refactor: Writer cleanup

dataclass_io/_lib/file.py

@@ -1,11 +1,47 @@
 from dataclasses import dataclass
+from enum import Enum
+from enum import unique
 from io import TextIOWrapper
 from typing import IO
 from typing import Optional
 from typing import TextIO
 from typing import TypeAlias
 
 ReadableFileHandle: TypeAlias = TextIOWrapper | IO | TextIO
+WritableFileHandle: TypeAlias = TextIOWrapper | IO | TextIO
+
+
+@unique
+class WriteMode(Enum):
+    """
+    The mode in which to open a file for writing.
+
+    Attributes:
+        value: The mode.
+        abbreviation: The short version of the mode (used with Python's `open()`).
+    """
+
+    value: str
+    abbreviation: str
+
+    def __new__(cls, value: str, abbreviation: str) -> "WriteMode":
+        enum = object.__new__(cls)
+        enum._value_ = value
+
+        return enum
+
+    # NB: Specifying the additional fields in the `__init__` method instead of `__new__` is
+    # necessary in order to construct `WriteMode` from only the value (e.g. `WriteMode("append")`).
+    # Otherwise, `mypy` complains about a missing positional argument.
+    # https://stackoverflow.com/a/54732120
+    def __init__(self, _: str, abbreviation: str = None):
+        self.abbreviation = abbreviation
+
+    WRITE = "write", "w"
+    """Write to a new file."""
+
+    APPEND = "append", "a"
+    """Append to an existing file."""
 
 
 @dataclass(frozen=True, kw_only=True)

dataclass_io/writer.py

@@ -1,61 +1,27 @@
 from csv import DictWriter
 from dataclasses import asdict
-from enum import Enum
-from enum import unique
-from io import TextIOWrapper
 from pathlib import Path
 from types import TracebackType
-from typing import IO
 from typing import Any
 from typing import Iterable
-from typing import TextIO
 from typing import Type
-from typing import TypeAlias
 
 from dataclass_io._lib.assertions import assert_dataclass_is_valid
 from dataclass_io._lib.assertions import assert_fieldnames_are_dataclass_attributes
 from dataclass_io._lib.assertions import assert_file_is_appendable
 from dataclass_io._lib.assertions import assert_file_is_writable
 from dataclass_io._lib.dataclass_extensions import DataclassInstance
 from dataclass_io._lib.dataclass_extensions import fieldnames
-
-WritableFileHandle: TypeAlias = TextIOWrapper | IO | TextIO
-
-
-@unique
-class WriteMode(Enum):
-    """
-    The mode in which to open the file.
-
-    Attributes:
-        value: The mode.
-        abbreviation: The short version of the mode (used with Python's `open()`).
-    """
-
-    value: str
-    abbreviation: str
-
-    def __new__(cls, value: str, abbreviation: str) -> "WriteMode":
-        enum = object.__new__(cls)
-        enum._value_ = value
-
-        return enum
-
-    # NB: Specifying the additional fields in the `__init__` method instead of `__new__` is
-    # necessary in order to construct `WriteMode` from only the value (e.g. `WriteMode("append")`).
-    # Otherwise, `mypy` complains about a missing positional argument.
-    # https://stackoverflow.com/a/54732120
-    def __init__(self, _: str, abbreviation: str = None):
-        self.abbreviation = abbreviation
-
-    WRITE = "write", "w"
-    """Write to a new file."""
-
-    APPEND = "append", "a"
-    """Append to an existing file."""
+from dataclass_io._lib.file import WritableFileHandle
+from dataclass_io._lib.file import WriteMode
 
 
 class DataclassWriter:
+    _dataclass_type: type[DataclassInstance]
+    _fieldnames: list[str]
+    _fout: WritableFileHandle
+    _writer: DictWriter
+
     def __init__(
         self,
         path: Path,
@@ -88,9 +54,9 @@ def __init__(
                 May not be used together with `include_fields`.
 
         Raises:
-            FileNotFoundError: If the input file does not exist.
-            IsADirectoryError: If the input file path is a directory.
-            PermissionError: If the input file is not readable.
+            FileNotFoundError: If the output file does not exist when trying to append.
+            IsADirectoryError: If the output file path is a directory.
+            PermissionError: If the output file is not writable (or readable when trying to append).
             TypeError: If the provided type is not a dataclass.
         """
 
@@ -101,6 +67,7 @@ def __init__(
 
         assert_dataclass_is_valid(dataclass_type)
 
+        self._fieldnames: list[str]
         if include_fields is not None and exclude_fields is not None:
             raise ValueError(
                 "Only one of `include_fields` and `exclude_fields` may be specified, not both."
@@ -122,7 +89,6 @@ def __init__(
 
         self._dataclass_type = dataclass_type
         self._fout = path.open(write_mode.abbreviation)
-
         self._writer = DictWriter(
             f=self._fout,
             fieldnames=self._fieldnames,