Create a function for add_new_field in Python

Question

I am trying to create a function for a task I mentioned earlier for adding a new field to some files that are stored in one folder. I read about the function method in Python and followed but it does not work. I would be happy if someone could guide me about it.

def newfield(infile,outfile):
    infile.readlines()
    outfile = ["%s\t%s" %(item.strip(),2) for item in infile]
    outfile.write("\n".join(output))
    outfile.close()
    return outfile

and then I try to call it:

a = open("E:/SAGA/data/2006last/test/325145404.all","r")
b = open("E:/SAGA/data/2006last/test/325145404_edit.all","w")
newfield(a,b)

Moreover, when we create a function, should we call it in a same python shell or we can same it as a file and then call it?

What doesn't work? What output did you expect to get, and what output did you find instead? What does your input look like? — Martijn Pieters
– Martijn Pieters, Commented Apr 3, 2013 at 11:36
it gives this error:Traceback (most recent call last): File "<pyshell#21>", line 1, in <module> newfield(a,b); File "<pyshell#19>", line 4, in newfield outfile.write("\n".join(output)) AttributeError: 'list' object has no attribute 'write' — mari mm
– mari mm, Commented Apr 3, 2013 at 11:44

GP89 · Accepted Answer · 2013-04-03 11:39:11Z

1

Looks like the problem is you're assigning the list you build to the same name as the output file. You're also reading all of the lines out of the file and not assigning the result anywhere, so when your list comprehension iterates over it, it's already at the end of the file. So you can drop the readlines line.

def newfield(infile,outfile):
    output = ["%s\t%s" %(item.strip(),2) for item in infile]
    outfile.write("\n".join(output))
    outfile.close()
    return outfile

answered Apr 3, 2013 at 11:39

GP89

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12 Comments

mari mm Over a year ago

I tried ur suggestion. when I call this function as : a = open("E:/SAGA/data/2006last/test/325155405.all","r") b = open("E:/SAGA/data/2006last/test/325155405_edit.all","w") newfield(a,b); it does nt create a new outfile

GP89 Over a year ago

Is there an error? Also did you start a new python prompt after updating the code?

mari mm Over a year ago

yes the error is : Traceback (most recent call last): File "<pyshell#50>", line 1, in <module> newfield(a,b); File "<pyshell#46>", line 3, in newfield outfile.write("\n".join(output)) AttributeError: 'list' object has no attribute 'write'

mari mm Over a year ago

maybe my problem is the way to call it. I wrote the function in python GUI and called afterwards in the same page,is it correct?

GP89 Over a year ago

That error looks to be from the orignal code you posted, rather than the code above

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Andreas Schlapsi · Accepted Answer · 2013-04-03 12:05:50Z

0

The line infile.readlines() consumes the whole input file. All lines are read and forgotten because they are not assigned to a variable.

The construct ["%s\t%s" %(item.strip(),2) for item in infile] is a list comprehension. This expression returns a list. By assigning it to the variable outfile the old value of outfile - probably a file object - is forgotten and replaced by the list returned by the list comprehension. This list object does not have a write method. By assigning the list to another variable the file object in outfile is preserved.

Try this:

def newfield(infile,outfile):
    items = ["%s\t%s" %(item.strip(),2) for item in infile]
    outfile.write("\n".join(items))
    outfile.close()
    return outfile

edited Apr 3, 2013 at 12:05

answered Apr 3, 2013 at 11:38

Andreas Schlapsi

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6 Comments

mari mm Over a year ago

I tried ur suggestion. when I call this function as : a = open("E:/SAGA/data/2006last/test/325155405.all","r") b = open("E:/SAGA/data/2006last/test/325155405_edit.all","w") newfield(a,b); it gives this error :<closed file 'E:/SAGA/data/2006last/test/325145404_ed.all', mode 'w' at 0x02CE8650>

mari mm Over a year ago

sorry would u plz explain more what do u exactly mean?I do not get u.

mari mm Over a year ago

thank you for ur help. I tried the new code u recommended. when I call ,I receive this error: <closed file 'E:/SAGA/data/2006last/test/325145404_ed_2.all', mode 'w' at 0x02CE86A8> what do u think it refers to?

Andreas Schlapsi Over a year ago

Do you use the returned file after calling the function?

mari mm Over a year ago

what does this error means? File "<pyshell#81>", line 3, in newfield1 outfile.write("\n".join(items)) ValueError: I/O operation on closed file

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