv file using python

Question

I am trying to add content to my existing csv file. meaning I would not like to remove any existing content but just like to add new lines based on list of lists.

However, in the final output I don't see any new row being added in the csv. Also the variable is returning empty list values (please see the comment on the code line).

Here i am asking the user first the number of entries. then value for each entry (each row value for each column). Then I am simple appending it to a final list i.e total_number_of_rows which should look like: [[x,x,x][xy,xy,xy].

In the final code I am trying to write the values from the two lists to the CSV file for example the final output should look like:

id, name, color
oldvalue1, oldvalue2, oldvalues3  #assuming this is an already existing row in the csv file
x,x,x
xy,xy,xy


number_of_entries = int(input("how many entries will you need to enter:"))
each_new_row = []
total_number_of_rows = []
for i in range(number_of_entries):
    new_id = input("add new new_id")
    new_name = input ("add new date")
    new_color = input ("add new color")
    each_new_row.extend((new_id,new_name,new_color))
    print("each_new_row",each_new_row)
    total_number_of_rows.append(each_new_row)
    print("total number of rows", total_number_of_rows) # this is showing [[1,1,red],[2,2,blue]]
    each_new_row.clear()
print("total_number_of_rows", total_number_of_rows) ## this is showing [[],[]]


  
with open('file.csv', 'a',newline ="") as f:
      
        writer = csv.writer(f)
        writer.writerows(total_number_of_rows)
    #not seeing any new row added

i am populating the total_number_of_rows with values using the list method. somehow that is not going through. Even if i add print("total_number_of_rows", total_number_of_rows) right after each_new_row.clear() i.e directly below it, it shows the same results — trillion
– trillion, Commented Apr 7, 2021 at 21:44
# this is showing [[1,1,red],[2,2,blue]] that's not true, you should check that again. Maybe you will notice what the problem is then. — mkrieger1
– mkrieger1, Commented Apr 7, 2021 at 21:45
it's something with the each_new_row.clear() that's causing the issue. If you comment that out both total_number_of_rows print statments work correctly, but then your lists aren't separated correctly into sublists — JD2775
– JD2775, Commented Apr 7, 2021 at 21:48
It may show [[1,1,red]] or [[2,2,blue], [2,2,blue]], but not [[1,1,red], [2,2,blue]]. — mkrieger1
– mkrieger1, Commented Apr 7, 2021 at 21:49
@JD2775 exactly i tried to remove it, can you suggest how can i solve that issue ? — trillion
– trillion, Commented Apr 7, 2021 at 21:51

mkrieger1 · Accepted Answer · 2021-04-07 21:53:51Z

1

each_new_row = []
# ...
for i in range(number_of_entries):
    # ...
    total_number_of_rows.append(each_new_row)
    # ...
    each_new_row.clear()

This appends the same list multiple times to total_number_of_rows. At the end you clear this list, and that's why total_number_of_rows contains only empty lists (it's the same empty list multiple times).

Instead, you want to create a new empty list in every iteration instead of reusing the same list over and over.

for i in range(number_of_entries):
    each_new_row = []      # do this
    # ...
    total_number_of_rows.append(each_new_row)
    # ...
    #each_new_row.clear()  # don't do this

answered Apr 7, 2021 at 21:53

mkrieger1

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