csv writer in Python with custom quoting

问题:

I'm looking for a way to define custom quoting with csv.writer in Python. There are 4 built-in ways to qoute values:

csv.QUOTE_ALL, csv.QUOTE_MINIMAL, csv.QUOTE_NONNUMERIC, csv.QUOTE_NONE

However I need a quoting mechanism which will emulate Postgres' FORCE QUOTE *, i.e. it will quote all non-None values. With csv.QUOTE_ALL Python will turn None into '' but I would like to have empty string instead.

Is it possible to do that with built-in csv module ( I'm not interested in hacks, I'm already doing that :P )? Or am I forced to write/get some custom csv parser?

And generally: is it possible to write custom quoting mechanism for csv module?

回答1:

Disable csv quoting and add the quotes yourself:

def quote(col):
    if col is None:
        return ''
    # uses double-quoting style to escape existing quotes
    return '"{}"'.format(str(col).replace('"', '""'))

writer = csv.writer(fileobj, quoting=csv.QUOTE_NONE, escapechar='', quotechar='')

for row in rows:
    writer.writerow(map(quote, row))

By setting both escapechar and quotechar to empty strings you avoid the module quoting your already-quoted values.

The above works as long as you don't use the delimiter in the csv values.

Note that by this time it would just be easier to write comma-delimited lines yourself:

with open(filename, 'w'), fd:
    for row in rows:
        fd.write(','.join(map(quote, row)) + '\r\n')

回答2:

I've written my own csv writer which does exactly what I want:

class PostgresCSVWriter(object):
    def __init__(self, stream, quotechar="\"", delimiter=",", escapechar="\\"):
        self.stream = stream
        self.quotechar = quotechar
        self.delimiter = delimiter
        self.escapechar = escapechar
        self.buffer_size = 16384

    def _convert_value(self, obj):
        if obj is None:
            return ""
        value = str(obj)
        value = value.replace(self.quotechar, self.quotechar+self.quotechar)
        value = value.replace(self.delimiter, self.escapechar+self.delimiter)
        return self.quotechar+value+self.quotechar

    def _convert_row(self, row):
        return self.delimiter.join(self._convert_value(v) for v in row) + "\r\n"

    def writerow(self, row):
        self.stream.write(self._convert_row(row))

    def writerows(self, rows):
        data = ""
        counter = 0
        for row in rows:
            buf = self._convert_row(row)
            data += buf
            counter += len(buf)
            if counter >= self.buffer_size:
                self.stream.write(data)
                data = ""
                counter = 0

        if data:
            self.stream.write(data)

If anyone sees any problem with it, then please let me know. I'm still looking for a solution with csv module though.