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.

Moses Koledoye · Accepted Answer · 2016-06-23 19:26:27Z

2

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.

answered Jun 23, 2016 at 19:26

Moses Koledoye

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5 Comments

Luca Over a year ago

Okay. I think I can fix that by switching the places of the two for loops. Thank you.

Moses Koledoye Over a year ago

@LucaDelSignore Yes, you can. But you can also store the items from the reader in a list, if the csv file isn't too large

Luca Over a year ago

Actually, the real CSV file is very large.

Moses Koledoye Over a year ago

Oh, then might be better to switch the loops

Luca Over a year ago

Thanks man. I appreciate the help and not getting down voted into oblivion for being inexperienced.

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