Writing files in Python

Pure Python

The most basic way to write files in Python is to simply open a file with write access:

f = open('file.txt', 'wb')

and to then call the write method to write to the file:

f.write("Hello World")

Line returns have to be explicitly included using \n:

f.write("Line 1\n")
f.write("line 2\n")

And files should be closed with:


The best way to write out variables with this technique is to use string formatting which is described in more detail here. The basic command to format variables into a string is:

format % variables

where format is a string containing the format statements and variables is a tuple of the values, for example:

>>> print "%s %5.2f %10.4e" % ("name", 3.4, 1.e-10)
name  3.40 1.0000e-10

We can use this when writing out files, so if we have two lists or arrays of values a and b we can do:

a = [1,2,3,4,5]
b = [2,6,4,3,2]

f = open('file.txt', 'wb')
for i in range(len(a)):
    f.write("%i %5.2f\n" % (a[i], b[i]))

which will produce a file containing:

1  2.00
2  6.00
3  4.00
4  3.00
5  2.00


Numpy provides a function called savetxt that makes it easy to write out arrays to files. Given two lists or arrays a and b as above, one can simply do:

import numpy as np
a = [1,2,3,4,5]
b = [2,6,4,3,2]
np.savetxt('file_numpy.txt', zip(a, b), fmt="%i %5.2f")

which produces exactly the same output as above and avoids the for loop.