Writing then reading in-memory bytes (BytesIO) gives a blank result

Question

I wanted to try out the python BytesIO class.

As an experiment I tried writing to a zip file in memory, and then reading the bytes back out of that zip file. So instead of passing in a file-object to gzip, I pass in a BytesIO object. Here is the entire script:

from io import BytesIO
import gzip

# write bytes to zip file in memory
myio = BytesIO()
with gzip.GzipFile(fileobj=myio, mode='wb') as g:
    g.write(b"does it work")

# read bytes from zip file in memory
with gzip.GzipFile(fileobj=myio, mode='rb') as g:
    result = g.read()

print(result)

But it is returning an empty bytes object for result. This happens in both Python 2.7 and 3.4. What am I missing?

Solution 1

You need to seek back to the beginning of the file after writing the initial in memory file...

myio.seek(0)

Solution 2

How about we write and read gzip content in the same context like this?

#!/usr/bin/env python

from io import BytesIO
import gzip

content = b"does it work"

# write bytes to zip file in memory
gzipped_content = None
with BytesIO() as myio:
    with gzip.GzipFile(fileobj=myio, mode='wb') as g:
        g.write(content)
    gzipped_content = myio.getvalue()

print(gzipped_content)
print(content == gzip.decompress(gzipped_content))

Comments

[TomTom101]

Buffers filled by matplotlib savefig() also do need this before they can be sent by an application server. Thanks for ending hours of research!

[dz902]

Or use getvalue(). These little things are scattered all over the place!

[caot]

getvalue() didn't work for me except the seek(0)

[juan]

I don't think g.close() is needed here, right?

[Gatsby Lee]

@juan you're right. since it is in context, g.close() is not necessary. I removed it. Thank you.