What does "input() already active" mean in python fileinput module?

Question

I'm trying to read 2nd line in text.txt file:

import fileinput

x = 0
for line in fileinput.input([os.path.expandvars("$MYPATH/text.txt")]):
        if x < 3:
            x += 1
            if x == 2:
                mydate = line
fileinput.close()
print "mydate : ", mydate

But I get an error:

Traceback (most recent call last):
  File "/tmp/tmpT8RvF_.py", line 4, in <module>
    for line in fileinput.input([os.path.expandvars("$MYPATH/text.txt")]):
  File "/usr/lib64/python2.6/fileinput.py", line 102, in input
    raise RuntimeError, "input() already active"
RuntimeError: input() already active

What is wrong above?

Martijn Pieters · Accepted Answer · 2012-12-10 11:40:55Z

5

To get the second line from the fileinput.input() iterable, just call .next() twice:

finput = fileinput.input([os.path.expandvars("$MYPATH/text.txt")])
finput.next()  # skip first line
mydate = finput.next()  # store second line.

You can also use the itertools.islice() function to select just the second line:

import itertools

finput = fileinput.input([os.path.expandvars("$MYPATH/text.txt")])
mydate = itertools.islice(finput.next(), 1, 2).next()  # store second line.

Both methods ensure that no more than two lines are ever read from the input.

The .input() function returns a global singleton object, that the other functions operate on. You can only run one fileinput.input() instance at a time. Make sure you called fileinput.close() before you open a new input() object.

You should use the fileinput.FileInput() class instead to create multiple instances.

edited Dec 10, 2012 at 11:40

answered Dec 10, 2012 at 11:29

Martijn Pieters

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7 Comments

alwbtc Over a year ago

The file is really huge, i need to read 2nd line only. Is your suggestion above sutiable for this?

alwbtc Over a year ago

I tried the first method, but I still get "input() already active" error.

Martijn Pieters Over a year ago

@alwbtc: Both methods only read enough to give you the first two lines (using a read buffer). Neither method will read the whole file.

alwbtc Over a year ago

I tried fileinput.FileInput() instead of fileinput.input() and it worked.

alwbtc Over a year ago

OK, but I create only 1 instance if I use your first method, with fileinput.input(). Why do I still get "input() already active" error?

|

mihaicc · Accepted Answer · 2012-12-10 12:09:48Z

1

The python way is:

with open('text.txt', 'r') as file:
    file.next() #as Martjin stated in his response, skips 1st line
    mydate = file.next()

In this example only 2 lines are read from the file, and it's closed automatically.

For your exact question: My python is Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41) and it doesn't raise that error for your example.That error would appear because you're not consuming all the lines before you reopen the input.

edited Dec 10, 2012 at 12:09

answered Dec 10, 2012 at 11:46

mihaicc

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2 Comments

alwbtc Over a year ago

I don't want to consume all lines, text file is really huge.

mihaicc Over a year ago

I stated that only the 2 first lines are read from the file, the processing is always at most 2 lines, no matter the length of the file

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