Basic Python loop question - problem reading a list from a text file

Question

I'm trying to read a list of items from a text file and format with square brackets and separators like this: ['item1','item2', .... 'last_item'] but I'm having trouble with the beginning and end item for which I always get: ...,'last_item','], so I do not want the last ,' to be there.

In python I've write:

out_list = "['"
for line in open(file_in):  
    out_list += line                #append the item to the list
    out_accession_list += "','"     #add the separator
out_accession_list += "]"           #add the final closed bracket
return out_list

I realize that this is a basic loop question, but I can't think of the best way to do it. Should I use a try final statement, should it be a while loop, or should I count the number of lines first and then use a loop with a range?

Help much appreciated.

Thanks, John

Why do you have so many variables? Why out_list and out_accession_list? Why so many variables? — S.Lott
– S.Lott, Commented Nov 9, 2009 at 21:14

Soviut · Accepted Answer · 2009-11-09 20:29:11Z

4

Read in all your lines and use the string.join() method to join them together.

lines = open(file_in).readlines()

out_list = "['" + "','".join(lines) + "']"

Additionally, join() can take any sequence, so reading the lines isn't necessary. The above code can be simplified as:

out_list = "['" + "','".join(open(file_in)) + "']"

edited Nov 9, 2009 at 20:29

answered Nov 9, 2009 at 20:20

Soviut

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3 Comments

Soviut Over a year ago

True, however, I wanted to be verbose to illustrate things better.

Ned Deily Over a year ago

Your one-liner doesn't strip the end-of-lines: "['item1\n','item2\n','item3\n']".

Soviut Over a year ago

You're right, but the example gives enough guidance that the OP should be able to figure it out.

Ned Batchelder · Accepted Answer · 2009-11-09 20:21:01Z

1

out_list = []
for line in open(file_in):
    out_list.append("'" + line + "'")
return "[" + ",".join(out_list) + "]"

answered Nov 9, 2009 at 20:21

Ned Batchelder

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2 Comments

Soviut Over a year ago

Why are you doing all the string building in the append? Just use "','" as your join string.

Ned Batchelder Over a year ago

Yeah, you're right, that'd be better. Soviut has the better answer.

jldupont · Accepted Answer · 2009-11-09 20:19:10Z

0

You "right strip" for "," the result before adding the last "]".

e.g. use the string.rstrip(",")

answered Nov 9, 2009 at 20:19

jldupont

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Comments

Jonathan Feinberg · Accepted Answer · 2009-11-09 20:22:08Z

0

Or

result = "['"
for line in open(file_in):
    if len(result) > 0:
        result += "','" 
    result += line
result += "']"
return result

answered Nov 9, 2009 at 20:22

Jonathan Feinberg

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inspectorG4dget · Accepted Answer · 2009-11-09 20:27:03Z

0

def do(filepath):
    out = []
    for line in open(filepath, 'r'):
        out.append("'" + line.strip() + "'")
    return out.__str__()

answered Nov 9, 2009 at 20:27

inspectorG4dget

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Ned Deily · Accepted Answer · 2009-11-09 20:53:49Z

0

Your desired output format is exactly Python's standard printable representation of a list. So an easy solution is to read the file, create a list with each line as a string element (stripping the end-of-line of each), and call the Python built-in function repr to produce a string representation of the list:

>>> repr([line.rstrip() for line in open(file_in)])
"['item1', 'item2', 'item3']"

edited Nov 9, 2009 at 20:53

answered Nov 9, 2009 at 20:22

Ned Deily

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