Python CSV Reader Control Flow

Question

I was fairly surprised to see this functionality in Python's CSV reader.

with open(sys.argv[1]) as csvfile:
    reader = csv.DictReader(csvfile)
    for i in range(3):
        sys.stdout.write('A ')
        for row in reader:
            sys.stdout.write('B ')

#sys.argv[1] is a 3 row csv file

I would normally expect code like this to print out something like:

A B B B A B B B A B B

But instead I get:

A B B A A

This seems to violate the basic flow control properties of for loops as I understand them. I mainly suspect that there is an unusual property of this iterator that results in this behavior. Any explanation would helpful and very much appreciated. Thank you.

Answer 1

Your reader object from csv.DictReader is a generator. It got exhausted in the first iteration of the outer for . So in the following iterations, the inner for loop has no items to execute the loop.