如何从 HTML 页面提供 Python 泡菜文件

Question

I am trying to allow for downloading a Python pickle file from a Flask app through

import pickle
from flask import Flask, render_template_string


app = Flask(__name__)

template = """
<button onclick="download_file()" data-trigger-update-context="false">Download</button>
<script>
    function download_file() {
        mime_type = '{{ mime_type }}';
        var blob = new Blob(['{{ file_content }}'], { type: mime_type });
        var dlink = document.createElement('a');
        dlink.download = 'pickle.pkl';
        dlink.href = window.URL.createObjectURL(blob);
        dlink.onclick = function (e) {
            // revokeObjectURL needs a delay to work properly.
            var that = this;
            setTimeout(function () {
                window.URL.revokeObjectURL(that.href);
            }, 1500);
        };
        document.body.appendChild(dlink);
        dlink.click();
        dlink.remove();
    }
</script>
"""


@app.route("/")
def download():
    return render_template_string(
        template,
        file_content=pickle.dumps("text"),
        mime_type="application/octet-stream",
    )

While downloading the file works fine, the downloaded file itself seems corrupted as I get the following error while reading it

Python 3.7.6 | packaged by conda-forge | (default, Mar 23 2020, 23:03:20) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.14.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import pickle                                                                                                                                                                                              

In [2]: with open("pickle.pkl", "rb") as f: 
   ...:     pickle.load(f) 
   ...:                                                                                                                                                                                                            
---------------------------------------------------------------------------
UnpicklingError                           Traceback (most recent call last)
<ipython-input-2-b5282f4164d8> in <module>
      1 with open("pickle.pkl", "rb") as f:
----> 2     pickle.load(f)
      3 

UnpicklingError: unpickling stack underflow

Any hint on the issue with the download script?

Thanks for your help.

Answer 1

Basically, you need to change just two things:

• In the download() method, you need to convert the serialized bytes to a list of Integers.
• Then, you need to change the JavaScript code to read this list of numbers.

So, your code should look like this:

import pickle
from flask import Flask, render_template_string


app = Flask(__name__)

template = """
<button onclick="download_file()" data-trigger-update-context="false">Download</button>
<script>
    function download_file() {
        let bytes_array = new Uint8Array({{file_content}}); //<--- add this
        mime_type = '{{ mime_type }}';
        var blob = new Blob([bytes_array], { type: mime_type }); //<-- change this
        var dlink = document.createElement('a');
        dlink.download = 'pickle.pkl';
        dlink.href = window.URL.createObjectURL(blob);
        dlink.onclick = function (e) {
            // revokeObjectURL needs a delay to work properly.
            var that = this;
            setTimeout(function () {
                window.URL.revokeObjectURL(that.href);
            }, 1500);
        };
        document.body.appendChild(dlink);
        dlink.click();
        dlink.remove();
    }
</script>
"""


@app.route("/")
def download():
    return render_template_string(
        template,
        file_content=list(pickle.dumps("text")),  # change this
        mime_type="application/octet-stream",
    )


if __name__ == '__main__':
    app.run(debug = True)

Now, you can read the pickled file using pickle.load() just like so:

import pickle

with open("pickle.pkl", 'rb') as fin:
    print(pickle.load(fin))
# prints: text