Save list to csv with two decimal digits in python?

Question

I have data which looks like

[('A',5.3748),('B',8.324203),('C',3.492)]

I'm saving it to a csv with the following lines:

with open('output.csv','wb') as fp:
    a = csv.writer(fp,delimiter = ",")
    a.writerows(data)

Works like a charm except that I would like to display my values with only two decimal digits. Earlier in my code I zip two lists to get the data. So I could make the change before if need be.

Thanks!

Answer 1

If you can isolate the number you want to round, you can format it to two decimal places and store it as a string like this:

"%.2f" % myNum

Answer 2

You can use the string formatting syntax:

with open('output.csv','wb') as fp:
    a = csv.writer(fp,delimiter = ",")
    a.writerows(map(lambda t: (t[0], "%.2f" % t[1]), data))

The trick is here:

>> "%.2f" % a_number

will print the number with two decimal digits, while

map(lambda t: (t[0], "%.2f" % t[1]), data)

will apply this transformation to every tuples in the list data. (see http://docs.python.org/2/library/functions.html#map)

Answer 3

from itertools import imap
with open('output.csv','wb') as fp:
  a = csv.writer(fp,delimiter = ",")
  a.writerows(imap(lambda x: (x[0], round(x[1], 2)), data))

Answer 4

Try formatting your decimals using str.format():

temp = [('A',5.3748),('B',8.324203),('C',3.492)]
newlist = []
for a, b in temp:
    newlist.append((a, "{0:.2f}".format(b)))

As a list comprehension, you get:

data = [('A',5.3748),('B',8.324203),('C',3.492)]
data = [(a, "{0:.2f}".format(b)) for a, b in temp]

As @JonClements noted, you can replace "{0:.2f}".format(b) with format(b, '.2f'). In this case, that's probably the more readable approach.

Answer 5

You can also use python's round() method.

round(5.3748, 2)

The advantage of using this with a csv writer is that it maintains it's number type instead of being converted to a string. This affects options like quoting non-numericals in the csv file.

If you don't need to worry about such formatting options I personally prefer formatting it to a string, since it aligns the correct number of decimal places even if they are zero. I.e. 5.3748 to "5.3700" instead of 5.37

Answer 6

You can format the list elements as follow

linux:~ # python
>>> l=[('A',5.3748),('B',8.324203),('C',3.492)]
>>> l=[ (x[0], "%.2f" % x[1]) for x in l]
>>> l
[('A', '5.37'), ('B', '8.32'), ('C', '3.49')]