Tired of validating and manipulating uploaded files manually? Try this instead.
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README.rst

Flask-Transfer

Validate, process and persist file uploads easily through a single object instead of cramming all that stuff into your routes.

Tired of this?

@app.route('/upload', methods=['GET', 'POST'])
def handle_upload():
    form = FileUploadForm()

    if form.validate_on_submit():
        filehandle = form.uploaded.data
        allowed_exts = ('md', 'txt', 'rst')
        if not os.path.splitext(filehandle.filename)[1] in allowed_exts:
            raise SomeError('Unallowed extension!')
        if filehandle.read() != b'Hello World!':
            raise SomeError('File contents not allowed!')
        filehandle.seek(0)

        username = g.current_user.name
        upload_dir = current_app.config['UPLOAD_DIR']
        full_path = os.path.join(upload_dir, username, secure_filename(filehandle.filename))
        filehandle.save(full_path)
        flash("Uploaded {}!".format(filehandle.filename), 'success')
        return redirect(url_for('handle_upload'))
    else:
        return render_template('upload.html', form=form)

That's a mess. Your test runner is literally running away from you. There's like four or five different things going on in this single route! Argh.

There's a better way

from flask_transfer import Transfer, UploadError
from flask_transfer.validators import AllowedExts

TextFileTransfer = Transfer(validators=[AllowedExts('md', 'rst', 'txt')])

@TextFileTransfer.destination
def save_to_user_dir(filehandle, metadata):
    username = g.current_user.name
    upload_path = current_app.config['UPLOAD_DIR']
    full_path = os.path.join(upload_dir, username, secure_filename(filehandle.filename))
    filehandle.save(full_path)


@TextFileTransfer.validator
def check_file_contents(filehandle, metadata):
    if filehandle.read() != metadata['allowed_contents']:
        raise UploadError('File contents not allowed!')
    filehandle.seek(0)
    return True


@app.route('/upload', methods=['GET', 'POST'])
def handle_upload():
    form = FileUploadForm()

    if form.validate_on_submit():
        filehandle = form.uploaded.data
        TextFileTransfer.save(filehandle, metadata={'allowed_contents': b'Hello World!'})
        flash('Uploaded {}!'.format(filehandle.filename), 'success')
        return redirect(url_for('handle_upload'))
    else:
        return render_template('upload.html', form=form)

Aaaah. Sure, it's a little bit more code. But it's separated out into bits and pieces. It's easy to test each bit and the intent in the route is very clear.

More Power

Flask-Transfer supplies hooks for validation, preprocessing and postprocessing file uploads via decorators. If you need to always create thumbnails of uploaded images, you can supply a callable to MyTransfer.preprocessor or MyTransfer.postprocessor that'll do that for you.

And validation beyond just simple extension checking is at your fingertips as well. Perhaps, you've limited your user to a certain amount of disk space and they should be told to delete data before uploading more. Write a simple function to check current disk usage and if the upload would exceed the cap. Then hook it to your Transfer object with MyTransfer.validator.

Finally, persisting files is easy! Maybe you're running on Heroku and can't rely on the local filesystem. Just write a callable that'll pass the file to your S3 bucket! Hook it in with MyTransfer.destination. Flask-Transfer handles using string paths and writable objects as destinations as well.

Check out the quickstart for some more information, as well!

Todo

There's still quite a bit to do. For example, better error handle. Perhaps a tighter integration with Flask, or running the opposite way and cleaving the already few dependencies on werkzeug to become framework independent.

Contributions

Given the infancy of this project, pull requests and issue are more than welcome. Just add yourself to the authors file, write some tests for the added or change functionality and submit it!