How to use validators on FileField content

Question

In my model, I want to use a validator to analyze the content of a file, the thing I can not figure out is how to access the content of the file to parse through it as the file has not yet been saved (which is good) when the validators are running.

I'm not understanding how to get the data from the value passed to the validator into a file (I assume I should use tempfile) so I can then open it and evaluate the data.

Here's a simplified example, in my real code, I want to open the file and evaluate it with csv.

in Models.py

class ValidateFile(object):
    ....
    def __call__(self, value):
        # value is the fieldfile object but its not saved
        # I believe I need to do something like:
        temp_file = tempfile.TemporaryFile()
        temp_file.write(value.read())
        # Check the data in temp_file
    ....

class MyItems(models.Model):
    data = models.FileField(upload_to=get_upload_path,
                            validators=[FileExtensionValidator(allowed_extensions=['cv']),
                            ValidateFile()])

Thanks for the help!

Answer 1

Take a look how this is done in the ImageField implementation:

So your ValidateFile class may be something like this:

from io import BytesIO

class ValidateFile(object):

    def __call__(self, value):
        if value is None:
            #do something when None
            return None

        if hasattr(value, 'temporary_file_path'):
            file = value.temporary_file_path()
        else:
            if hasattr(value, 'read'):
                file = BytesIO(value.read())
            else:
                file = BytesIO(value['content'])

        #Now validate your file

Answer 2

No need for tempfile:

The value passed to a FileField validator is an instance of FieldFile, as already mentioned by the OP.

Under the hood, the FieldFile instance might already use a tempfile.NamedTemporaryFile (source), or it might wrap an in-memory file, but you need not worry about that:

To "evaluate the data" you can simply treat the FieldFile instance as any Python file object.

For example, you could iterate over it:

def my_filefield_validator(value):
    # note that value is a FieldFile instance
    for line in value:
        ... # do something with line

The documentation says:

In addition to the API inherited from File such as read() and write(), FieldFile includes several methods that can be used to interact with the underlying file: ...

and the FieldFile class provides

... 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.

An example of such an underlying file implementation is the InMemoryUploadedFile docs/source.

Also from the docs:

The File class is a thin wrapper around a Python file object with some Django-specific additions

Also note: class-based validators vs function-based validators