使用if条件列出理解

Question

The following line of Python code

values = ", ".join(["\"%s\"" % x for x in row])

takes a list of elements in "row" to create a comma-separated string "values", while each value is put in double quotes, eg: "New York", "5", "", "3.2"

However, since the result is to be part of a mysqldump file, empty fields ("") should become NULL (without double quotes).

Therefore, I'd like to learn how to change that expression so that it is checked whether x is empty (""), in which case NULL should be appended to the values string.

Thanks

Answer 1

values = ", ".join('"%s"' % x if x else 'NULL' for x in row)

For example:

>>> row = ["foo", "", "bar"]
>>> values = ", ".join('"%s"' % x if x else 'NULL' for x in row)
>>> values
'"foo", NULL, "bar"'

Thanks to DSM for pointing out that it's of course even better to change the list comprehension into a generator expression - this makes it faster because it's no longer necessary to construct a list that will be discarded immediately afterwards.

Answer 2

values = ", ".join(["NULL" if x == "" else "\"%s\"" % x for x in row])

Answer 3

To make this a bit more robust, I'd utilise the csv module to handle embedded field and string delimiters... (just an adhoc example)

from StringIO import StringIO
import csv

csvrec = StringIO()
csvout = csv.writer(csvrec, quoting=csv.QUOTE_ALL)
items = [1, 'hello, there, how are you?', 3.2, 'it\'s a "cat" I say!', '', '']
csvout.writerow([col or '\t' for col in items]) # maybe col != '' ?
print csvrec.getvalue().replace('"\t"', 'NULL').strip()
# "1","hello, there, how are you?","3.2","it's a ""cat"" I say!",NULL,NULL