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

stackoverflow.com › questions › 26879981 › writing-then-reading-in-memory-bytes-bytesio-gives-a-blank-result

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

myio.seek(0)

Answer from mgilson on Stack Overflow

Python

docs.python.org › 3 › library › io.html

io — Core tools for working with streams

Its subclasses, BufferedWriter, BufferedReader, and BufferedRWPair buffer raw binary streams that are writable, readable, and both readable and writable, respectively. BufferedRandom provides a buffered interface to seekable streams. Another BufferedIOBase subclass, BytesIO, is a stream of in-memory bytes.

DigitalOcean

digitalocean.com › community › tutorials › python-io-bytesio-stringio

Python io.BytesIO and io.StringIO: Memory File Guide | DigitalOcean

August 3, 2022 - There are many ways in which we can use the io module to perform stream and buffer operations in Python. We will demonstrate a lot of examples here to prove the point. Let’s get started. Just like what we do with variables, data can be kept as bytes in an in-memory buffer when we use the io module’s Byte IO operations. Here is a sample program to demonstrate this: import io stream_str = io.BytesIO(b"JournalDev Python: \x00\x01") print(stream_str.getvalue())

Stack Overflow

stackoverflow.com › questions › 26879981 › writing-then-reading-in-memory-bytes-bytesio-gives-a-blank-result

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

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1 of 3

The issue is that you are positioned at the end of the stream. Think of the position like a cursor. Once you have written b' world', your cursor is at the end of the stream. When you try to .read(), you are reading everything after the position of the cursor - which is nothing, so you get the empty bytestring.

To navigate around the stream you can use the .seek method:

>>> import io
>>> in_memory = io.BytesIO(b'hello', )
>>> in_memory.write(b' world')
>>> in_memory.seek(0)  # go to the start of the stream
>>> print(in_memory.read())
b' world'

Note that, just like a filestream in write ('w') mode, the initial bytes b'hello' have been overwritten by your writing of b' world'.

.getvalue() just returns the entire contents of the stream regardless of current position.

2 of 3

this is a memory stream but still a stream. The position is stored, so like any other stream if you try to read after having written, you have to re-position:

import io
in_memory = io.BytesIO(b'hello')
in_memory.seek(0,2)   # seek to end, else we overwrite
in_memory.write(b' world')
in_memory.seek(0)    # seek to start
print( in_memory.read() )

prints:

b'hello world'

while in_memory.getvalue() doesn't need the final seek(0) as it returns the contents of the stream from position 0.

GeeksforGeeks

geeksforgeeks.org › python › convert-from-_io-bytesio-to-a-bytes-like-object-in-python

Convert from '_Io.Bytesio' to a Bytes-Like Object in Python - GeeksforGeeks

July 23, 2025 - This method reads and returns the underlying byte data stored in the BytesIO buffer.

Beautiful Soup

tedboy.github.io › python_stdlib › generated › generated › io.BytesIO.html

io.BytesIO — Python Standard Library

class io.BytesIO¶ · Create a buffered I/O implementation using an in-memory bytes buffer, ready for reading and writing.

GeeksforGeeks

geeksforgeeks.org › python › stringio-and-bytesio-for-managing-data-as-file-object

Stringio And Bytesio For Managing Data As File Object - GeeksforGeeks

July 24, 2025 - BytesIO is like a virtual file ... data (bytes) instead of text. It lets you perform operations on these bytes, such as reading and writing, as if you were interacting with a regular file....

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Andrew Wheeler

andrewpwheeler.com › 2022 › 11 › 02 › using-io-objects-in-python-to-read-data

Using IO objects in python to read data | Andrew Wheeler

November 2, 2022 - # Example 2, grabbing zipped contents import zipfile from io import BytesIO census_url = 'https://www2.census.gov/programs-surveys/acs/summary_file/2019/data/2019_5yr_Summary_FileTemplates.zip' req = requests.get(census_url) # Can use BytesIO for this content zf = zipfile.ZipFile(BytesIO(req.content)) The zipfile library would be equivalent to reading/extracting a zipfile already on disk.

Stack Overflow

stackoverflow.com › questions › 59365055 › convert-file-into-bytesio-object-using-python

Convert file into BytesIO object using python - Stack Overflow

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1 of 1

Opening a file with open and mode read-binary already gives you a Binary I/O object.

Documentation:

The easiest way to create a binary stream is with open() with 'b' in the mode string:

f = open("myfile.jpg", "rb")

So in normal circumstances, you'd be fine just passing the file handle wherever you need to supply it. If you really want/need to get a BytesIO instance, just pass the bytes you've read from the file when creating your BytesIO instance like so:

from io import BytesIO

with open(filepath, "rb") as fh:
    buf = BytesIO(fh.read())

This has the disadvantage of loading the entire file into memory, which might be avoidable if the code you're passing the instance to is smart enough to stream the file without keeping it in memory. Note that the example uses open as a context manager that will reliably close the file, even in case of errors.

Runebook.dev

runebook.dev › en › docs › python › library › io › io.BytesIO

python - BytesIO Gotchas and Workarounds: Fixing Empty Reads and TypeErrors

import io buffer = io.BytesIO(b"Data to read twice.") # First read (cursor moves to end) first_read = buffer.read() print(f"1st read: {first_read}") # Second read attempt (cursor is still at the end) second_read = buffer.read() print(f"2nd read (empty): {second_read}") # Fix: Move the cursor back to the beginning buffer.seek(0) # Third read (works now!) third_read = buffer.read() print(f"3rd read (fixed): {third_read}") io.BytesIO strictly handles bytes. You cannot directly write regular Python strings (str) to it.

Code-learner

code-learner.com › home › python stringio and bytesio example

Python StringIO And BytesIO Example ·

March 21, 2021 - Python StringIO and BytesIO are methods that manipulate string and bytes data in memory, this makes memory data manipulation use the consistent API as read and write files. StringIO is used to operate string data, and if you want to manipulate binary data, you need to use BytesIO.

Learning Machine

gallon.me › using-the-bytesio-class-in-python.html

Using the BytesIO Class in Python - Learning Machine

February 14, 2025 - ```python # Writing binary data to a file: with open('output.bin', 'wb') as f: f.write(b'\x00\x01\x02') # Reading binary data from a file: with open('output.bin', 'rb') as f: data = f.read() print(data) # Output: b'\x00\x01\x02' ``` In-memory File-like Object: BytesIO acts like a file that exists in memory.

GitHub

github.com › python › cpython › blob › main › Modules › _io › bytesio.c

cpython/Modules/_io/bytesio.c at main · python/cpython

BytesIO objects are not connected to a TTY-like device. ... Current file position, an integer. ... Read at most size bytes, returned as a bytes object.

Author python

Python Assets

pythonassets.com › posts › what-is-io-bytesio-useful-for

What Is `io.BytesIO` Useful For? | Python Assets

July 19, 2024 - This means you can read from and write to it just like a file, but without creating any actual files on disk. In Python, file-like objects are objects that implement methods like read(), write(), and seek(), allowing you to interact with data ...

ProgramCreek

programcreek.com › python › example › 1734 › io.BytesIO

Python Examples of io.BytesIO

:rtype : ``list`` ''' decoder = avro.io.BinaryDecoder(io.BytesIO(rawbytes)) reader = avro.io.DatumReader(avro.schema.parse(AVSC)) try: return reader.read(decoder)[list.__name__] except Exception as exc: logging.getLogger('SPOT.INGEST.COMMON.SERIALIZER')\ .error('[{0}] {1}'.format(exc.__class__.__name__, exc.message)) return []

AskPython

askpython.com › home › python io module: the complete practical reference

Python IO Module: The Complete Practical Reference - AskPython

February 16, 2023 - In this article, we learned about using the Python IO module, and it’s two main classes – io.BytesIO and io.StringIO for reading and writing byte and string data onto a buffer.

Python.org

discuss.python.org › ideas

Add search methods from bytes/bytearray objects to io.BytesIO - Ideas - Discussions on Python.org

November 5, 2022 - I worked heavily with BytesIO objects in one of my applications, and recently I thought that it would be useful to be able to look through the object for specific values. For this I did something like: file.read().find(text), which I think is creating a copy of the entire bytes object.

GeeksforGeeks

geeksforgeeks.org › python › python-stringio-and-bytesio-compared-with-open

Python Stringio and Bytesio Compared With Open() - GeeksforGeeks

July 23, 2025 - In this example, below code shows how to use BytesIO to handle binary data in memory, first by writing binary data to a BytesIO object, then by reading the data using open() to write it to a temporary binary file (temp_file.bin).