samredai commented on a change in pull request #4021:
URL: https://github.com/apache/iceberg/pull/4021#discussion_r798082837
##########
File path: python/src/iceberg/io/base.py
##########
@@ -24,7 +24,40 @@
"""
from abc import ABC, abstractmethod
-from typing import Union
+from typing import Protocol, Union, runtime_checkable
+
+
+@runtime_checkable
+class InputStream(Protocol):
+ def read(self, n: int) -> bytes:
+ ...
Review comment:
These are the equivalent of
[SeekableInputStream](https://github.com/apache/iceberg/blob/master/python_legacy/iceberg/api/io/seekable_input_stream.py#L19)
and
[PositionOutputStream](https://github.com/apache/iceberg/blob/master/python_legacy/iceberg/api/io/position_output_stream.py#L19)
in the legacy python client. It's just less strict here in that any class with
the same structure is automatically a subclass.
I think this highlights the behavior:
```py
from typing import Protocol, runtime_checkable
@runtime_checkable
class A(Protocol):
def foo(self):
...
def bar(self):
...
class B:
def foo(self):
...
def bar(self):
...
class C:
def foo(self):
...
def bar(self):
...
def baz(self):
...
class D:
def foo(self):
...
isinstance(B(), A) # True
isinstance(C(), A) # True
isinstance(D(), A) # False
```
So an input file or output file class could have more methods, as long as it
has the required methods set in the protocol which should be the case for any
file-like object that's based on
[RawIOBase](https://docs.python.org/3/library/io.html#io.RawIOBase) (inherits
from IOBase and adds a `read`, `readall`, `readinto`, and `write` method).
The reason I renamed a few of the methods from the legacy client was also to
be in-line with `RawIOBase`. Here's an example of how urllibs HTTPResponse
object matches this protocol:
```py
import urllib.request
response = urllib.request.urlopen('http://python.org/')
isinstance(response, InputStream) # True
print(dir(response))
```
output:
```
[...
'begin',
'chunk_left',
'chunked',
'close',
'closed',
'code',
'debuglevel',
'detach',
'fileno',
'flush',
'fp',
'getcode',
'getheader',
'getheaders',
'geturl',
'headers',
'info',
'isatty',
'isclosed',
'length',
'msg',
'peek',
'read',
'read1',
'readable',
'readinto',
'readinto1',
'readline',
'readlines',
'reason',
'seek',
'seekable',
'status',
'tell',
'truncate',
'url',
'version',
'will_close',
'writable',
'write',
'writelines']
```
One of the remaining questions though is where do we set this `isinstance`
check, or do we do it at all? The rest of the library will be assuming this
interface so do we leave it to every `InputFile` and `OutputFile`
implementations to do an `isinstance` check? Or do we do the `isinstance` check
in other parts of the library wherever `FileIO.new_input_file(...)` is called,
for example. This feels like it might be a tradeoff to this approach in
addition to lack of python 3.7 support.
(sorry for the super long comment)
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