idiomatic analogue of Perl's: while (<>) { ... }

[Sahil Tandon]

I've been tasked with converting some programs from Perl -> Python, and
am (as will soon be obvious) new to the language. A few archive/google
searches were inconclusive on a consensus approach, which is OK, but I
just wonder if there is a more Python-esque way to do the following in
Python 2.7.1:

%%
# unbuffer STDOUT
sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)

# process input, line-by-line, and print responses after parsing input
while 1:
rval = parse(raw_input())
if rval == None:
print('foo')
else:
print('bar')
%%

This works, but while reading the documentation, I thought of using 'for
line in fileinput.input()' in lieu of 'while 1:' construct. This does
not work when debugging the program on the command line -- the script
appears to just hang no matter what is typed into STDIN. I believe this
is because of some internal buffering when using fileinput. Is there a
recommended way to disable such buffering? Am I taking a totally wrong
approach?

Feel free to just link me to previous discussions on the topic(s) if I
have missed them. Please be gentle with your cluebats.

Thanks,

 

[Steven D'Aprano]

I've been tasked with converting some programs from Perl -> Python, and
am (as will soon be obvious) new to the language. A few archive/google
searches were inconclusive on a consensus approach, which is OK, but I
just wonder if there is a more Python-esque way to do the following in
Python 2.7.1:

%%
# unbuffer STDOUT
sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)

I've never bothered with unbuffered stdout, but that looks fine to me.

I'm not sure if it is necessary though, because print seems to automatically
flush the buffer after each line in my testing. Unless you're printing
repeatedly to the same line, I'm not sure unbuffered stdout is helpful.

# process input, line-by-line, and print responses after parsing input
while 1:
rval = parse(raw_input())
if rval == None:
print('foo')
else:
print('bar')
%%

"while True" is considered slightly more idiomatic (readable), but
otherwise, that seems fine.

This works, but while reading the documentation, I thought of using 'for
line in fileinput.input()' in lieu of 'while 1:' construct. This does
not work when debugging the program on the command line -- the script
appears to just hang no matter what is typed into STDIN. I believe this
is because of some internal buffering when using fileinput. Is there a
recommended way to disable such buffering? Am I taking a totally wrong
approach?

I'm not sure anything about fileinput is exactly *recommended*, it's kinda
discouraged on account of being a bit slow. See help(fileinput) at the
interactive prompt.

For what it's worth, the default buffersize for fileinput.input is 0, so if
that doesn't do what you want, I don't think fileinput is the right
solution.

 

[Anssi Saari]

I've been tasked with converting some programs from Perl -> Python, and
am (as will soon be obvious) new to the language.

If it's any help, I have usually done handling of standard input line by
line with this kind of thing:

for inputline in sys.stdin:

 

[Sahil Tandon]

[Thanks to everyone who responded]

I've never bothered with unbuffered stdout, but that looks fine to me.

I'm not sure if it is necessary though, because print seems to automatically
flush the buffer after each line in my testing. Unless you're printing
repeatedly to the same line, I'm not sure unbuffered stdout is helpful.

I found it necessary because without reopening sys.stdout with buffering
explicitly turned off, I would have to manually flush the buffer after
each print. This is because the program must reply (via writing to
STDOUT) after parsing each line read via STDIN. If I neither disable
buffering nor manually flush after each print, the program just hangs
instead of printing right away.

"while True" is considered slightly more idiomatic (readable), but
otherwise, that seems fine.

Ah, thanks -- I've changed '1' to 'True'.

I'm not sure anything about fileinput is exactly *recommended*, it's kinda
discouraged on account of being a bit slow. See help(fileinput) at the
interactive prompt.

For what it's worth, the default buffersize for fileinput.input is 0, so if
that doesn't do what you want, I don't think fileinput is the right
solution.

Got it. Based on your and others' response, I will stick with my
existing approach.

 

[Dan Sommers]

"while True" is considered slightly more idiomatic (readable), but
otherwise, that seems fine.

Arguably more readable, but arguably less idomatic, is to describe the
actual condition that controls the loop in a string (non-empty strings
are equivalent to True in this context):

while "there is more input":
rval = parse(raw_input())
if real is None:
print('foo')
else:
print('bar')

(Although now that I've said that, this looks like an infinite loop
unless parse, raw_input, or print raises an exception.)

Dan