Extract multipart parsing into _parse_multipart_files helper (#2958)

Refactor five image validators to eliminate duplicate multipart parsing
code by extracting it into a reusable helper with @beartype decoration.
Also simplify validate_image_is_image by directly accessing the stream
from the files dict.

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

Author: adamtheturtle

src/mock_vws/_query_validators/image_validators.py

@@ -7,6 +7,7 @@
 
 from beartype import beartype
 from PIL import Image
+from werkzeug.datastructures import FileStorage, MultiDict
 from werkzeug.formparser import MultiPartParser
 
 from mock_vws._query_validators.exceptions import (
@@ -19,19 +20,19 @@
 
 
 @beartype
-def validate_image_field_given(
+def _parse_multipart_files(
     *,
     request_headers: Mapping[str, str],
     request_body: bytes,
-) -> None:
-    """Validate that the image field is given.
+) -> MultiDict[str, FileStorage]:
+    """Parse the multipart body and return the files section.
 
     Args:
         request_headers: The headers sent with the request.
         request_body: The body of the request.
 
-    Raises:
-        ImageNotGivenError: The image field is not given.
+    Returns:
+        The files parsed from the multipart body.
     """
     email_message = EmailMessage()
     email_message["Content-Type"] = request_headers["Content-Type"]
@@ -42,6 +43,28 @@ def validate_image_field_given(
         boundary=boundary.encode(encoding="utf-8"),
         content_length=len(request_body),
     )
+    return files
+
+
+@beartype
+def validate_image_field_given(
+    *,
+    request_headers: Mapping[str, str],
+    request_body: bytes,
+) -> None:
+    """Validate that the image field is given.
+
+    Args:
+        request_headers: The headers sent with the request.
+        request_body: The body of the request.
+
+    Raises:
+        ImageNotGivenError: The image field is not given.
+    """
+    files = _parse_multipart_files(
+        request_headers=request_headers,
+        request_body=request_body,
+    )
     if files.get(key="image") is not None:
         return
 
@@ -64,14 +87,9 @@ def validate_image_file_size(
     Raises:
         RequestEntityTooLargeError: The image file size is too large.
     """
-    email_message = EmailMessage()
-    email_message["Content-Type"] = request_headers["Content-Type"]
-    boundary = email_message.get_boundary(failobj="")
-    parser = MultiPartParser()
-    _, files = parser.parse(
-        stream=io.BytesIO(initial_bytes=request_body),
-        boundary=boundary.encode(encoding="utf-8"),
-        content_length=len(request_body),
+    files = _parse_multipart_files(
+        request_headers=request_headers,
+        request_body=request_body,
     )
     image_part = files["image"]
     image_value = image_part.stream.read()
@@ -105,14 +123,9 @@ def validate_image_dimensions(
         BadImageError: The image is given and is not within the maximum width
             and height limits.
     """
-    email_message = EmailMessage()
-    email_message["Content-Type"] = request_headers["Content-Type"]
-    boundary = email_message.get_boundary(failobj="")
-    parser = MultiPartParser()
-    _, files = parser.parse(
-        stream=io.BytesIO(initial_bytes=request_body),
-        boundary=boundary.encode(encoding="utf-8"),
-        content_length=len(request_body),
+    files = _parse_multipart_files(
+        request_headers=request_headers,
+        request_body=request_body,
     )
     image_part = files["image"]
     image_value = image_part.stream.read()
@@ -142,14 +155,9 @@ def validate_image_format(
     Raises:
         BadImageError: The image is given and is not either a PNG or a JPEG.
     """
-    email_message = EmailMessage()
-    email_message["Content-Type"] = request_headers["Content-Type"]
-    boundary = email_message.get_boundary(failobj="")
-    parser = MultiPartParser()
-    _, files = parser.parse(
-        stream=io.BytesIO(initial_bytes=request_body),
-        boundary=boundary.encode(encoding="utf-8"),
-        content_length=len(request_body),
+    files = _parse_multipart_files(
+        request_headers=request_headers,
+        request_body=request_body,
     )
     image_part = files["image"]
     pil_image = Image.open(fp=image_part.stream)
@@ -175,17 +183,11 @@ def validate_image_is_image(
     Raises:
         BadImageError: Image data is given and it is not an image file.
     """
-    email_message = EmailMessage()
-    email_message["Content-Type"] = request_headers["Content-Type"]
-    boundary = email_message.get_boundary(failobj="")
-    parser = MultiPartParser()
-    _, files = parser.parse(
-        stream=io.BytesIO(initial_bytes=request_body),
-        boundary=boundary.encode(encoding="utf-8"),
-        content_length=len(request_body),
+    files = _parse_multipart_files(
+        request_headers=request_headers,
+        request_body=request_body,
     )
-    image_part = files["image"]
-    image_file = image_part.stream
+    image_file = files["image"].stream
 
     try:
         Image.open(fp=image_file)