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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?
948
DictReader method you can read the CSV file as a list of dictionaries, keyed by the entries in the header row. However, csv. DictReader needs a file and does not directly accept strings. You may think you need to use FLASK methods to first save the uploaded file, get the new file name and location, open it using csv.
Common issues with importing a CSV (and the work-arounds to fix them) The most common CSV import errors include: The file size is too large - The CSV import tool of the program you're using might have a file size requirement. To reduce the file size, you can delete unnecessary data values, columns, and rows.
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)
Important note: This answer is relevant only for platforms where SpooledTemporaryFile is available.
Further to iLuveTux answer, you can save the redundant read() call by replacing the following string-based stream creation:
stream = io.StringIO(f.stream.read().decode("UTF8"), newline=None)
with:
stream = io.TextIOWrapper(f.stream._file, "UTF8", newline=None)
Example:
stream = io.TextIOWrapper(f.stream._file, "UTF8", newline=None)
csv_input = csv.reader(stream)
print(csv_input)
for row in csv_input:
print(row)
Further information:
Werkzeug default stream for form data parser is SpooledTemporaryFile (as of 1.0.1), from which you can obtain the underlying buffer using its _file memeber.
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