How to write binary and asci data into a file in python?

Question

I have this wired protocol I am implementing and I have to write binary and ASCII data into the same file, how can I do this at the same time or at least the result in the end will be the file with mixed asci and binary data ?

I know that open("myfile", "rb") does open myfile in the binary mode, except that I can't find a solution how to go about this!

You can just write ASCII data to a binary file still. Are you on Python 2 or 3 here? — Martijn Pieters
– Martijn Pieters, Commented May 21, 2014 at 11:25
Then just write your ASCII data directly. The only difference between w and wb is how newlines are translated. — Martijn Pieters
– Martijn Pieters, Commented May 21, 2014 at 11:29
The end file will have ASCII and the binary data, the real question is how can the other program on the other part read the binary and the ASCII and see the difference ? Do you see what I mean here ? — e-nouri
– e-nouri, Commented May 21, 2014 at 11:38
That's a different question altogether. That's a file format design question. How will the other side know what the binary data means? ASCII is just binary data too; it's all zeros and ones in the end. — Martijn Pieters
– Martijn Pieters, Commented May 21, 2014 at 11:39

Martijn Pieters · Accepted Answer · 2014-05-21 11:31:32Z

1

Under Python 2, the only difference binary mode makes is how newlines are translated when writing; \n would be translated to the platform-dependant line separator.

In other words, just write your ASCII byte strings directly to your binary file, there is no difference between your ASCII data and the binary data as far as Python is concerned.

answered May 21, 2014 at 11:31

Martijn Pieters

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James Mills · Accepted Answer · 2014-05-21 11:29:28Z

1

What you write to a file is in fact "bytes". Python 2 or 3 (*Just that in Python2 it was str and we changed this to be more clear and explicit in Python 3 to bytes).

So:

with open("file.ext", "w") as f:
    f.write(b"some bytes")

Example:

bash-4.3$ python
Python 2.7.6 (default, Apr 28 2014, 00:50:45) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> with open("file.ext", "w") as f:
...     f.write(b"some bytes")
... 
>>> 
bash-4.3$ cat file.ext
some bytesbash-4.3$

Normally you would use an encoding if you are dealing with Unicode strings (str in Python 3, unicode in Python 2). e.g:

s = "Hello World!"

with open("file.ext", "w") as f:
    f.write(s.encode("utf-8"))

Note: As mentioned in the comments; open(filename, "wb") doesn't really do what you think it does - it just affects how newlines are treated.

answered May 21, 2014 at 11:29

James Mills

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1 Comment

e-nouri Over a year ago

Thanks for the clarification @James, the "wb" only affects how the system handle new lines. I up voted your answer!

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