how do I "peek" into the next line?

[les_ander]

Hi,
suppose I am reading lines from a file or stdin.
I want to just "peek" in to the next line, and if it starts
with a special character I want to break out of a for loop,
other wise I want to do readline().

Is there a way to do this?
for example:
while 1:
line=stdin.peek _nextline()
if not line: break
if line[0]=="|":
break:
else:
x=stdin.nextlin e()
# do something with x

thanks

[Skip Montanaro]

les> suppose I am reading lines from a file or stdin. I want to just
les> "peek" in to the next line, and if it starts with a special
les> character I want to break out of a for loop, other wise I want to
les> do readline().

Create a wrapper around the file object:

class PeekFile:
def __init__(self, f):
self.f = f
self.last = None

def push(self, line):
self.last = line

def readline(self):
if self.last is not None:
line = self.last
self.last = None
return line
return self.f.readline ()

def __getattr__(sel f, attr):
return getattr(self.f, attr)

Then use it like so:

stdin = PeekFile(stdin)
while 1:
line = stdin.readline( )
if not line:
break
if line[0] == "|":
stdin.push(line )
break
else:
# do something with line

Of course, this could be tested (which my example hasn't been), PeekFile can
probably can be written as a subclass of the file class, and made more
generally robust (handle filenames as inputs instead of file objects,
support write modes, etc), but this should give you the general idea.

Skip

[Jeffrey Maitland]

le*******@yahoo .com writes:

Hi,
suppose I am reading lines from a file or stdin.
I want to just "peek" in to the next line, and if it starts
with a special character I want to break out of a for loop,
other wise I want to do readline().

Is there a way to do this?
for example:
while 1:
line=stdin.peek _nextline()
if not line: break
if line[0]=="|":
break:
else:
x=stdin.nextlin e()
# do something with x

thanks

--
http://mail.python.org/mailman/listinfo/python-list

Well what you can do is read the line regardless into a testing variable.

here is some sample code (writting this off the topof my head so syntax
might be off some)

import re

file = open("test.txt" , 'r')

variablestr = '' #this would be your object.. in my example using a string
for the file data

file.seek(0,2)
eof = file.tell() #what this is the position of the end of the file.
file.seek(0,0)

while file.tell() != eof:
testline = file.readline()
if re.match("#", testline) == True:
break
else:
variablestr += testline

file.close()

now if I was concerned with being at the beging of the testline that it read
in what you can do is in the if is something like:
file.seek((file .tell() - len(testline)), 0)
and that will move you back to the beginging of that line which is where the
readline from the previous call left you before the "peek".

hope that helps some..

Jeff

[Jeffrey Maitland]

Jeffrey Maitland writes:

le*******@yahoo .com writes:

Hi,
suppose I am reading lines from a file or stdin.
I want to just "peek" in to the next line, and if it starts
with a special character I want to break out of a for loop,
other wise I want to do readline().

Is there a way to do this?
for example:
while 1:
line=stdin.peek _nextline()
if not line: break
if line[0]=="|":
break:
else:
x=stdin.nextlin e()
# do something with x

thanks

--
http://mail.python.org/mailman/listinfo/python-list

Well what you can do is read the line regardless into a testing variable.

here is some sample code (writting this off the topof my head so syntax
might be off some)

import re

file = open("test.txt" , 'r')

variablestr = '' #this would be your object.. in my example using a string
for the file data

file.seek(0,2)
eof = file.tell() #what this is the position of the end of the file.
file.seek(0,0)

while file.tell() != eof:
testline = file.readline()
if re.match("#", testline) == True:
break
else:
variablestr += testline

file.close()

now if I was concerned with being at the beging of the testline that it
read in what you can do is in the if is something like:
file.seek((file .tell() - len(testline)), 0)
and that will move you back to the beginging of that line which is where
the readline from the previous call left you before the "peek".

hope that helps some..

Jeff
--
http://mail.python.org/mailman/listinfo/python-list

I noticed something in my code.

re.match("#", testline) == True isn't possible it would be more like.
re.match("#", testline) != None

[Fredrik Lundh]

Jeffrey Maitland wrote:

file.seek(0,2)
eof = file.tell() #what this is the position of the end of the file.
file.seek(0,0)

while file.tell() != eof:
no no no. that's not how you read all lines in a text file -- in any
programming language.

in recent versions of Python, use:

for testline in file:

to loop over all lines in a given file.
if re.match("#", testline) == True:
break

ahem. RE's might be nice, but using them to check if a string
starts with a given string literal is a pretty lousy idea.

if testline[0] == "#":
break

works fine in this case (when you use the for-loop, at least).

if testline might be empty, use startswith() or slicing:

if testline.starts with("#"):
break

if testline[:1] == "#":
break

(if the thing you're looking for is longer than one character, startswith
is always almost the best choice)

if you insist on using a RE, you should use a plain if statement:

if re.match(patter n, testline):
break # break if it matched

</F>

[Craig Ringer]

On Tue, 2004-12-14 at 03:09, le*******@yahoo .com wrote:

Hi,
suppose I am reading lines from a file or stdin.
I want to just "peek" in to the next line, and if it starts
with a special character I want to break out of a for loop,
other wise I want to do readline().

Assuming there's a good reason, such as monster lines, not to just read
the next line anyway, I'd suggest read()ing the next character then
seek()ing back by one character to restore the file position.

def peekChar(fileob j):
ch = fileobj.read(1)
fileobj.seek(-1,1)
return ch

--
Craig Ringer

[Peter Otten]

le*******@yahoo .com wrote:

Hi,
suppose I am reading lines from a file or stdin.
I want to just "peek" in to the next line, and if it starts
with a special character I want to break out of a for loop,
other wise I want to do readline().

Is there a way to do this?
for example:
while 1:
line=stdin.peek _nextline()
if not line: break
if line[0]=="|":
break:
else:
x=stdin.nextlin e()
# do something with x

thanks

The itertools approach:

(some preparations)

import itertools
file("tmp.txt", "w").write( """\ .... a
.... b
.... c
.... |
.... d
.... e
.... f
.... """)

(1) You are only interested in lines after and including the "special" line:
def predicate(line) : .... return not line.startswith ("|")
.... for line in itertools.dropw hile(predicate, file("tmp.txt") ): .... print line,
....
|
d
e
f

(2) You want both the "head" and the "tail", where the tail includes the
"special" line:
lines = iter(file("tmp. txt"))
for line in lines: .... if line.startswith ("|"):
.... lines = itertools.chain ([line], lines)
.... break
.... print "head", line,
....
head a
head b
head c for line in lines: .... print "tail", line,
....
tail |
tail d
tail e
tail f

Peter

[Steven Bethard]

Skip Montanaro wrote:

les> suppose I am reading lines from a file or stdin. I want to just
les> "peek" in to the next line, and if it starts with a special
les> character I want to break out of a for loop, other wise I want to
les> do readline().

Create a wrapper around the file object:
[snip]
Of course, this could be tested (which my example hasn't been)

If you'd like something that works similarly and has been tested a bit,
try one of my recipes for peeking into an iterator:

http://aspn.activestate.com/ASPN/Coo.../Recipe/304373

You also might consider writing an iterator wrapper for a file object:

class strangefileiter (object): .... def __init__(self, file):
.... self.itr = iter(file)
.... def __iter__(self):
.... while True:
.... next = self.itr.next()
.... if not next or next.rstrip('\n ') == "|":
.... break
.... yield next
.... file('temp.txt' , 'w').write("""\ .... some text
.... some more
.... |
.... not really text""") for line in strangefileiter (file('temp.txt ')): .... print repr(line)
....
'some text\n'
'some more\n' file('temp.txt' , 'w').write("""\ .... some text
.... some more
....
.... really text""") for line in strangefileiter (file('temp.txt ')):

.... print repr(line)
....
'some text\n'
'some more\n'
'\n'
'really text'
Steve

[les_ander]

OK, I am sorry , I did not explain my problem completely.
I can easily break from the loop when I see the character in a line
that I just read; however my problem involves putting back the line I
just read since if I have seen this special character, I have read one
line too many. Let me illustrate
suppose the file has 3 lines

line1.line1.lin e1

line2.line2.li ne

line3.line3.lin e3

now suppose I have read the first line already.
then I read the second line and notice
that there is a ">" in front (my special character)
then I want the put back the second line into the
file or the stdin.

An easy way is if i read all the lines in to an array
and then I can put back the line with the special
character back into this array. However,
this file I am readding is huge! and I can read
it all in one swoop (not enough memory).

for x in stdin:
if x[0]==">":
#### put back the x some how... <-----
break
else:
print x

I hope this is clear
thanks

[Steven Bethard]

le*******@yahoo .com wrote:

now suppose I have read the first line already.
then I read the second line and notice
that there is a ">" in front (my special character)
then I want the put back the second line into the
file or the stdin.

Amended iterator class example using my peekable recipe:

class strangefileiter (object): .... def __init__(self, file):
.... self.iter = peekable(file)
.... def __iter__(self):
.... while True:
.... next = self.iter.peek( )
.... if not next or next.rstrip('\n ') == "|":
.... break
.... yield self.iter.next( )
.... file('temp.txt' , 'w').write("""\ .... some text
.... some more
.... |
.... not really text""") f = strangefileiter (file('temp.txt '))
for line in f: .... print repr(line)
....
'some text\n'
'some more\n' remainder = f.iter
for line in remainder:

.... print repr(line)
....
'|\n'
'not really text'

Note that because I wrap the file iterator with a peekable[1] object,
the lines aren't consumed when I test them. So to access the remaining
lines, I just use the iter field of the strangefileiter object.

Steve

[1] http://aspn.activestate.com/ASPN/Coo.../Recipe/304373