numpy.savetxt(fname, X, fmt='%.18e', delimiter=' ', newline='\n', header='', footer='', comments='# ', encoding=None)[source]

Save an array to a text file.


fname : filename or file handle

If the filename ends in .gz, the file is automatically saved in compressed gzip format. loadtxt understands gzipped files transparently.

X : 1D or 2D array_like

Data to be saved to a text file.

fmt : str or sequence of strs, optional

A single format (%10.5f), a sequence of formats, or a multi-format string, e.g. ‘Iteration %d – %10.5f’, in which case delimiter is ignored. For complex X, the legal options for fmt are:

  1. a single specifier, fmt=’%.4e’, resulting in numbers formatted like ‘ (%s+%sj)’ % (fmt, fmt)
  2. a full string specifying every real and imaginary part, e.g. ‘ %.4e %+.4ej %.4e %+.4ej %.4e %+.4ej’ for 3 columns
  3. a list of specifiers, one per column - in this case, the real and imaginary part must have separate specifiers, e.g. [‘%.3e + %.3ej’, ‘(%.15e%+.15ej)’] for 2 columns

delimiter : str, optional

String or character separating columns.

newline : str, optional

String or character separating lines.

New in version 1.5.0.

header : str, optional

String that will be written at the beginning of the file.

New in version 1.7.0.

footer : str, optional

String that will be written at the end of the file.

New in version 1.7.0.

comments : str, optional

String that will be prepended to the header and footer strings, to mark them as comments. Default: ‘# ‘, as expected by e.g. numpy.loadtxt.

New in version 1.7.0.

encoding : {None, str}, optional

Encoding used to encode the outputfile. Does not apply to output streams. If the encoding is something other than ‘bytes’ or ‘latin1’ you will not be able to load the file in NumPy versions < 1.14. Default is ‘latin1’.

New in version 1.14.0.

See also

Save an array to a binary file in NumPy .npy format
Save several arrays into an uncompressed .npz archive
Save several arrays into a compressed .npz archive


Further explanation of the fmt parameter (%[flag]width[.precision]specifier):


- : left justify

+ : Forces to precede result with + or -.

0 : Left pad the number with zeros instead of space (see width).

Minimum number of characters to be printed. The value is not truncated if it has more characters.
  • For integer specifiers (eg. d,i,o,x), the minimum number of digits.
  • For e, E and f specifiers, the number of digits to print after the decimal point.
  • For g and G, the maximum number of significant digits.
  • For s, the maximum number of characters.

c : character

d or i : signed decimal integer

e or E : scientific notation with e or E.

f : decimal floating point

g,G : use the shorter of e,E or f

o : signed octal

s : string of characters

u : unsigned decimal integer

x,X : unsigned hexadecimal integer

This explanation of fmt is not complete, for an exhaustive specification see [R285].


[R285](1, 2) Format Specification Mini-Language, Python Documentation.


>>> x = y = z = np.arange(0.0,5.0,1.0)
>>> np.savetxt('test.out', x, delimiter=',')   # X is an array
>>> np.savetxt('test.out', (x,y,z))   # x,y,z equal sized 1D arrays
>>> np.savetxt('test.out', x, fmt='%1.4e')   # use exponential notation

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