Python 3 In Memory Zipfile Error. string argument expected, got 'bytes'

Question

I have the following code to create an in memory zip file that throws an error running in Python 3.

from io import StringIO from pprint import pprint import zipfile in_memory_data = StringIO() in_memory_zip = zipfile.ZipFile( in_memory_data, "w", zipfile.ZIP_DEFLATED, False) in_memory_zip.debug = 3 filename_in_zip = 'test_filename.txt' file_contents = 'asdf' in_memory_zip.writestr(filename_in_zip, file_contents)

To be clear this is only a Python 3 problem. I can run the code fine on Python 2. To be exact I'm using Python 3.4.3. The stack trace is below:

Traceback (most recent call last): File "in_memory_zip_debug.py", line 14, in <module> in_memory_zip.writestr(filename_in_zip, file_contents) File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/zipfile.py", line 1453, in writestr self.fp.write(zinfo.FileHeader(zip64)) TypeError: string argument expected, got 'bytes' Exception ignored in: <bound method ZipFile.__del__ of <zipfile.ZipFile object at 0x1006e1ef0>> Traceback (most recent call last): File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/zipfile.py", line 1466, in __del__ self.close() File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/zipfile.py", line 1573, in close self.fp.write(endrec) TypeError: string argument expected, got 'bytes'

Jareq · Accepted Answer · 2019-07-29 07:20:21Z

78

ZipFile writes its data as bytes, not strings. This means you'll have to use BytesIO instead of StringIO on Python 3.

The distinction between bytes and strings is new in Python 3. The six compatibility library has a BytesIO class for Python 2 if you want your program to be compatible with both.

edited Jul 29, 2019 at 7:20

Jareq

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answered Aug 18, 2015 at 14:29

Wander Nauta

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2 Comments

Dan Loewenherz Over a year ago

This is confusing because the error message from the OP implies the exact opposite problem. He's providing a string value since that's what the error message states is the expected argument type.

Tyrannas Over a year ago

except OP is using writestr method and not write, so it should indeed be a string and not bytes, or am I missing something ?

alexcb · Accepted Answer · 2018-07-18 18:33:56Z

The problem is that io.StringIO() is being used as the memory buffer, when it needs to be io.BytesIO. The error is occurring because the zipfile code is eventually calling the StringIO().Write() with bytes when StringIO expects a string.

Once it's changed to BytesIO(), it works:

from io import BytesIO from pprint import pprint import zipfile in_memory_data = BytesIO() in_memory_zip = zipfile.ZipFile( in_memory_data, "w", zipfile.ZIP_DEFLATED, False) in_memory_zip.debug = 3 filename_in_zip = 'test_filename.txt' file_contents = 'asdf' in_memory_zip.writestr(filename_in_zip, file_contents)

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