fix(fields.py): fix File field bug when get an empty value (#301)

* fix(fields.py): fix `File` field bug when get an empty value

* perf(fields.py): compatible with `python3.8`

* Update changelog

---------

Co-authored-by: Steven Loria <sloria1@gmail.com>

CHANGELOG.rst

@@ -1,6 +1,14 @@
 Changelog
 =========
 
+1.2.1 (unreleased)
+******************
+
+Bug fixes:
+
+* Fix `File` field when it receives an empty value (:pr:`301`, :issue:`apiflask/apiflask#551`).
+  Thanks :user:`uncle-lv`.
+
 1.2.0 (2024-02-05)
 ******************
 

src/flask_marshmallow/fields.py

@@ -10,6 +10,7 @@
 
 import re
 import typing
+from collections.abc import Sequence
 
 from flask import current_app, url_for
 from marshmallow import fields, missing
@@ -225,6 +226,17 @@ def __init__(self, *args, **kwargs):
 
     default_error_messages = {"invalid": "Not a valid file."}
 
+    def deserialize(
+        self,
+        value: typing.Any,
+        attr: typing.Optional[str] = None,
+        data: typing.Optional[typing.Mapping[str, typing.Any]] = None,
+        **kwargs,
+    ):
+        if isinstance(value, Sequence) and len(value) == 0:
+            value = missing
+        return super().deserialize(value, attr, data, **kwargs)
+
     def _deserialize(self, value, attr, data, **kwargs):
         from werkzeug.datastructures import FileStorage
 

tests/test_fields.py

@@ -2,6 +2,7 @@
 
 import pytest
 from flask import url_for
+from marshmallow import missing
 from marshmallow.exceptions import ValidationError
 from werkzeug.datastructures import FileStorage
 from werkzeug.routing import BuildError
@@ -150,6 +151,9 @@ def test_file_field(ma, mockauthor):
     result = field.deserialize(fs, mockauthor)
     assert result == fs
 
+    result = field.deserialize("", mockauthor)
+    assert result is missing
+
     with pytest.raises(ValidationError, match="Field may not be null."):
         field.deserialize(None, mockauthor)