Exporting 4 byte floats with Python

Question

I would like to export floating point numbers with a Python script into a binary file, and read the binary file with a C++ program, and interpret the bytes as floats. (On x86/IEEE754 machines. Without conversion. Just read data from a file and call it a float array.)

If I understand correctly Python uses floats that are the same as double in C. So 8 bytes. Is there a way to make this work? To export 4 byte floats from Python that will be the same representation as 4 byte floats in C++?

What Python actually uses is an implementation detail. CPython, though, will use whatever your machine's native double type is. — chepner
– chepner, Commented Nov 25, 2019 at 16:22

Botje · Accepted Answer · 2019-11-25 16:24:43Z

2

The array Python module allows you to dump your float array in one go:

import array
def dump_numbers(filename, numbers):
    with open(filename, 'wb') as fp:
        array.array('f', numbers).tofile(fp)

answered Nov 25, 2019 at 16:24

Botje

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chepner · Accepted Answer · 2019-11-25 16:21:27Z

2

The struct module takes care of this.

>>> import struct
>>> struct.pack("f", 3.14)  # 4-byte float
b'\xc3\xf5H@'
>>> struct.pack("g", 3.14)  # 8-byte double
b'\x1f\x85\xebQ\xb8\x1e\t@'

See the documentation for more information about how to control the endianness of the result.

answered Nov 25, 2019 at 16:21

chepner

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