ex 5.3

reader.py

@@ -1,23 +1,33 @@
 import csv
-from typing import Iterable, Optional, Sequence
+from typing import Any, Callable, Iterable, Mapping, Optional, Sequence
+
+
+def convert_csv(
+    lines: Iterable[str],
+    conv: Callable[[Sequence[Any], Sequence[str]], Any],
+    headers: Optional[Sequence[str]] = None,
+) -> Sequence[Any]:
+    rows = csv.reader(lines)
+    if headers is None:
+        headers = next(rows)
+    records = list(map(lambda row: conv(row, headers), rows))
+    return records
 
 
 def csv_as_dicts(
     lines: Iterable[str], types: Sequence[type], headers: Optional[Sequence[str]] = None
-):
-    records = []
-    rows = csv.reader(lines)
-    if headers is None:
-        headers = next(rows)
-    for row in rows:
-        record = {name: func(val) for name, func, val in zip(headers, types, row)}
-        records.append(record)
-    return records
+) -> Sequence[Mapping[str, Any]]:
+    """Parse CSV lines into a list of dictionaries."""
+
+    def _conv(row, hdrs):
+        return {name: func(val) for name, func, val in zip(hdrs, types, row)}
+
+    return convert_csv(lines, _conv, headers)
 
 
 def read_csv_as_dicts(
     filename: str, types: Sequence[type], headers: Optional[Sequence[str]] = None
-):
+) -> Sequence[Mapping[str, Any]]:
     """
     Read CSV data into list of dictionaries with optional type conversion.
     """
@@ -25,18 +35,16 @@ def read_csv_as_dicts(
         return csv_as_dicts(file, types, headers)
 
 
-def csv_as_instances(lines: Iterable[str], cls: type, has_headers: bool = True):
-    records = []
-    rows = csv.reader(lines)
-    if has_headers:
-        next(rows)
-    for row in rows:
-        record = cls.from_row(row)
-        records.append(record)
-    return records
+def csv_as_instances(
+    lines: Iterable[str], cls: type, headers: Optional[Sequence[str]] = None
+) -> Sequence[Any]:
+    """Parse CSV lines into a list of of class instances."""
+    return convert_csv(lines, lambda row, _: cls.from_row(row), headers)
 
 
-def read_csv_as_instances(filename: str, cls: type, has_headers: bool = True):
+def read_csv_as_instances(
+    filename: str, cls: type, has_headers: bool = True
+) -> Sequence[Any]:
     """
     Read CSV data into list of instances.
     """