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?

Answer 1

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.

Answer 2

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.