wtf_validators.py

from wtforms.validators import ValidationError

def FileSizeLimit(max_size_in_mb):
    max_bytes = max_size_in_mb*1024*1024
    def file_length_check(form, field):
        if len(field.data.read()) > max_bytes:
            raise ValidationError(f"File size must be less than {max_size_in_mb}MB")
        field.data.seek(0)
    return file_length_check