File I/O

NumPy provides functions for reading and writing numeric data to simple files in a regular column layout.

These I/O functions offer a very convenient way to load and store data in a human-readable format. Comments and column delimiters are handled automatically, so usually one can read any data file in a column layout into a NumPy array.

Assume we have the following measurement data in a file called xy-coordinates.dat. As
you can see it also contains an invalid data point that is commented out as
well as one data point with an undefined value (nan).

# x y
 -5.000000 25.131953
 -3.888889 15.056032
 -2.777778 7.261712
# -1.666667 -99999 << invalid data!
 -0.555556 -0.141217
 0.555556 0.176612
 1.666667 2.833694
 2.777778 nan
 3.888889 14.979309
 5.000000 25.299547

One can read the data into a NumPy array with a single loadtxt() function
call:

xy = numpy.loadtxt('xy-coordinates.dat')

print(xy)
# output:
# [[ -5. 25.131953]
# [ -3.888889 15.056032]
# [ -2.777778 7.261712]
# [ -0.555556 -0.141217]
# [ 0.555556 0.176612]
# [ 1.666667 2.833694]
# [ 2.777778 nan]
# [ 3.888889 14.979309]
# [ 5. 25.299547]]

Comment lines are stripped away (both the header as well as the invalid data
row) and the undefined value (nan) is automatically recognised. The datatype
of the NumPy array is also automatically chosen based on the values.

If we want to write the data back to another file, this can be done with the
writetxt() function. One can also format the output e.g. by providing a
header comment (header) or by defining the number format (fmt) or column
delimiter (delimiter).

args = {
 'header': 'XY coordinates',
 'fmt': '%7.3f',
 'delimiter': ','
}
numpy.savetxt('output.dat', xy, **args)

If we look into the output file, we can see that data has been written in a
nicely formatted column layout with the header we provided:

# XY coordinates
 -5.000, 25.132
 -3.889, 15.056
 -2.778, 7.262
 -0.556, -0.141
 0.556, 0.177
 1.667, 2.834
 2.778, nan
 3.889, 14.979
 5.000, 25.300