How can I handle file upload errors in Flask-WTF?

How can I handle file upload errors in Flask-WTF?

You can handle file upload errors in Flask-WTF by using the FileField and adding custom validation to check for errors.

What kind of validations can I implement?

You can check for file size, file type, and whether the file was actually uploaded. Use validators for these checks.

Can you show me a code example?

Sure! Here's a simple example:

from flask_wtf import FlaskForm
from wtforms import FileField, SubmitField
from wtforms.validators import DataRequired, ValidationError

class UploadForm(FlaskForm):
    file = FileField('File', validators=[DataRequired()])
    submit = SubmitField('Upload')

    def validate_file(self, field):
        if field.data:
            if field.data.size > 16 * 1024 * 1024:  # 16 MB limit
                raise ValidationError('File is too large.')
            if not field.data.filename.endswith(('.png', '.jpg', '.jpeg')):
                raise ValidationError('File type not allowed.')

What should I do if the upload fails?

You can catch the validation errors and display them to the user using Flask's flash messages or by rendering them in your template.