In modern versions of Python one can have static type analysis using function annotations, according to PEP 484. This is made easy through the typing module.
Now I'm wondering how I would give a "type hint" towards a "filestream".
def myfunction(file: FILETYPE):
pass
with open(fname) as file:
myfunction(file)
What would I insert as FILETYPE?
Using print(type(file)) returns <class '_io.TextIOWrapper'> which isn't clear at all.
Isn't there a generic "file" type?
You can use typing.IO, typing.TextIO, and typing.BinaryIO to represent different types of I/O streams. To quote the documentation:
сlass typing.IO
class typing.TextIO
class typing.BinaryIOGeneric type
IO[AnyStr]and its subclassesTextIO(IO[str])andBinaryIO(IO[bytes])represent the types of I/O streams such as returned byopen().
typing.IO, typing.TextIO, and typing.BinaryIO is deprecated since version 3.8, will be removed in version 3.12 per typing documentation.
typing.io that mypy would respect (io.IOBase and it's children).I think you want io.IOBase, "[t]he abstract base class for all I/O classes, acting on streams of bytes."
Note that this includes also in-memory streams like io.StringIO and io.BytesIO. Read the documentation on the module io for details.
_io._base and io.base though?io.IOBase is the best type hint towards a "bytestream" and every file-like object that can be created using the standard library is an instance of it. If IOBase does not match your idea of a bytestream or you have a use case where it's not a good type hint you might want to edit your question and explain why._BufferedIOBase which derives from _IOBaseio.BytesIO also inherits from io.IOBase.Either this:
from typing import TextIO # or IO or BinaryIO
def myfunction(file: TextIO ):
pass
OR this
from typing import TYPE_CHECKING
if TYPE_CHECKING:
from typing import TextIO # or IO or BinaryIO
def myfunction(file: 'TextIO'):
pass
The second approach would avoid to import the class during execution. Although python would still have to import TYPE_CHECKING during execution, it is a good practice to avoid importing classes for type hinting only: (1) doesn't get executed (just parsed), and (2) it could avoid cyclic imports.
typeshed has a SupportsRead protocol:
from __future__ import annotations
from typing import TYPE_CHECKING, AnyStr
if TYPE_CHECKING:
from _typeshed import SupportsRead
def takes_readable_str(fo: SupportsRead[str]):
return fo.read()
def takes_readable_bytes(fo: SupportsRead[bytes]):
return fo.read()
def takes_readable_any(fo: SupportsRead[AnyStr]):
return fo.read()
_typeshed should not be used. Is not the module intended only for developers of type checker tools?
if TYPE_CHECKING.
StringIO?