csv: more precise types, remove TODO (#3581)

Author: hauntsaninja

stdlib/2and3/_csv.pyi

@@ -1,6 +1,6 @@
 import sys
 
-from typing import Any, Iterable, Iterator, List, Optional, Sequence, Text
+from typing import Any, Iterable, Iterator, List, Optional, Protocol, Sequence, Text, Union
 
 QUOTE_ALL: int
 QUOTE_MINIMAL: int
@@ -39,9 +39,10 @@ class _writer:
         def writerows(self, rows: Iterable[Sequence[Any]]) -> None: ...
 
 
-# TODO: precise type
-def writer(csvfile: Any, dialect: Any = ..., **fmtparams: Any) -> _writer: ...
-def reader(csvfile: Iterable[Text], dialect: Any = ..., **fmtparams: Any) -> _reader: ...
+class _Writer(Protocol):
+    def write(self, s: str) -> Any: ...
+def writer(csvfile: _Writer, dialect: Union[Dialect, str] = ..., **fmtparams: Any) -> _writer: ...
+def reader(csvfile: Iterable[Text], dialect: Union[Dialect, str] = ..., **fmtparams: Any) -> _reader: ...
 def register_dialect(name: str, dialect: Any = ..., **fmtparams: Any) -> None: ...
 def unregister_dialect(name: str) -> None: ...
 def get_dialect(name: str) -> Dialect: ...