replicating a row with changing a field in Python - csv

Question

I have a large csv file with two columns like this:
Id and vehicle
and I like to replicate the rows and if the vehicle is "truck", but instead put "car".
I have this code, but there is an error
which says
_csv.Error: iterator should return strings, not bytes (did you open the file in text mode?)
what does it mean? where I am wrong?
infilename = r'external carriers.csv'
outfilename = r'outputCSV.csv'
with open(infilename, 'rb') as fp_in, open(outfilename, 'wb') as fp_out:
reader = csv.reader(fp_in, delimiter=",")
writer = csv.writer(fp_out, delimiter=",")
for row in reader:
if len(row) == 2:
if row == "truck":
writer.writerow = "car"

Answer 1

It's obvious, you have opened the file in text mode you need rt :
with open(infilename, 'rt') as fp_in, open(outfilename, 'wt') as fp_out:
Also if you want to check the vehicle type you need to check the row[1] which preserve your car name and then reassign it and write the row to your output file.Also note that you don't need to check the length of your rows since calling the len function can be terrible in term if your performance which has O(n) and for large files (specially with large rows) is very inefficient.
infilename = r'external carriers.csv'
outfilename = r'outputCSV.csv'
with open(infilename, 'rt') as fp_in, open(outfilename, 'wt') as fp_out:
reader = csv.reader(fp_in, delimiter=",")
writer = csv.writer(fp_out, delimiter=",")
for row1,row2 in reader:
if row2 == "truck":
writer.writerow([row1,'car'])