9.1.5 Examples

##
9.1.5 Examples

The simplest example of reading a CSV file:

    import csv
    reader = csv.reader(open("some.csv", "rb"))
    for row in reader:
        print row

Reading a file with an alternate format:

    import csv
    reader = csv.reader(open("passwd", "rb"), delimiter=':', quoting=csv.QUOTE_NONE)
    for row in reader:
        print row

The corresponding simplest possible writing example is:

    import csv
    writer = csv.writer(open("some.csv", "wb"))
    writer.writerows(someiterable)

Registering a new dialect:

    import csv

    csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)

    reader = csv.reader(open("passwd", "rb"), 'unixpwd')

A slightly more advanced use of the reader -- catching and reporting errors:

    import csv, sys
    filename = "some.csv"
    reader = csv.reader(open(filename, "rb"))
    try:
        for row in reader:
            print row
    except csv.Error, e:
        sys.exit('file %s, line %d: %s' % (filename, reader.line_num, e))

And while the module doesn't directly support parsing strings, it can easily be done:

    import csv
    for row in csv.reader(['one,two,three']):
        print row

The `csv` module doesn't directly support reading and writing Unicode, but it is 8-bit-clean save for some problems with ASCII NUL characters. So you can write functions or classes that handle the encoding and decoding for you as long as you avoid encodings like UTF-16 that use NULs. UTF-8 is recommended.

`unicode_csv_reader` below is a generator that wraps `csv.reader` to handle Unicode CSV data (a list of Unicode strings). `utf_8_encoder` is a generator that encodes the Unicode strings as UTF-8, one string (or row) at a time. The encoded strings are parsed by the CSV reader, and `unicode_csv_reader` decodes the UTF-8-encoded cells back into Unicode:

    import csv

    def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs):
        # csv.py doesn't do Unicode; encode temporarily as UTF-8:
        csv_reader = csv.reader(utf_8_encoder(unicode_csv_data),
                                dialect=dialect, **kwargs)
        for row in csv_reader:
            # decode UTF-8 back to Unicode, cell by cell:
            yield [unicode(cell, 'utf-8') for cell in row]

    def utf_8_encoder(unicode_csv_data):
        for line in unicode_csv_data:
            yield line.encode('utf-8')

For all other encodings the following `UnicodeReader` and `UnicodeWriter` classes can be used. They take an additional encoding parameter in their constructor and make sure that the data passes the real reader or writer encoded as UTF-8:

    import csv, codecs, cStringIO

    class UTF8Recoder:
        """
        Iterator that reads an encoded stream and reencodes the input to UTF-8
        """
        def __init__(self, f, encoding):
            self.reader = codecs.getreader(encoding)(f)

        def __iter__(self):
            return self

        def next(self):
            return self.reader.next().encode("utf-8")

    class UnicodeReader:
        """
        A CSV reader which will iterate over lines in the CSV file "f",
        which is encoded in the given encoding.
        """

        def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
            f = UTF8Recoder(f, encoding)
            self.reader = csv.reader(f, dialect=dialect, **kwds)

        def next(self):
            row = self.reader.next()
            return [unicode(s, "utf-8") for s in row]

        def __iter__(self):
            return self

    class UnicodeWriter:
        """
        A CSV writer which will write rows to CSV file "f",
        which is encoded in the given encoding.
        """

        def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
            # Redirect output to a queue
            self.queue = cStringIO.StringIO()
            self.writer = csv.writer(self.queue, dialect=dialect, **kwds)
            self.stream = f
            self.encoder = codecs.getincrementalencoder(encoding)()

        def writerow(self, row):
            self.writer.writerow([s.encode("utf-8") for s in row])
            # Fetch UTF-8 output from the queue ...
            data = self.queue.getvalue()
            data = data.decode("utf-8")
            # ... and reencode it into the target encoding
            data = self.encoder.encode(data)
            # write to the target stream
            self.stream.write(data)
            # empty queue
            self.queue.truncate(0)

        def writerows(self, rows):
            for row in rows:
                self.writerow(row)