Writing with Python's built-in .csv module

Question

[Please note that this is a different question from the already answered How to replace a column using Python’s built-in .csv writer module?]

I need to do a find and rep

Answer 1

the problem is you're trying to write to the same file you're reading from. write to a different file and then rename it after deleting the original.

Answer 2

Opening csv files as binary is just wrong. CSV are normal text files so You need to open them with

source = open("PALTemplateData.csv","r")
target = open("AnotherFile.csv","w")

The error

_csv.Error: iterator should return strings, not bytes (did you open the file in text mode?)

comes because You are opening them in binary mode.

When I was opening excel csv's with python, I used something like:

try:    # checking if file exists
    f = csv.reader(open(filepath, "r", encoding="cp1250"), delimiter=";", quotechar='"')
except IOError:
    f = []

for record in f:
    # do something with record

and it worked rather fast (I was opening two about 10MB each csv files, though I did this with python 2.6, not the 3.0 version).

There are few working modules for working with excel csv files from within python - pyExcelerator is one of them.

Answer 3

You cannot read and write the same file.

source = open("PALTemplateData.csv","rb")
reader = csv.reader(source , dialect)

target = open("AnotherFile.csv","wb")
writer = csv.writer(target , dialect)

The normal approach to ALL file manipulation is to create a modified COPY of the original file. Don't try to update files in place. It's just a bad plan.

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Edit

In the lines

source = open("PALTemplateData.csv","rb")

target = open("AnotherFile.csv","wb")

The "rb" and "wb" are absolutely required. Every time you ignore those, you open the file for reading in the wrong format.

You must use "rb" to read a .CSV file. There is no choice with Python 2.x. With Python 3.x, you can omit this, but use "r" explicitly to make it clear.

You must use "wb" to write a .CSV file. There is no choice with Python 2.x. With Python 3.x, you must use "w".

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Edit

It appears you are using Python3. You'll need to drop the "b" from "rb" and "wb".

Read this: http://docs.python.org/3.0/library/functions.html#open