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Tolerim
24 days ago

Reconstructed zip file retrieved via ajax from an incomplete get query.

Sorry for the inconvenience, but unfortunately, I am unable to provide a reproducible example as I work with a local server. However, I can explain what's bothering me. I'm currently working with a Flask API that's returning a zip file in the following manner: The file's name is derived from the path to the zip file. First, the API sends the file with a mimetype of "zip", source file name as its attachment filename and "asattachment" set to True. Then, the API opens the zip file in read mode and reads the file line by line, storing the result in the variable 'data'. Eventually, the API will return 'data', which is now a list of bytes, with headers that define the content type as "application/zip" and the content disposition as "attachment; filename={filename};". On the server side, I receive the file using the 'getpreview' function. This function creates a URL and uses the jQuery AJAX method to send a GET request to the Flask API. If the response from the API is not a zip file, it will print it on the browser. In contrast, if the 'getzip' parameter is set to 'yes', the response from the API is a zip file. The function then converts the response string to an array of bytes using the 'str2bytes' function and creates a URL for downloading the resulting blob. However, the complete file is not downloaded, and I receive an error message when attempting to unzip the file stating that 200K bytes are missing. Even though the console shows the expected binary output of the zip file from the response, the file saved on my browser is incomplete. Can you suggest what could be causing this issue? I've also attempted to save the file without converting it to bytes, but that produces even worse results as the file isn't recognized as binary, hence why I'm choosing to reconvert it.
Answers(2)
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Vogi
23 days ago
Verified Answer
I understand that you are having trouble downloading a zip file from a Flask API. It seems that the file is not being downloaded completely and you are receiving an error message when attempting to unzip the file. It is possible that the issue is caused by the way the file is being sent from the server. One possible solution could be to use the 'send_file' function instead of manually sending the file with headers. The 'send_file' function is a built-in Flask function that sends files from the server to the client with the correct headers. Here's an example of how you can use it: 
from flask import send_file

@app.route('/download')
def download():
    filename = 'example.zip'
    return send_file(filename, as_attachment=True)
This will send the file 'example.zip' with headers that define the content type as "application/zip" and the content disposition as "attachment; filename=example.zip;". If this doesn't solve your issue, please let me know and I'll try to help you further.
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Tolerim
24 days ago
The issue you are facing seems to be related to the way you are handling the response data in your JavaScript code. Specifically, when you receive the response from the Flask API, you are converting it to a string using the str2bytes function and then creating a new Blob object with the application/zip MIME type and the resulting byte array. However, this may lead to data loss or corruption if the response contains invalid characters or is not properly encoded. To ensure that you receive the complete and correct binary data from the API, you can modify your code to use the responseType: 'blob' option in your jQuery.ajax call, like this: 
url: query_string,
    method: 'GET',
    headers: { 'x-access-token': localStorage.getItem('token') },
    responseType: 'blob',
    success: function(blob) {
        var link = document.createElement('a');
        link.href=window.URL.createObjectURL(blob);
        filename = id + '_' + res_time + '_' + species + '_report.zip';
        link.download=filename;
        link.click();
    },
    error: function(xhr, status, error) {
        console.error(xhr, status, error);
        document.write('Failure to retrieve the report. Please refresh the page to start again or call BIOIT if it still fails')
    }
});
By setting the responseType option to 'blob', you can directly access the binary data of the response as a Blob object, without having to convert it to a string or byte array. You can then create a download link for the Blob object and trigger a download of the file.
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