Question about using "with"

[Steven W. Orr]

>From the tutorial, they said that the following construct will
automatically close a previously open file descriptor:

-------------------
#! /usr/bin/python
import sys

for nn in range ( 1, len(sys.argv ) ):
print "arg ", nn, "value = ", sys.argv[nn]
with open(sys.argv[nn]) as f:
for line in f:
print line,
------------------

but when I run it (with args) I get:

591 ./cat.py cat.py
File "./cat.py", line 6
with open(sys.argv[nn]) as f:
^
SyntaxError: invalid syntax
592 >

This example came from http://docs.python.org/tut/node10.html down in
section 8.7

Am I missing something?

TIA

--
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steveo at syslang.net

[Steven Bethard]

Re: Question about using "with&quot ;

Steven W. Orr wrote:

>From the tutorial, they said that the following construct will

automatically close a previously open file descriptor:
>
-------------------
#! /usr/bin/python
import sys
>
for nn in range ( 1, len(sys.argv ) ):
print "arg ", nn, "value = ", sys.argv[nn]
with open(sys.argv[nn]) as f:
for line in f:
print line,
------------------
>
but when I run it (with args) I get:
>
591 ./cat.py cat.py
File "./cat.py", line 6
with open(sys.argv[nn]) as f:
^
SyntaxError: invalid syntax
592 >
>
This example came from http://docs.python.org/tut/node10.html down in
section 8.7
>
Am I missing something?

You need to enable the with statement using a __future__ import::

>>with open('temp.txt' , 'w') as f:

<stdin>:1: Warning: 'with' will become a reserved keyword in Python 2.6
File "<stdin>", line 1
with open('temp.txt' , 'w') as f:
^
SyntaxError: invalid syntax

>>from __future__ import with_statement
>>with open('temp.txt' , 'w') as f:

.... f.write('hello' )
....

STeVe

[Peter Otten]

Re: Question about using &quot;with&quot ;

Steven W. Orr wrote:

>>From the tutorial, they said that the following construct will

automatically close a previously open file descriptor:
>
-------------------
#! /usr/bin/python
import sys
>
for nn in range ( 1, len(sys.argv ) ):
print "arg ", nn, "value = ", sys.argv[nn]
with open(sys.argv[nn]) as f:
for line in f:
print line,
------------------
>
but when I run it (with args) I get:
>
591 ./cat.py cat.py
File "./cat.py", line 6
with open(sys.argv[nn]) as f:
^
SyntaxError: invalid syntax
592 >
>
This example came from http://docs.python.org/tut/node10.html down in
section 8.7
>
Am I missing something?

The with-statement only works in Python 2.5, and you have to enable it
explicitly:

>>from __future__ import with_statement
>>with open("tmp.txt") as f:

.... print len(f.read())
....
15

>>f.read()

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file

Peter