Last element of csv reader in Python

Question

How do you detect the last row in a csv file? It is an iterator but I cannot find a way to detect the last row:

csvfile = open('file.csv', 'r')

reader = csv.DictReader(csvfile, fieldnames)

for idx, row in enumerate(reader, start=1):
    # How to detect last row?

Why do you need to know the last row? What are you doing with it that is so special? — , Jan 30 '15 at 02:37
@GamesBrainiac i confirm thats `.next()`. @Dane check this answer http://stackoverflow.com/a/11479202/2029818 — michalczukm, Jan 30 '15 at 02:39
@LegoStormtroopr I am converting csv to json. Last row should now have the trailing coma. — bman, Jan 30 '15 at 02:39

score 2 · Answer 1 · answered Jan 30 '15 at 02:41

2

You can always just convert to a list and take the last element of the list

csvfile = open('file.csv', 'r')

reader = csv.DictReader(csvfile, fieldnames)

all_lines = list(reader)
last_line = all_lines[-1]

answered Jan 30 '15 at 02:41

Greg

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This may cause problems if the file is very large as it will require reading the entire file into memory. – Jan 30 '15 at 02:59
I'm not disagreeing, its up to the user to define which solution best fits the needs of the situation – Greg Jan 30 '15 at 19:29

score 2 · Answer 2 · answered Jan 30 '15 at 02:58

As stated, using next() is probably the best bet

csvfile     = open('file.csv', 'r')

reader = csv.DictReader(csvfile, fieldnames)

mycsv = enumerate(reader, start=1)
idx, row = mycsv.next()
while row:
    try:
       next_set = mycsv.next()
       pass # process set of (idx, row) here
       idx, row = next_set
    except StopIteration:
       pass # process last row here
       idx, row = None # make sure to unset to kill the loop

score 1 · Accepted Answer · edited May 23 '17 at 12:24

When you want custom behavior, write a custom iterater. Adapting the second example from this answer:

Empty = object()

class BooleanIterator(object):
    """
    True/False tests:  True means there are items still remaining to be used
    """
    def __init__(self, iterator):
        self._iter = iter(iterator)
        self._get_next_value()
    def __next__(self):
        value = self._next_value
        self._get_next_value()
        if value is not Empty:
            return value
        raise StopIteration
    next = __next__                     # python 2.x
    def __bool__(self):
        return self._next_value is not Empty
    __nonzero__ = __bool__              # python 2.x
    def _get_next_value(self):
        self._next_value = next(self._iter, Empty)

In your code:

csvfile = open('file.csv', 'r')

reader = BooleanIterator(csv.DictReader(csvfile, fieldnames))

for idx, row in enumerate(reader, start=1):
    if reader:
        # not the last row
        # process normally
    else:
        # this is the last row!

For more examples of iterators see this answer.

Last element of csv reader in Python

3 Answers3