python - Not able to parse a .csv file uploaded using Flask

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python - Not able to parse a .csv file uploaded using Flask

posted Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

python - Not able to parse a .csv file uploaded using Flask

I am trying to upload a CSV file, work on it to produce results, and write back (download) a new CSV file containing the result. I am very new to Flask and I am not able to get a "proper" csv.reader object to iterate and work upon. Here is the code so far,

__author__ = 'shivendra'
from flask import Flask, make_response, request
import csv

app = Flask(__name__)

def transform(text_file_contents):
    return text_file_contents.replace("=", ",")


@app.route('/')
def form():
    return """
        <html>
            <body>
                <h1>Transform a file demo</h1>

                <form action="/transform" method="post" enctype="multipart/form-data">
                    <input type="file" name="data_file" />
                    <input type="submit" />
                </form>
            </body>
        </html>
    """

@app.route('/transform', methods=["POST"])
def transform_view():
    file = request.files['data_file']
    if not file:
        return "No file"

    file_contents = file.stream.read().decode("utf-8")
    csv_input = csv.reader(file_contents)
    print(file_contents)
    print(type(file_contents))
    print(csv_input)
    for row in csv_input:
        print(row)

    result = transform(file_contents)

    response = make_response(result)
    response.headers["Content-Disposition"] = "attachment; filename=result.csv"
    return response

if __name__ == "__main__":
    app.run(host='0.0.0.0', port=5001, debug=True)

The terminal output being

127.0.0.1 - - [12/Oct/2015 02:51:53] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [12/Oct/2015 02:51:59] "POST /transform HTTP/1.1" 200 -
4,5,6
<class 'str'>
<_csv.reader object at 0x105149438>
['1']
['', '']
['2']
['', '']
['3']
[]
['4']
['', '']
['5']
['', '']
['6']

Whereas the file I read is

What am I doing wrong to not get 2 lists representing 2 rows when I iterate the csv.reader object?

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1 Reply

深蓝 · Answer 1 · 2021-10-17T00:10:22+0000

OK, so there is one major problem with your script, csv.reader as noted here expects a file object or at least an object which supports the iterator protocol. You are passing a str which does implement the iterator protocol, but instead of iterating through the lines, it iterates through the characters. This is why you have the output you do.

First, it gives a singe character 1 which the csv.reader sees as a line with one field. After that the str gives another single character , which the csv.reader sees as a line with two empty fields (since the comma is the field seperator). It goes on like that throughout the str until it's exhausted.

The solution (or at least one solution) is to turn the str into a file-like object. I tried using the stream provided by flask.request.files["name"], but that doesn't iterate through the lines. Next, I tried using a cStringIO.StringIO and that seemed to have a similar issue. I ended up at this question which suggested an io.StringIO object in universal newlines mode which worked. I ended up with the following working code (perhaps it could be better):

__author__ = 'shivendra'
from flask import Flask, make_response, request
import io
import csv

app = Flask(__name__)

def transform(text_file_contents):
    return text_file_contents.replace("=", ",")


@app.route('/')
def form():
    return """
        <html>
            <body>
                <h1>Transform a file demo</h1>

                <form action="/transform" method="post" enctype="multipart/form-data">
                    <input type="file" name="data_file" />
                    <input type="submit" />
                </form>
            </body>
        </html>
    """

@app.route('/transform', methods=["POST"])
def transform_view():
    f = request.files['data_file']
    if not f:
        return "No file"

    stream = io.StringIO(f.stream.read().decode("UTF8"), newline=None)
    csv_input = csv.reader(stream)
    #print("file contents: ", file_contents)
    #print(type(file_contents))
    print(csv_input)
    for row in csv_input:
        print(row)

    stream.seek(0)
    result = transform(stream.read())

    response = make_response(result)
    response.headers["Content-Disposition"] = "attachment; filename=result.csv"
    return response

if __name__ == "__main__":
    app.run(host='0.0.0.0', port=5001, debug=True)

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python - Not able to parse a .csv file uploaded using Flask

python - Not able to parse a .csv file uploaded using Flask

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