Mutable vs Immutable Objects
Python objects are either mutable or immutable:
| Mutable | Immutable |
|---|---|
bytearray, list, dict, set |
str, bytes, int, tuple |
Key slicing behaviour:
- A slice of an immutable object (e.g.
bytes) always creates a new copy. - A slice of a mutable object also creates a new copy by default — unless you use
memoryview.
Memory View
memoryview exposes the underlying buffer of an object without copying it. This is useful for working with large binary data efficiently.
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buf = bytearray(b"hello world")
mv = memoryview(buf)
mv2 = mv[6:11] # zero-copy slice — still refers to buf[6:11]
mv2.tobytes() # => b"world" (copy only happens here)
A common use case is writing directly into a pre-allocated buffer:
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buf = bytearray(1024)
view = memoryview(buf)[100:] # no copy
n = sock.recv_into(view) # writes directly into buf[100:100+n]
- A
memoryviewof a mutable object is writable. - A
memoryviewof an immutable object is read-only.
Advanced: Casting to Other Types
You can reinterpret the raw bytes as a different element type using .cast(). For example, treating raw bytes as 32-bit integers:
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buf = bytearray(b'\x01\x00\x00\x00\x02\x00\x00\x00')
mv = memoryview(buf).cast('I') # unsigned 32-bit ints
list(mv) # => [1, 2]
