Get length of csv file without ruining reader?

Question

I am trying to do the following:

reader = csv.DictReader(open(self.file_path), delimiter='|')
reader_length = sum([_ for item in reader])
for line in reader:
    print line

However, doing the reader_length line, makes the reader itself unreadable. Note that I do not want to do a list() on the reader, as it is too big to read on my machine entirely from memory.

Answer 1

Use enumerate with a start value of 1, when you get to the end of the file you will have the line count:

for count,line in enumerate(reader,1):
    # do work
print count

Or if you need the count at the start for some reason sum using a generator expression and seek back to the start of the file:

  with open(self.file_path) as f:
        reader = csv.DictReader(f, delimiter='|')
        count = sum(1 for _ in reader)
        f.seek(0)
        reader = csv.DictReader(f, delimiter='|')
        for line in reader:
            print(line)