Writing to a binary file python

Question

I want to write something to a binary file using python.

I am simply doing:

import numpy as np

f = open('binary.file','wb')
i=4
j=5.55
f.write('i'+'j') #where do i specify that i is an integer and j is a double?

g = open('binary.file','rb')
first = np.fromfile(g,dtype=np.uint32,count = 1)
second = np.fromfile(g,dtype=np.float64,count = 1)

print first, second

The output is just: [] []

I know it is very easy to do this in Matlab "fwrite(binary.file, i, 'int32');", but I want to do it in python.

You do not write 4 and 5.55 into the file. You write 105 (the ASCII code of 'i') and 106 (the ASCII code of 'j'). — DYZ
– DYZ, Commented Mar 31, 2017 at 15:13
The line f.write('i'+'j') is writing the string 'ij' to a file. You will want to use struct.pack in order to properly encode your data as binary. — Random Davis
– Random Davis, Commented Mar 31, 2017 at 15:14
Since you are using numpy.fromfile to load the data, the most natural thing to do is use numpy.ndarray.tofile to store the data. (But note that the docs recommend using numpy.save and numpy.load instead.) — John Y
– John Y, Commented Mar 31, 2017 at 15:20

Community · Accepted Answer · 2017-05-23 12:02:02Z

You appear to be having some confusion about types in Python.

The expression 'i' + 'j' is adding two strings together. This results in the string ij, which is most likely written to the file as two bytes.

The variable i is already an int. You can write it to a file as a 4-byte integer in a couple of different ways (which also apply to the float j):

Use the struct module as detailed in how to write integer number in particular no of bytes in python ( file writing). Something like this:
```
import struct
with open('binary.file', 'wb') as f:
    f.write(struct.pack("i", i))
```
You would use the 'd' specifier to write j.
Use the numpy module to do the writing for you, which is especially convenient since you are already using it to read the file. The method ndarray.tofile is made just for this purpose:
```
i = 4
j = 5.55
with open('binary.file', 'wb') as f:
    np.array(i, dtype=np.uint32).tofile(f)
    np.array(j, dtype=np.float64).tofile(f)
```

Note that in both cases I use open as a context manager when writing the file with a with block. This ensures that the file is closed, even if an error occurs during writing.

Artagel · Accepted Answer · 2017-03-31 16:05:18Z

-1

That's because you are trying to write a string(edited) into a binary file. You also don't close the file before trying to read it again.
If you want to write ints or strings to a binary file try adding the below code:

import numpy as np
import struct

f = open('binary.file','wb')
i = 4
if isinstance(i, int):
    f.write(struct.pack('i', i)) # write an int
elif isinstance(i, str):
    f.write(i) # write a string
else:
    raise TypeError('Can only write str or int')

f.close()

g = open('binary.file','rb')
first = np.fromfile(g,dtype=np.uint32,count = 1)
second = np.fromfile(g,dtype=np.float64,count = 1)

print first, second

I'll leave it to you to figure out the floating number.

print first, second
[4] []

The more pythonic file handler way:

import numpy as np
import struct

with open ('binary.file','wb') as f:
    i = 4
    if isinstance(i, int):
        f.write(struct.pack('i', i)) # write an int
    elif isinstance(i, str):
        f.write(i) # write a string
    else:
        raise TypeError('Can only write str or int')

with open('binary.file','rb') as g:
    first = np.fromfile(g,dtype=np.uint32,count = 1)
    second = np.fromfile(g,dtype=np.float64,count = 1)

print first, second

edited Mar 31, 2017 at 16:05

answered Mar 31, 2017 at 15:16

Artagel

3961 silver badge12 bronze badges

7 Comments

Random Davis Over a year ago

Might it be better to use the with open('binary.file', 'wb') as f: syntax in this example to establish better practices?

Artagel Over a year ago

Sure, it solves the closing file issue. I was trying to keep it as close to the original code as possible. Edit: added with open...

David Nemeskey Over a year ago

I would either use Numpy's tofile and fromfile in pairs, or the struct module for both reading and writing. Also, he wasn't writing a sum to the file, but 'ij' as a string. Also, I am not sure this isn't a duplicate of e.g. stackoverflow.com/questions/37129257/….

Artagel Over a year ago

You are right David, I missed he had '' around the vars.

Mad Physicist Over a year ago

What about j?

|

Collectives™ on Stack Overflow

Writing to a binary file python

2 Answers 2

Comments

7 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

7 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related