writing data to a file in a specific format using Python

Question

I am trying to write numeric data from a function return in the format:

[1,2,3,4,5][10,11]
[6,7,8,9,10][13,14]
...

The problem is that I want the text file to be in the format:

1,2,3,4,5[::tab::]10,11
6,7,8,9,10[::tab::]13,14
...

At the moment I am using

with open(outfilename,'a') as outfile:
    outfile.writelines("%s%s\n"%(major,minor))

I would be thankful for help please. I am still totally confused by lists, reading and writing to text files, and achieving specific formatting of data. I always end up with unwanted [] or parentheses.

Rubens · Accepted Answer · 2012-12-23 17:05:15Z

1

You can simply write strings using the elements from the list, joining each element with a ,, or the separator you want; then, print the lists using a \t between them.

with open(outfilename, 'at') as outfile:

    major = ",".join(str(i) for i in major)
    minor = ",".join(str(i) for i in minor)

    outfile.writelines("%s\t%s\n" % (major, minor))

The magic behind the following line is very simple, actually:

major = ",".join(str(i) for i in major)

Python offers a feature named list comprehension, that allows you to perform an action over the elements of a list with a very clear and straightforward syntax.

For example, if you have a list l = [1, 2, 3, 4, 5], you can run over the list, converting the elements from integers to strings, calling the method str():

do _something_ with _element_ foreach _element_ in list _l_
          str(element)           for       element in l

When you do this, each resultant string will be created at a time, and you have a generator in this statement:

(str(element) for element in l)

As the method join works by iterating on a list of elements, or receiving this elements from a generator, you can have:

delimiter = ","
delimiter.join(str(element) for element in l)

Hope it clarifies the operation above.

edited Dec 23, 2012 at 17:05

answered Dec 23, 2012 at 15:05

Rubens

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4 Comments

user1478335 Over a year ago

So simple and yet I have such difficulty with the logic of this. Thank you. Can you suggest any book or text where I can learn these logical steps. I genuinely have trouble with this. Thank you for your fantastic help.

Rubens Over a year ago

Did this solution work for you? If so, accept the answer; I'll add an explanation on the ",".join(str(i) for i in major step, but the more I can tell you is to poke around with python using the interative python shell ipython, and consult the python documentation whenever you get in trouble: docs.python.org/2 or docs.python.org/3.

user1478335 Over a year ago

Thank you. On the same issue of reading a file in the format 1,2,3,4,5 tab 1,2. I want to return the highest value of the group with two numbers. Using split('\t')[1], I get the second group. If I use [1][0], I get the lowest number ( the data are sorted), but if I use [1][1] I get an error and if I use [1][-1], its fine with single digit numbers, but returns the second of the digits of a two digit number i.e. 10 returns 0 and 11 returns 1. I guess I still don't understand how to 'target' slicing properly

Rubens Over a year ago

@user1478335 You are writing a file here, not reading. I guess it's better to create an appropriate question, with a description of what you're trying to do, and what you've tried so far, followed by the wrong output you're getting.

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