SimpleUploadedFile cannot be re-opened

[Denis Washington]

Summary

When a SimpleUploadedFile is closed, it calls close() on the underlying BytesIO, which means that the next call to open() fails (ValueError: I/O operation on closed file.). This is unlike the conceptually similar ContentFile, which explicitly overrides close() to do nothing, avoiding this issue.

How to Reproduce

The following script reproduces the issue:

from django.core.files.base import ContentFile
from django.core.files.uploadedfile import SimpleUploadedFile


def read_twice(file):
    with file.open() as f:
        print(f.read())
    with file.open() as f:
        print(f.read())


# Works as expected
read_twice(ContentFile(b"test data"))

# ValueError: I/O operation on closed file.
read_twice(SimpleUploadedFile("test.txt", b"test data"))

Expected Behavior

SimpleUploadedFile should follow ContentFile in making close() a no-op.

Ideally, the same should be done by its superclass InMemoryUploadedFile, at least if its file is a BytesIO or StringIO. (Skipping closing unconditionally could be risky here because the init method accepts any file-like object in principle.)

[Vishy Algo]

This appears to be a fundamental file handling problem: once a BytesIO object is created and passed to parent classes, it cannot be reconstructed if abruptly closed. Since it's an InMemoryUploadedFile, we could save the content and recreate the file object via open(), allowing the context manager to clean it up automatically.

But, currently InMemoryUploadedFile accepts file object while construction, we have to add it to the SimpleUploadedFile somewhat similar to this,

django/core/files/uploadedfile.py

@@ -113 +113 @@
         self.field_name = field_name
 
     def open(self, mode=None):
+        if self.closed:
+            self.file = BytesIO(self.content)
         self.file.seek(0)
         return self
 
@@ -132 +134 @@
     """
 
     def __init__(self, name, content, content_type="text/plain"):
-        content = content or b""
+        self.content = content or b""
         super().__init__(
-            BytesIO(content), None, name, content_type, len(content), None, None
+            BytesIO(self.content), None, name, content_type, len(self.content), None, None
         )

I've verified this change with a unit test, and it works as expected. I believe using InMemoryUploadedFile would have been more appropriate than SimpleUploadedFile, as it enables easy reconstruction of the file from its content throughout the request lifecycle for all in-memory file wrappers.

I'd appreciate any second opinions or suggestions on this approach.

[Jacob Walls]

Thanks, I'm a little sympathetic to your report, since it seems like a straightforward violation of the docs for File.open(), which claim it handles re-opening, and shouldn't be complicated to fix, but I'm going to respect the prior triage decision here in #33023. Feel free to start a forum discussion if you'd like to gather consensus to revisit.