Specific question about readlines versus xreadlines

[Pernell Williams]

Hi all:

Thank you for your responses. I have a more specific question about
"file.seek() and file.readline()" versus "file.seek() and file.xreadlines".

When I have the following code:

#!/usr/bin/env python

#opening the file for reading

str = 'myShortFile'

file = open(str, 'r')

counter = 0

#outer loop using file.readline

while 1:

line = file.readline()

if line == '':

break

data = line

print data

file.seek(12)

# end while

And the input file is:

abc

def

ghi

The above code with the readline statement is:

abc

In other words, the code reads the first line of the file, goes to position
12 (which is the end of the last line of the file), and then during the
second iteration of the while loop, the position of the file is at 12.

Now, with xreadlines(), it is a different story. I have this code:

#!/usr/bin/env python

#opening the file for reading

str = 'myShortFile'

file = open(str, 'r')

counter = 0

#outer loop using file.xreadlines

for line in file.xreadlines():

data = line

print data

file.seek(12)

# end while

and if I have the same input file as above, I get this output:

abc

def

ghi

In other words, even though I have the "file.seek(12)" statement in the
loop, the position is NOT changed to 12 during the 2nd iteration of the
while loop.

Is there ANYTHING I can do in this "xreadlines()" loop to make it go to
position 12 in the second iteration?

Thanks for your help!

-Pernell


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[Erik Max Francis]

Pernell Williams wrote:

Thank you for your responses. I have a more specific question about
"file.seek() and file.readline()" versus "file.seek() and
file.xreadlines".

The latter won't work, since F.xreadlines buffers its reads.

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-- Louis Wu

[Garry Knight]

Erik gave you the correct answer. I just thought I'd point out a few other
things about your code:

In message <c3**********@news01.intel.com>, Pernell Williams wrote:

#opening the file for reading

str = 'myShortFile'
Did you know that str() is a Python function that returns the string
representation of an object. To see how it works, run Python interactively
and pass it some objects:
print str(3)
print str('hello')
d = {"one": 1, "two": 2, "three": 3}
print str(d)
In other words, it's not a good idea to redefine str in your code as it
hides the original function making it unusable. Also...
file = open(str, 'r')
In Python file() is a function that happens to be an alias of open(). So if
you redefine it like this you can't do:
f = file('myFile', 'r')
This is OK if you only intend to use open() but redefining existing Python
names is still arguably not good practice.
while 1:

line = file.readline()

if line == '':

break
The usual way to read lines from a file is like this (assuming you followed
my previous suggestion and stopped using 'file' for the filename:
for line in f.readlines():
# process line
This will loop until line == '' so you don't even need the break statement.
And you can make it shorter still:
for line in f:
# process line
And you can shorten the whole process further still by combining the open
with the read:
for line in open(filename):
# process line

I realise that your example is specifically comparing and contrasting the
readline() function with the xreadlines() function, but I wasn't sure if
you were aware of the different ways of doing the same thing. If you were,
please ignore the above paragraph: hopefully it will prove useful to
newcomers to Python.
data = line

print data

I'm curious as to why you didn't just do 'print line' rather than assigning
it to 'data' first.

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Garry Knight
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Linux registered user 182025