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Iterating over input using either 'for line in fileinput.input():'
or 'for line in sys.stdin:' has the following unexpected behavior:
no matter how many lines of input the process reads, the loop body is not
entered until either (1) at least 8193 chars have been read and at least one of
them was a newline, or (2) EOF is read (i.e. the read() system call returns
zero bytes).
The behavior I expect instead is what
"for line in iter(sys.stdin.readline, ''):" does: that is, the loop body is
entered for the first time as soon as a newline or EOF is read.
Furthermore strace reveals that this well-behaved alternative code does
sensible input buffering, in the sense that the underlying system call being
made is read(0,buf,8192), thereby allowing it to get as many characters as are
available on input, up to 8192 of them, to be buffered and used in subsequent
loop iterations. This is familiar and sensible behavior, and is what I think
of as "input buffering".
I anticipate there will be responses to this bug report of the form "this is
documented behavior; the fileinput and sys.stdin iterators do input buffering".
To that, I say: no, these iterators' unfriendly behavior is *not* input
buffering in any useful sense; my impression is that someone may have
implemented what they thought the words "input buffering" meant, but if so,
they really botched it.
This bug is most noticeable and harmful when using a filter written in python
to filter the output of an ongoing process that may have long pauses between
lines of output; e.g. running "tail -f" on a log file. In this case, the
python filter spends a lot of time in a state where it is paused without
reason, having read many input lines that it has not yet processed.
If there is any suspicion that the delayed output is due to the previous
program in the pipeline buffering its output instead, strace can be used on the
python filter process to confirm that its input lines are in fact being read in
a timely manner. This is certainly true if the previous process in the
pipeline is "tail -f", at least on my ubuntu linux system.
To demonstrate the bug, run each of the following from the bash command line.
This was observed using bash 4.3.11(1), python 2.7.6, and python 3.4.3,
on ubuntu 14.04 linux.
----------------------------------------------
{ echo a;echo b;echo c;sleep 1;} | python2.7 -c $'import fileinput,sys\nfor line in fileinput.input(): sys.stdout.write("line: "+line)'
 # result (BAD): pauses for 1 second, prints the three lines, returns to prompt
{ echo a;echo b;echo c;sleep 1;} | python2.7 -c $'import sys\nfor line in sys.stdin: sys.stdout.write("line: "+line)'
 # result (BAD): pauses for 1 second, prints the three lines, returns to prompt
{ echo a;echo b;echo c;sleep 1;} | python2.7 -c $'import sys\nfor line in iter(sys.stdin.readline, ""): sys.stdout.write("line: "+line)'
 # result (GOOD): prints the three lines, pauses for 1 second, returns to prompt
{ echo a;echo b;echo c;sleep 1;} | python3.4 -c $'import fileinput,sys\nfor line in fileinput.input(): sys.stdout.write("line: "+line)'
 # result (BAD): pauses for 1 second, prints the three lines, returns to prompt
{ echo a;echo b;echo c;sleep 1;} | python3.4 -c $'import sys\nfor line in sys.stdin: sys.stdout.write("line: "+line)'
 # result (GOOD): prints the three lines, pauses for 1 second, returns to prompt
{ echo a;echo b;echo c;sleep 1;} | python3.4 -c $'import sys\nfor line in iter(sys.stdin.readline, ""): sys.stdout.write("line: "+line)'
 # result (GOOD): prints the three lines, pauses for 1 second, returns to prompt
----------------------------------------------
Notice the 'for line in sys.stdin:' behavior is apparently fixed in python 3.4.
So the matrix of behavior observed above can be summarized as follows:
 2.7 3.4
for line in fileinput.input(): BAD BAD
for line in sys.stdin: BAD GOOD
for line in iter(sys.stdin.readline, ""): GOOD GOOD
Note that adding '-u' to the python args makes no difference in behavior, in
any of the above 6 command lines.
Finally, if I insert "strace -T" before "python" in each of the 6 command lines
above, it confirms that the python process is reading the 3 lines of input
immediately in all cases, in a single read(..., ..., 4096 or 8192) which seems
reasonable.

Possibly related to http://bugs.python.org/issue1633941 .
Note that the matrix of GOOD and BAD versions and input methods is
exactly the same for this bug as for that one. To verify: run
each of the 6 python commands I mentioned on its own, being sure to type
at least one line of input ending in newline before hitting ctrl-D -- if it exits after one ctrl-D it's GOOD; having to type a second ctrl-D is BAD.

For fileinput see issue15068.

Issue 15068 as been fixed in 3.5+ and 2.7, and it looks like it fixes the fileinput aspect of this bug. That leaves the sys.stdin aspect, which only affects Python 2, and I think is a duplicate of Issue 1633941.

For those who find this in the future, the simplest workaround for the:
for line in sys.stdin:
issue on Python 2 is to replace it with:
for line in iter(sys.stdin.readline, ''):
The problem is caused by the way file.__next__'s buffering behaves, but file.readline doesn't use that code (it delegates to either fgets or a loop over getc/getc_unlocked that never overbuffers beyond the newline). Two-arg iter lets you make an iterator that calls readline each time you want a line, and considers a return of '' (which is what readline returns when you hit EOF) to terminate iteration.