Python 3.14+iofunction

io.Reader

protocols

The io.Reader[T] protocol: an abstract class with a read() method, used for annotations and structural isinstance/issubclass checks. Python 3.14+.

Syntax

io.Reader[T]

It landed only in Python 3.14 — on 3.13 and earlier touching io.Reader raises AttributeError, so portable code still falls back on older annotations such as IO[str]. The check is structural: isinstance only looks for a read() method, and the type parameter is not verified at runtime, so Reader[str] and Reader[bytes] are indistinguishable there.

Examples

import io
r = io.StringIO('hello world')
print(isinstance(r, io.Reader))    # → True
print(r.read(5))                   # → hello
print(isinstance(42, io.Reader))   # → False
print(io.Reader[str])              # → io.Reader[str]

See also

Common errors

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