Re: Resource leaks from FieldFile's implicit open?

I think that the documentation is not entirely clear. I wouldn't say that the object "wrapped by" FieldFile is "a wrapper around the result of" Storage.open(). It is actually FieldFile.open() that is a wrapper around Storage.open(). Therefore I think I'd rephrase as follows:

FieldFile is a subclass of File. While File is a wrapper around Python's built-in file object, FieldFile uses the underlying Storage.

As far as I can see by reading the code, you can write code like this:

with some_file_field:
    ...

Inside the "with" block you will probably access `some_file_field.file`. Apparently accessing that attribute is what opens the file (but accessing the `size` attribute also seems to be opening it, which is probably suboptimal.)

I'm not certain about files that don't exist. You might want to wrap the "with" block in a try-except, but I'm not certain about which exceptions you want to catch. It's probably ValueError and IOError.

Regards,

Antonis

Antonis Christofides  +30-6979924665 (mobile)

On 01/02/2022 21.22, Carsten Fuchs wrote:

Dear group,    despite a lot of reading and searching I still don't understand how FieldFiles work.    The documentation at https://docs.djangoproject.com/en/4.0/ref/models/fields/#django.db.models.fields.files.FieldFile says:    

The API of FieldFile mirrors that of File, with one key difference: The object wrapped by the class is not necessarily a wrapper around Python's built-in file object. Instead, it is a wrapper around *** the result of the Storage.open() method ***, which may be a File object, or it may be a custom storage's implementation of the File API.  

  (I added markers to indicate the portion that I have trouble with.)    As far as I understand this, whenever a `FieldFile` is instantiated, `Storage.open()` is called. Thus, if the underlying storage is the `FileSystemStorage`, a Python file object is opened.    However, how is this file handle closed again?    It looks as if it stays open until garbage collected, that is, possibly for a very long time.    Another consequence besides the potential resource leak seems to be that the implicit open will raise an exception if the file that the `FieldFile` refers to doesn't exist. As I don't exactly understand when this opening occurs, a lot of code must be wrapped in try-except blocks to handle this?    Best regards,  Carsten