Python 2.5.2- what was instead of 'with' s

问题:

I wrote my code for python 2.7 but the server has 2.5. How do i rewrite the next code so it will run in python 2.5.2:

gzipHandler = gzip.open(gzipFile)

try:
    with open(txtFile, 'w') as out:
        for line in gzipHandler:
            out.write(line)
except: 
    pass

Right now, when i try to run my script I get this error:

Warning: 'with' will become a reserved keyword in Python 2.6 Traceback (most recent call last): File "Main.py", line 7, in from Extractor import Extractor File "/data/client/scripts/Extractor.py", line 29 with open(self._logFile, 'w') as out: ^ SyntaxError: invalid syntax

Thanks, Ron.

回答1:

In Python 2.5, you actually can use the with statement -- just import it from __future__:

from __future__ import with_statement

回答2:

If you can't, or don't want to use with, then use finally:

gzipHandler = gzip.open(gzipFile)
out = open(txtFile, 'w')
try:
    for line in gzipHandler:
        out.write(line)
finally:
    out.close()
    gzipHandler.close()

The cleanup code in the finally clause will always be excecuted, whether an exception is raised, or not.

回答3:

The "old" version of the code inside your try/except block would be:

out = open(txtFile, 'w')
for line in gzipHandler:
    out.write(line)
out.close()

The with open() ... context manager is effectively the same thing here. Python closes files automatically when their objects are garbage collected (see question 575278 for details), so out will be closed when the function it's in stops executing for some reason. Furthermore, the OS will close the file when the Python process terminates should it fail catastrophically for some reason before out.close() gets executed.

The with open() context manager will expand to approximately:

out = open(txtFile, 'w')
try:
    for line in gzipHandler:
        out.write(line)
finally:
    out.close()

See the above link to "context manager" for an explanation. So how does it work? It opens the file, executes your block of code, then explicitly closes the file. How does the "old" version I describe work? It opens the file, executes your block of code, then implicitly closes the file when its scope is finished or when the Python process terminates.

Save but for the "explicit" vs "implicit" parts, the functionality is identical.