I need to write a long list of ints and floats with Python the same way fwrite would do in C - in a binary form.
This is necessary to create input files for another piece of code I am working with.
What is the best way to do this?
2 Answers
You can do this quite simply with the struct module.
For example, to write a list of 32-bit integers in binary:
import struct
ints = [10,50,100,2500,256]
with open('output', 'w') as fh:
data = struct.pack('i' * len(ints), *ints)
fh.write(data)
Will write '\n\x00\x00\x002\x00\x00\x00d\x00\x00\x00\xc4\t\x00\x00\x00\x01\x00\x00'
Comments
Have a look at numpy: numpy tofile:
With the array-method 'tofile' you can write binary-data:
# define output-format
numdtype = num.dtype('2f')
# write data
myarray.tofile('filename', numdtype)
Another way is to use memmaps: numpy memmaps
# create memmap
data = num.memmap('filename', mode='w+', dtype=num.float, offset=myoffset, shape=(my_shape), order='C')
# put some data into in:
data[1:10] = num.random.rand(9)
# flush to disk:
data.flush()
del data
structmodule.