Open the file in binary mode with "rb" and call read() to load its contents as a bytes object. Use bytearray if you need to change individual bytes.
Read the entire file with open()
Use a context manager so Python closes the file when the block ends:
with open("data.bin", "rb") as file:
data = file.read()
data contains the file’s raw contents as a bytes object. The "b" in "rb" selects binary mode, so Python does not decode the contents into text. This is appropriate for arbitrary binary data and for cases where you deliberately need the underlying bytes of a text-formatted file. Python’s open() documentation describes the file modes.
Use pathlib for a concise whole-file read
If you do not need to keep an open file object for other operations, Path.read_bytes() reads and returns the file contents as bytes:
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from pathlib import Path
data = Path("data.bin").read_bytes()
The method opens and closes the file for the operation. See the Path.read_bytes() documentation.
Choose between bytes and bytearray
Although “byte array” is often used informally, Python’s ordinary binary file reads return bytes, an immutable sequence. If your code needs to modify byte values, convert the result to bytearray:
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mutable_data = bytearray(data)
mutable_data[0] = 0x41
That example changes the first byte to the value 0x41. The conversion creates mutable byte storage. If an API accepts buffer objects and you only need to access existing binary data without copying it, a memoryview may be suitable. Python documents these binary sequence and buffer types in its standard types reference.
Read a large file in bounded chunks
Calling read() without a size loads the whole file into memory. For a large file, process a chunk at a time instead of retaining the complete contents:
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with open("large.bin", "rb") as file:
while chunk := file.read(64 * 1024):
process(chunk)
Replace process(chunk) with the operation your application needs. The loop ends when the read returns an empty bytes object at end of file. A sized stream read requests up to the specified number of bytes; it may return fewer, so do not assume each read fills the requested size. For stream details, see Python’s io documentation.
When the complete contents are still needed
You can collect chunks and join them after reading, but this still requires memory for the complete result:
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chunks = []
with open("large.bin", "rb") as file:
while chunk := file.read(64 * 1024):
chunks.append(chunk)
data = b"".join(chunks)
Fill a reusable mutable buffer with readinto()
When you want to reuse a preallocated writable buffer, use readinto(). It reads into a bytes-like object such as a bytearray and returns the number of bytes read:
buffer = bytearray(64 * 1024)
with open("large.bin", "rb") as file:
while count := file.readinto(buffer):
process(memoryview(buffer)[:count])
Only the first count bytes of the buffer contain data from that read; the final read may be shorter than the buffer. Consult the stream API documentation for readinto().
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Decode the data separately if you need text
Binary mode returns bytes without decoding. If the file is actually text and your next step is text processing, decode the bytes using the file’s known encoding, for example data.decode("utf-8") when UTF-8 is the correct encoding. Do not assume arbitrary binary contents are valid text.
Use an in-memory binary stream when needed
If you already have bytes in memory but need an object that behaves like a binary file, wrap them in io.BytesIO. It provides a file-like binary stream around bytes data; it is not needed for a straightforward disk-file read. See the BytesIO documentation.
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